A rotor sail and a method of operating a rotor sail

Abstract

A rotor sail 2 suitable for use as a wind propulsion device on a vessel (1, Fig 1) such as a ship. The rotor sail has a rotational axis 9 and comprises: an elongate cylindrical rotor body 8 having a p

Description

Field of the Invention The present invention relates to a rotor sail of a type configured to use the Magnus effect for propulsion. The invention also relates to a method of operating a rotor sail on a vessel. Background Rotor sails are known in the art and have been proposed previously for use in propelling vessels such as ships or boats through the water. Such rotor sails are also sometimes referred to as “Flettner Rotors” (so-named after the German engineer Anton Flettner, who was the first to build a ship using such rotor sails for propulsion) or “Magnus Rotors” (so-named after the Magnus effect which such rotor sails utilise). A so-called “rotor ship” or “Flettner ship” is a ship equipped with at least one such rotor sail as a form of propulsion. Despite having first been proposed for use as a form of ship propulsion nearly 100 years ago, rotor sails are now experiencing something of a revival as the shipping industry seeks to reduce its reliance on fossil fuels. Marine engineers are now turning to rotor sails as a means to harness the wind’s energy for propulsion, for example in so-called Wind Assisted Ship Propulsion (WASP) systems where wind propulsion is used to supplement the propulsive force produced by internal combustion engines in order to reduce fuel consumption and thereby improve economy and reduce emissions. As will be understood by those of skill in the art, a conventional rotor sail comprises a large cylindrical rotor body which is powered so as to rotate about its (vertically orientated) axis and thereby exploit the Magnus effect. The Magnus effect arises when the rotor body rotates about its axis in a moving airstream arising from the wind. Considering the boundary layer of air passing over the surface of the rotating rotor body, it can be understood that the rotor body will cause the air to slow on a first side of the rotor body as its surface moves against the motion of the air, and will cause the air to speed up on the opposite side of the rotor body as its surface moves in the same direction as the motion of the air. In accordance with Bernoulli’s theorem, this results in an increase in air pressure on the first side of the rotor body and a decrease in air pressure on the opposite side, giving rise to respective high pressure and low-pressure sides. This pressure difference across the rotor body creates a lift force on the rotor sail towards the low-pressure side. On a rotor vessel, in optimum wind directions, this lift force is resisted by the hull of the vessel and a component of the force is used to propel the vessel forwards. As will be appreciated, the manner in which the Magnus effect is harnessed by conventional rotor sails means that they are most effective in producing forwards thrust when the resulting lift force is aligned with the longitudinal direction of travel of the vessel. This in turn means that conventional rotor sails provide the greatest useful thrust in crosswind conditions in which the apparent wind incident on the rotor is directed from the side of the vessel, because in such conditions the difference in air pressure caused by the Magnus effect will arise between the aft side (high pressure) and forward side (low pressure) of the rotor body. Indeed, most conventional rotor sails produce the greatest thrust when the apparent wind direction is actually a few degrees aft of beam. The performance of conventional rotor sails is known to reduce considerably as the apparent wind moves forward or aft from this ideal beam wind direction towards a headwind or tailwind direction, and can be significantly affected by vessel speed. Whilst this drop-off in performance in sub-optimal wind directions can be mitigated to some degree by adjusting the speed of rotation of the rotor body, conventional rotor sails nevertheless have a rather small effective optimal operating range in terms of apparent wind angle. Conventional rotor sails also suffer from disadvantages in headwind conditions, and indeed also in tailwind conditions. As will be appreciated, in headwind conditions the rotor is unable to contribute thrust to the vessel because it is unable to generate a lift force having a component aligned with the direction of travel of the vessel. Therefore, in such conditions conventional rotor sails are typically disabled such that their rotor bodies do not rotate. When disabled in this manner, the rotor body thus acts as a simple static cylinder (effectively a bluff body), creating a turbulent wake and generating only drag acting against the direction of travel. Furthermore, in such circumstances vortex shedding can occur, producing an oscillating flow downwind of the rotor body in which vortices detach periodically from each side of the static rotor body, thereby forming a so-called Karman vortex street. This can have serious consequences on the structural integrity of a rotor sail in high wind conditions because vortex shedding can cause the rotor to oscillate violently. Similar problems due to vortex shedding can also occur in tailwind conditions although, as will be appreciated, in tailwind conditions the drag force produced by the static rotor will at least produce some useful thrust in the direction of travel of the vessel. Additionally, the present inventors have observed that even in beam wind conditions, conventional rotor sails are rather poorly optimised, because airflow still struggles to remain attached to the curved surface of the rotor body and thus tends to separate from the rotor body in a turbulent fashion earlier than desired. Thus, even in good beam wind conditions, conventional rotor sails often produce high drag forces. Conventional rotor sails also permit very little control of their lift force in response to changes in the apparent wind direction, with the only meaningful control option being adjustment of rotor spin speed. Furthermore, if a convention rotor sail is subject to a power failure rendering its rotor body inoperable (i.e., non-rotating) during use, even in optimum beam wind conditions, then it will act as a simple static cylinder (effectively a bluff body) and will create a turbulent wake with the potential for vortex shedding and its associated problems, in the same manner as noted above in headwind conditions. The present invention has been devised in light of the above considerations. In one aspect, the present invention seeks to provide an improved rotor sail. In another aspect, the present invention seeks to provide an improved method of operating a rotor sail on a vessel. Summary of the Invention According to a first aspect, there is provided a rotor sail having a rotational axis and comprising: an elongate cylindrical rotor body having a peripheral wall rotatable about the rotational axis; and a flap arrangement adjacent the rotor body, the flap arrangement having: a pair of leading edges proximate the peripheral wall and substantially parallel to the rotational axis; a pair of trailing edges distal to the peripheral wall; a pair of aerosurfaces, each aerosurface interconnecting a respective said leading edge and a respective said trailing edge; and a pair of notional chord lines, each chord line interconnecting a respective said leading edge and a respective said trailing edge in transverse cross-section; the flap arrangement being configured or configurable such that: said aerosurfaces are oppositely and outwardly directed relative to one another and said chord lines are convergent in a direction away from the rotor body .It has been found that by providing a rotor sail with a flap arrangement of the type defined above, the flap arrangement acts to block flow of air around the downwind side of the rotor body, from the high-pressure region towards the low-pressure region, thereby presenting a preferable smoother flow path that promotes flow attachment to the low-pressure region of the rotor body for longer. The resulting longer and smoother flow path lowers pressure in the low-pressure region and increases pressure in the high-pressure region relative to a conventional rotor sail, thereby increasing the lift force produced. This can be considered to mimic a larger and more cambered wing sail with an additional Gurney flap. Additionally, it has been found that the flap arrangement defined above promotes less turbulent flow separation, meaning that it effectively reduces the drag force produced, relative to a conventional rotor sail, which further contributes to an increased resulting net force vector. Additional benefits arising from the resultant reduction in turbulence include reduced oscillation and vibration in use, which can be advantageous for the long-term durability of the rotor sail and passenger comfort, and may permit a reduction in the weight of materials used to construct the rotor sail. In some proposals, said aerosurfaces are substantially concave. It is to be appreciated, however, that the aerosurfaces may alternatively be substantially planar, for example. Advantageously, in some proposals, said flap arrangement may be concentrically adjustable relative to the rotational axis within a range of positions about the rotor body. A rotor sail of the configuration proposed provides improved aerodynamic performance in ideal beam wind conditions, and also facilitates a wide range of adjustment to tune the rotor sail to account for changes in apparent wind direction, thereby allowing the lift force produced by the sail to be ‘steered’ as required (for example into alignment with the course direction of the vessel) for any given apparent wind direction. The proposed rotor sail may be thought of as a hybrid proposal combining aspects of conventional rotor sails with aspects of wing sails. The flap arrangement may be positioned to cooperate with the rotor body to define an improved and adjustable aerodynamic profile -effectively making the rotor sail more ‘wing-like’. The proposed rotor sail also facilitates adjustment of the position of the flap arrangement about the rotational axis of the rotor body, and this adjustment can be considered similar to adjustment of the camber profile of a wing sail, thereby permitting optimization of the lift force produced, both in terms of intensity and direction. It is proposed that the rotor sail may be actively controlled in use (for example under the control of one or more computer devices) by adjusting the position and / or configuration of the flap arrangement, optionally in combination with adjustment of the rotational speed of the rotor body, to maximise performance in any given wind condition. Optionally, said flap arrangement may comprise a single flap defining both leading edges and both trailing edges. The single flap may be configured such that the trailing edges are circumferentially spaced-apart about the rotational axis. Alternatively, according to a second aspect, the flap arrangement may comprise a pair of discrete flaps. In such an arrangement, each said flap may have a respective said leading edge, a respective said trailing edge, a respective said aerosurface, and a respective said chord line, wherein the flaps are independently concentrically adjustable relative to the rotational axis within a range of flap positions about the rotor body. Said range of flap positions may include at least one operational configuration in which said flaps are positioned such that their aerosurfaces are oppositely and outwardly directed relative to one another and their chord lines are convergent in a direction away from the rotor body. It has been found that by positioning the flaps in an operational configuration as defined above, the flaps act to block flow of air around the downwind side of the rotor body, from the high-pressure region towards the low-pressure region, thereby presenting a preferable smoother flow path that promotes flow attachment to the low-pressure region of the rotor body for longer. The resulting longer and smoother flow path lowers pressure in the low-pressure region and increases pressure in the high-pressure region relative to a conventional rotor sail, thereby increasing the lift force produced. This can be considered to mimic a larger and more cambered wing sail with an additional gurney flap. Additionally, it has been found that the ability to position the flaps to adopt an operational configuration as defined above promotes less turbulent flow separation, meaning that the flaps effectively reduce the drag force produced, relative to a conventional rotor sail, which further contributes to an increased resultant force vector. Additional benefits arising from the resultant reduction in turbulence include reduced oscillation and vibration in use, which can be advantageous for the long-term durability of the rotor sail and passenger comfort, and may permit a reduction in the weight of materials used to construct the rotor sail. Furthermore, positioning the flaps to adopt an operational configuration as defined above has been found to significantly reduce the likelihood of vortex shedding occurring in the event of a power failure rendering the rotor body inoperable (i.e., non-rotating). Indeed, it has been found that in such circumstances the rotor sail of the present invention is still able to generate useful lift force because, with the flaps positioned in the above-noted operational position and the rotor body static, the rotor sail will resemble a standard wing profile. Preferably, each flap is mounted for pivotal adjustment about the rotational axis through a range of flap positions in which the trailing edge of the flap is centred on said rotational axis. Optionally, each trailing edge is spaced from the rotational axis of the rotor body by a distance of between 1.8R and 2.2R, where R denotes the radius of the rotor body. In such a configuration the length of the flap arrangement (e.g., the length of each flap) extending radially away from the rotor body is generally comparable to the radius of the rotor body itself (i.e., between 0.8R and 1.2R). In some embodiments, each trailing edge is spaced from the rotational axis of the rotor body by a distance of approximately 2R. In such a configuration the length of the flap arrangement extending radially away from the rotor body is approximately equal to the radius of the rotor body itself. Each aerosurface may have a region proximate a respective leading edge which lies substantially tangential to the peripheral wall of the rotor body. In the case of an arrangement having two adjustable flaps, this ensures that wherever each flap is positioned about the rotational axis of the rotor body, its aerosurface will extend smoothly from the adjacent region of the peripheral wall of the rotor body, thereby minimizing flow separation and / or turbulence in the region of the flap’s leading edge. Optionally, each aerosurface comprises a region proximate a respective trailing edge which lies approximately radial to the peripheral wall of the rotor body. In some embodiments, the flap arrangement (e.g., each flap) is spaced from the peripheral wall of the rotor body by a gap having a radial dimension of approximately 0.01 R to 0.02R, where R denotes the radius of the rotor body. A small gap of this configuration between the peripheral wall of the rotor body and the flap arrangement permits rotation of the rotor body in use such that its peripheral wall may move freely past the flap arrangement. The gap is preferably sufficiently small to minimise flow separation and turbulence. Optionally, each leading edge has a curved profile having a radius of approximately 0.01 R to 0.02R, where R denotes the radius of the rotor body. By providing a radius to the leading edges of the flap arrangement in this manner, aerodynamic robustness may be improved, and flow separation minimised. Optionally, the flap arrangement may be configured or configurable (e.g., via adjustment) such that the trailing edges subtend an angle of between 5° and 25° from the rotational axis of the rotor body. In some proposals the trailing edges may subtend an angle of between 10° and 20° from the rotational axis of the rotor body in the operational configuration. In particular embodiments the trailing edges may subtend an angle of approximately 15° from the rotational axis of the rotor body in the operational configuration Optionally, in proposals in which the flap arrangement comprises a pair of discrete flaps, the above-noted operational configuration may be such that the trailing edges of the flaps subtend an angle of between 5° and 25° from the rotational axis of the rotor body. In some proposals the trailing edges of the flaps may subtend an angle of between 10° and 20° from the rotational axis of the rotor body in the operational configuration. In particular embodiments the trailing edges of the flaps may subtend an angle of approximately 15° from the rotational axis of the rotor body in the operational configuration. Configuring the rotor sail such that the flaps may be positioned in this manner allows the adoption, in use, of flap positions which may be optimised for a wide range of operating conditions such as, for example, different apparent wind directions. Optionally, the flap arrangement may be configured or configurable (e.g., via adjustment) such that the chord lines converge at an angle of between 35° and 55°. In some proposals the flap arrangement may be configured or configurable such that the chord lines converge at an angle of between 40° and 50°. In particular embodiments, the chord lines may converge at an angle of 45° in said operational configuration. Optionally, in proposals in which the flap arrangement comprises a pair of discrete flaps, the operational configuration may be such that the chord lines of the flaps converge at an angle of between 35° and 55°. In some proposals the operational configuration may be such that the chord lines of the flaps converge at an angle of between 40° and 50°. In particular embodiments, the chord lines of the flaps may converge at an angle of 45° in said operational configuration. Configuring the rotor sail such that the flaps may be positioned in this manner allows the adoption, in use, offlap positions which may be optimised for a wide range of operating conditions such as, for example, different apparent wind directions. It is to be appreciated that in proposals in which the flap arrangement comprises a pair of discrete flaps, the range offlap positions may further include (e.g., additional) configurations in which the flaps are positioned such that their chord lines are divergent in a direction away from the rotor body. For example, in some proposals the range of flap positions may include an alternate operational configuration in which the flaps are positioned such that their trailing edges subtend an angle of between 140° and 180° from the rotational axis of the rotor body. In some particular embodiments the flaps may be positioned in an alternate operational configuration in which their trailing edges subtend an angle of approximately 160° from the rotational axis of the rotor body. Configuring the rotor sail such that the flaps may be positioned in this manner, with their chord lines divergent, allows the adoption in use of flap positions which may increase (e.g., maximise) the windage area of the rotor sail. This may be useful, for example, in tailwind conditions, where the flaps can be positioned generally diametrically opposite one another on respective sides of the rotor body, thereby providing an increased windage area in combination with the rotor body. In such circumstances, it is envisaged that the rotor body may be held static (i.e., not rotating) such that the rotor sail may act as a passive drag device (akin to a parachute) to generate thrust aligned with the dead-ahead direction of the vessel in tailwind conditions. However, in some proposals it is recognised that there may be some advantage to rotating the rotor body at very low speeds in tail wind conditions for mechanical reasons, for example to avoid potentially damaging stress being placed on the bearings of drive motors of the rotation mechanism and / or to aid in the avoidance of ice build up in very cold conditions. In proposals in which the flap arrangement comprises a pair of discrete flaps, the range of flap positions may include a feathered configuration in which the flaps are positioned such that their trailing edges subtend an angle of between 30° and 50° from the rotational axis of the rotor body. In some proposals, the feathered configuration may be such that the trailing edges of the flaps subtend an angle of approximately 40° from the rotational axis of the rotor body. The feathered configuration may be such that the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another and the chord lines of the flaps are convergent in a direction away from the rotor body. In some proposals, the feathered configuration may be such that the chord lines of the flaps converge at an angle of between 10° and 30°. In particular embodiments, the feathered configuration may be such that the chord lines of the flaps converge at an angle of approximately 20°. Configuring the rotor sail such that the flaps may be positioned in a feathered configuration having any / all of the aforementioned characteristics allows the adoption, in use, of flap positions which may be optimised for headwind conditions (or indeed in beam wind conditions in the event that the rotor sail is disabled or otherwise inoperable). Positioning the flaps in the feathered configuration in such conditions, with the flaps extending away from the rotor body in a downwind direction, serves to significantly reduce drag in comparison to a conventional rotor sail in such conditions. Furthermore, in this configuration the flaps additionally serve to minimise vortex shedding which, as noted above, can represent a problem with conventional rotor sails in headwind conditions or in other conditions when inoperable. In such circumstances, it is envisaged that the rotor body may be held static (i.e., not rotating) in a similar manner to that in which conventional rotor sails are operated in headwind conditions. However, in some proposals it is recognised that there may be some advantage to rotating the rotor body at very low speeds in headwind conditions for mechanical reasons, for example to avoid potentially damaging stress being placed on the bearings of drive motors of the rotation mechanism and / or to aid in the avoidance of ice build-up in very cold conditions. According to a third aspect, there is provided a method of operating a rotor sail on a vessel, the method comprising: providing a rotor sail according to the first or second aspects in an upstanding position on the vessel such that said rotational axis is substantially orthogonal to a longitudinal axis of the vessel; determining an apparent wind angle value representative of the angle between the direction of apparent wind incident on the rotor sail and a dead-ahead direction of the vessel coincident with said longitudinal axis; and positioning said flap arrangement relative to said rotor body in dependence on said apparent wind angle value. Optionally, the method may comprise providing a rotor sail according to the second aspect, wherein said step of positioning the flap arrangement involves positioning said discrete flaps relative to said rotor body in dependence on said apparent wind angle value. Optionally, a range of apparent wind angles from 0° to 180° may be divided into a plurality of apparent wind angle sectors, and a plurality of distinct flap configurations may be defined within said range of flap positions, each said distinct flap configuration corresponding to a respective said sector. The method may involve monitoring said apparent wind angle value, determining in which of said sectors the determined apparent wind angle falls, and positioning the flaps in the corresponding flap configuration in response thereto. For example, said range of apparent wind angles may be divided into seven said wind angle sectors, and the method may involve positioning said flaps in one of seven respective distinct flap configurations. In a particular example, in response to determination of an apparent wind angle value of between 0° and 3° (e.g., representative of headwind conditions) said flaps may be positioned in a feathered configuration. Optionally, rotation of the rotor body about the rotational axis may be substantially prevented. The feathered configuration may be such that the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another and the chord lines of the flaps are convergent in a substantially downwind direction away from the rotor body. Optionally, the feathered configuration may be such that the chord lines of the flaps each make a substantially equal angle of between 5° and 15° to the direction of apparent wind incident on the rotor sail. In some embodiments of the method the feathered configuration is such that the chord lines of the flaps each make an angle of approximately 10° to the direction of apparent wind incident on the rotor sail. The feathered configuration may be such that the trailing edges of the flaps are equispaced from a notional midline substantially parallel with the direction of apparent wind incident on the rotor sail. Configuring the rotor sail in the above-noted feathered configuration (and optionally preventing rotation of the rotor body) serves to optimise the rotor sail for headwind conditions. Positioning the flaps in the feathered configuration in such conditions, with the flaps extending away from the rotor body in a downwind direction, serves to significantly reduce drag in comparison to a conventional rotor sail in such conditions. Furthermore, in this configuration the flaps additionally serve to minimise vortex shedding which, as noted above, can represent a problem with conventional rotor sails in headwind conditions or in other conditions when inoperable. By way of example, in response to determination of an apparent wind angle value of between 4° and 12° the flaps may be positioned in a first operational configuration and the rotor body may be rotated about the rotational axis at a first rotational speed. Said first operational configuration may be such that: the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another; the chord lines of the flaps are convergent in a direction away from the rotor body; the chord line of a first windwardmost said flap makes an angle of between 10° and 20° to said direction of apparent wind incident on the rotor sail; and the chord line of the other leewardmost flap makes an angle of between 25° and 35° to said direction of apparent wind incident on the rotor sail. In some implementations of the method the first operational configuration may be such that the chord line of the first windwardmost flap makes an angle of approximately 15° to the direction of apparent wind incident on the rotor sail, and the chord line of the other leewardmost flap makes an angle of approximately 30° to the direction of apparent wind incident on the rotor sail. For example, said first rotational speed may be approximately 30 revolutions per minute. In some implementations of the method said first operational configuration is such that the trailing edges of said flaps are equispaced from a notional midline making an angle of: i) between 5° and 10°; or ii) between 7° and 8°; or iii) 7.5°, relative to said direction of apparent wind incident on the rotor sail. By way of example, in response to determination of an apparent wind angle value of between 13° and 24° the flaps may be positioned in a second operational configuration and the rotor body may be rotated about the rotational axis at a second rotational speed. The second operational configuration may be such that: the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another; the chord lines of the flaps are convergent in a direction away from the rotor body; the chord line of a first windwardmost flap makes an angle of between 10° and 20° to said direction of apparent wind incident on the rotor sail; and the chord line of the other leewardmost flap makes an angle of between 55° and 65° to said direction of apparent wind incident on the rotor sail. In some implementations of the method the second operational configuration may be such that the chord line the first windwardmost flap makes an angle of approximately 15° to the direction of apparent wind incident on the rotor sail, and the chord line of the other leewardmost flap makes an angle of approximately 60° to the direction of apparent wind incident on the rotor sail. By way of example, said second rotational speed may be approximately 60 revolutions per minute. In some implementations of the method said second operational configuration is such that the trailing edges of said flaps are equispaced from a notional midline making an angle of: i) between 35° and 40°; or ii) between 37° and 38°; or iii) 37.5°, relative to said direction of apparent wind incident on the rotor sail. By way of example, in response to determination of an apparent wind angle value of between 25° and 37° said flaps may be positioned in a third operational configuration and the rotor body may be rotated about the rotational axis at a third rotational speed. The third operational configuration may be such that: the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another; the chord lines of the flaps are convergent in a direction away from the rotor body; the chord line of a first windwardmost flap makes an angle of between 25° and 35° to said direction of apparent wind incident on the rotor sail; and the chord line of the other leewardmost flap makes an angle of between 70° and 80° to said direction of apparent wind incident on the rotor sail. In some implementations of the method the third operational configuration may be such that the chord line the first windwardmost flap makes an angle of approximately 30° to the direction of apparent wind incident on the rotor sail, and the chord line of the other leewardmost flap makes an angle of approximately 75° to the direction of apparent wind incident on the rotor sail. By way of example, said third rotational speed may be approximately 90 revolutions per minute. In some implementations of the method the third operational configuration is such that the trailing edges of said flaps are equispaced from a notional midline making an angle of: i) between 50° and 55°; or ii) between 52° and 53°; or iii) 52.5°, relative to said direction of apparent wind incident on the rotor sail. By way of example, in response to determination of an apparent wind angle value of between 38° and 60° the flaps may be positioned in a fourth operational configuration and the rotor body may be rotated about the rotational axis at a fourth rotational speed. The fourth operational configuration may be such that: the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another; the chord lines of the flaps are convergent in a direction away from the rotor body; the chord line of a first windwardmost flap makes an angle of between 40° and 50° to the direction of apparent wind incident on the rotor sail; and the chord line of the other leewardmost flap makes an angle of between 85° and 95° to the direction of apparent wind incident on the rotor sail. In some implementations of the method the fourth operational configuration may be such that the chord line of the first windwardmost flap makes an angle of approximately 45° to the direction of apparent wind incident on the rotor sail, and the chord line of the other leewardmost flap makes an angle of approximately 90° to the direction of apparent wind incident on the rotor sail. By way of example, said fourth rotational speed may be approximately 120 revolutions per minute. In some implementations of the method the fourth operational configuration is such that the trailing edges of the flaps are equispaced from a notional midline making an angle of: i) between 65° and 70°; or ii) between 67° and 68°; or iii) 67.5°, relative to the direction of apparent wind incident on the rotor sail. By way of example, in response to determination of an apparent wind angle value of between 61° and 178° the flaps may be positioned in a fifth operational configuration and the rotor body may be rotated about the rotational axis at a fifth rotational speed. The fifth operational configuration may be such that: the concave aerosurfaces of the flaps are oppositely and outwardly directed relative to one another; the chord lines of the flaps are convergent in a direction away from the rotor body; the chord line of a first windwardmost flap makes an angle of between 55° and 65° to the direction of apparent wind incident on the rotor sail; and the chord line of the other leewardmost flap makes an angle of between 100° and 110° to the direction of apparent wind incident on the rotor sail. In some implementations of the method the fifth operational configuration may be such that the chord line of said the windwardmost flap makes an angle of approximately 60° to the direction of apparent wind incident on the rotor sail, and the chord line of the other leewardmost flap makes an angle of approximately 105° to the direction of apparent wind incident on the rotor sail. By way of example, the fifth rotational speed may be approximately 120 revolutions per minute. In some implementations, the fifth operational speed may be substantially equal to the fourth operational speed. The method may involve rotating the rotor body about the rotational axis at substantially the same rotation speed in: i) conditions in which the apparent wind angle value is between 61° and 178°; and ii) conditions in which the apparent wind angle value is between 38° and 60°. In some implementations of the method the fifth operational configuration is such that the trailing edges of said flaps are equispaced from a notional midline making an angle of: i) between 80° and 85°; or ii) between 82° and 83°; or iii) 82.5°, relative to the direction of apparent wind incident on the rotor sail. The rotor body may be rotated in a clockwise direction in response to apparent wind being incident on the port side of the vessel. The rotor body may be rotated in a counterclockwise direction in response to apparent wind being incident on the starboard side of the vessel. By way of example, in response to determination of an apparent wind angle value of between 179° and 180° the flaps may be positioned in a drag configuration and rotation of the rotor body about the rotational axis may be substantially prevented. The drag configuration may be such that the flaps are positioned substantially diametrically opposite one another across the rotor body with their concave aerosurfaces directed substantially downwind and thus generally away from the apparent wind incident on the rotor sail. Each flap may thereby present a surface opposite its respective concave aerosurface to the apparent wind. Optionally, the drag configuration may be such that the chord lines of the flaps are divergent in an upwind direction away from the rotor body. The drag configuration may be such that the trailing edges of the flaps subtend an angle of between 140° and 180° from the rotational axis of the rotor body. In some implementations of the method the drag configuration is such that the trailing edges of the flaps subtend an angle of approximately 160° from the rotational axis of the rotor body. The drag configuration may be such that the trailing edges of the flaps are equispaced from a notional midline substantially parallel with the direction of apparent wind incident on the rotor sail. The method of the invention may involve substantially continuously monitoring wind incident on the rotor sail during use, determining said apparent wind angle value therefrom, and adjusting the position of said flaps (and optionally also the rotational speed of the rotor body) in response to a change in said apparent wind angle value. The method may involve operation of a plurality of said rotor sails. For example, a ship may be equipped with a plurality of substantially identical rotor sails in accordance with the first aspect of the invention. In such implementations, the positions of the flaps and the rotational speed of each rotor sail may be adjusted in dependence on the apparent wind angle value representative of the apparent wind incident on the respective rotor sail. The method may therefore involve selecting different flap positions and rotor speeds for individual rotor sails. For example, in an installation comprising two rows of rotor sails arranged longitudinally along a ship, then in beam wind conditions the rotor sails of one (downwind) row may positioned in a wind shadow of the other (upwind) row, meaning that rotor sails in the downwind row will experience different wind conditions (e.g., a different apparent wind direction) to those in the upwind row. By adjusting the flap positions and rotational speed of the rotor sails individually, each rotor sail may be optimised for its local wind condition. According to a fourth aspect, there is provided: a rotor sail having a rotational axis and comprising: an elongate cylindrical rotor body having a peripheral wall rotatable about the rotational axis; and a pair of discrete elongate flaps arranged adjacent the rotor body, each flap having: a respective leading edge proximate the peripheral wall and substantially parallel to the rotational axis; a respective trailing edge distal the peripheral wall and substantially parallel to the rotational axis; a respective concave aerosurface interconnecting said leading and trailing edges; and a notional chord line interconnecting said leading and trailing edges in transverse cross-section; the flaps being independently concentrically adjustable relative to the rotational axis within a range of flap positions about the rotor body, wherein said range of flap positions includes at least one operational configuration in which said flaps are positioned such that their concave aerosurfaces are oppositely and outwardly directed relative to one another and their chord lines are convergent in a direction away from the rotor body. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and / or combined with any other feature or parameter described herein. Summary of the Figures So that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view illustrating a rotor ship equipped with four rotor sails in accordance with the invention; Figure 2 is a schematic perspective view showing some components of a rotor sail in accordance with the invention, the rotor sail being shown in broken view to illustrate features in transverse cross-section; Figure 3 is a schematic transverse cross-sectional type view of the rotor sail, showing its flaps in an example operational configuration; Figure 4 is a schematic transverse cross-sectional type view similar to that of Figure 3; Figure 5 is a schematic cross-sectional type view, to an enlarged scale, showing part of the rotor sail in more detail; Figure 6 is a schematic transverse cross-sectional type view, similar to that of Figure 3, but which shows the flaps in an alternate operational configuration; Figure 7 is a schematic perspective showing a possible alternative type of flap; Figure 8 is a schematic perspective view from above showing two further alternative types of flap; Figure 9 is a schematic plan view from above showing the rotor sail in an example operational configuration similar to that of Figure 3, with part of the rotor sail denoted in transparency; Figure 10 is a schematic side elevational view showing the uppermost region of the rotor sail of Figure 9; Figure 11 is a schematic plan view similar to that of Figure 9, but which shows the sail’s flaps in an alternate operational configuration similar to that of Figure 6; Figure 12 is a schematic perspective view from above showing a rotor sail in accordance with the present invention, with the sail’s rotor body being shown in transparency; Figure 13 is a schematic side elevational view showing the rotor sail of figure 12, with the sail’s rotor body again shown in transparency; Figure 14 is a schematic perspective view showing the flaps of the rotor sail and part of a mechanism to reposition the flaps relative to the sail’s rotor body; Figure 15 is a schematic perspective view showing the upper region of the flaps and another part of the mechanism; Figure 16 is a schematic perspective view showing further detail of the mechanism shown in Figure 14; Figure 17 is a schematic illustration showing a rotor sail vessel in headwind conditions; Figures 18a and 18b are schematic illustrations showing the rotor sail configured for the headwind conditions of Figure 17, in which the flaps are shown in a feathered configuration; Figure 19 is a schematic illustration similar to that of Figure 17, but which shows the vessel subject to wind conditions characterised by a first non-zero apparent wind angle; Figures 20a and 20b are schematic illustrations showing the rotor sail configured for the wind conditions of Figure 19, in which the flaps are shown in a first operational configuration; Figure 21 is a schematic illustration showing the vessel subject to wind conditions characterised by a second non-zero apparent wind angle; Figures 22a and 22b are schematic illustrations showing the rotor sail configured for the wind conditions of Figure 21, in which the flaps are shown in a second operational configuration; Figure 23 is a schematic illustration showing the vessel subject to wind conditions characterised by a third non-zero apparent wind angle; Figures 24a and 24b are schematic illustrations showing the rotor sail configured for the wind conditions of Figure 23, in which the flaps are shown in a third operational configuration; Figure 25 is a schematic illustration showing the vessel subject to wind conditions characterised by a fourth non-zero apparent wind angle; Figures 26a and 26b are schematic illustrations showing the rotor sail configured for the wind conditions of Figure 25, in which the flaps are shown in a fourth operational configuration; Figures 27a, 27b and 27c are schematic illustrations showing the vessel in wind conditions characterised by various further apparent wind angles; Figures 28a and 28b are schematic illustrations showing the rotor sail configured for the wind conditions of Figures 27a, 27b and 27c, in which the flaps are shown in a fifth operational configuration; Figure 29 is a schematic illustration showing the vessel in tailwind conditions; Figures 30a and 30b are schematic illustrations showing the rotor sail configured for the tailwind conditions of Figure 29, in which the flaps are shown in a drag configuration; Figure 31 is a schematic illustration showing division of a range of apparent wind directions divided into a number of apparent wind angle sectors; Figure 32 is a schematic side elevational view showing a rotor sail in accordance with the invention having a different form of flap; and Figure 33 schematic transverse cross-sectional type view showing an alternative rotor sail having a flap arrangement comprising a single flap instead of two flaps. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. Turning initially to Figure 1, there is illustrated a vessel 1 in the form of a so-called ‘rotor ship’ equipped with four substantially identical rotor sails 2 in accordance with an embodiment of the present invention. As illustrated, the rotor sails 2 are arranged at the corners of a notional rectangle and comprise a pair of rotor sails 2 located towards the bow 3 of the vessel 1 and another pair of rotor sails 2 located towards the stern 4 of the vessel (but nevertheless forward of the vessel’s superstructure 5). It is to be appreciated, however, that alternative arrangements and numbers of rotor sails 2 are possible; for example, installations are envisaged in which all the rotor sails 2 may be aligned along the length of the vessel 1 (for example along a central longitudinal axis of the vessel 2, or alternatively along an axis substantially parallel to the central longitudinal axis of the vessel 2). It is also envisaged that a vessel 1 may be equipped with a single rotor sail 2. As is conventional, each rotor sail 2 is mounted so as to extend generally vertically above the deck 6 of the vessel and terminates in an end plate 7 at its uppermost (free) end. As is also conventional, and as will be explained in more detail below, each rotor sail 2 comprises an elongate cylindrical rotor body 8 arranged for rotation about a respective rotational axis 9 (only one such rotational axis 9 being illustrated in Figure 1) oriented so as to be orthogonal to a longitudinal axis of the vessel 1, and thus substantially vertical in flat water conditions in which the deck is substantially horizontal. Turning now to consider Figures 2 to 9, further features of each rotor sail 2 will now be described. Figure 2 illustrates a single rotor sail 2 in schematic broken view, denoting a central length of the rotor sail 2 having been removed, so that some components can be seen in transverse cross-section. As will be observed, the rotor body 8 has a peripheral wall 10 which is rotatable about the rotational axis 9. As will be appreciated, in common with conventional rotor sails the rotor body 8 and thus also its peripheral wall 10 will be rotated in use about the rotational axis 9, for example via the use of one or more electrical motors (not shown in Figure 2). In addition to the rotor body 8, the rotor sail 2 also comprises a flap arrangement 11 arranged adjacent the rotor body 8 so as to aerodynamically interact with the peripheral wall 10 thereof. In the particular proposal illustrated, the flap arrangement 11 comprises a pair of discrete elongate flaps 11a, 11b arranged adjacent the rotor body 8. In the arrangement illustrated, the flap arrangement 11 (and thus each flap 11a, 11b thereof) has vertical height approximately equal to vertical height of the rotor body 8, such that the flap arrangement 11 extends all of the way along the side of the rotor body 8. However, examples are envisaged in which the flap arrangement 11 may have a reduced vertical height such that it extends only part way along the side of the rotor body 8. As will be explained in more detail below, in the illustrated example the flaps 11a, 11b are independently concentrically adjustable relative to the rotational axis 9 so that they may selectively adopt a plurality of positions relative to the rotor body 8. In Figure 2, the flaps 11a, 11b are shown in positions in which they are adjacent one another so as to be substantially abutting. As illustrated more clearly in the schematic transverse cross-sectional view of Figure 3, each flap 11a, 11b comprises a respective leading edge 12a, 12b proximate the peripheral wall 10 of the rotor body 8. Both leading edges 12a, 12b lie substantially parallel to the rotational axis 9 of the rotor body 10. Additionally, each flap 11a, 11b also comprises a respective trailing edge 13a, 13b distal to the peripheral wall 10 of the rotor body 8. Both trailing edges 13a, 13b are also arranged to lie substantially parallel to the rotational axis 9 of the rotor body 10. A respective concave aerosurface 14a, 14b interconnects the leading and trailing edges of each flap 11a, 11b. As further illustrated in Figure 3, a respective notional chord line 15a, 15b may also be considered to interconnect the leading and trailing edges of each flap 11a, 11b in transverse cross-section. As indicated above, the flaps 11a, 11b are independently concentrically adjustable about the rotational axis 9 so that they may adopt a range of positions relative to the rotor body 8 and each other. It is envisaged that in use in steady-state wind conditions, each flap 11a, 11b will remain substantially static in an operational configuration, whilst the rotor body 9 will rotate (i.e., spin) about the rotational axis 9. Figure 3 illustrates the two flaps 11a, 11b in one such operational configuration in which they are circumferentially spaced apart from one another about the rotor body 8. In more detail, it will be observed that the operational configuration illustrated in Figure 3 is such that the flaps 11a, 11b are positioned with their aerosurfaces 14a, 14b oppositely and outwardly directed relative to one another, and with their chord lines 15a, 15b convergent in a direction away from the rotor body 8 (as denoted by arrow 16). It is to be noted, therefore, that although the flaps 11a, 11b may each adopt a wide range of positions about the rotor body 8 and its rotational axis 9, the arrangement is configured such that the range of possible flap positions includes at least one operational configuration in which the flaps 11a, 11b are positioned with their aerosurfaces 14a, 14b oppositely and outwardly directed relative to one another, and with their chord lines 15a, 15b convergent in a direction 16 away from the rotor body 8 as illustrated. As may be understood most clearly from Figure 4, each flap 11a, 11b is mounted for pivotal adjustment about the rotational axis 9 of the rotor body through a range of flap positions in which the respective trailing edges 13a, 13b remain centred on the rotational axis 9. With reference again to Figure 3, the arrangement may be configured such that the range of possible flap positions includes an operational configuration in which the trailing edges 13a, 13b of the flaps 11a, 11b subtend an angle 0 from the rotational axis 9 of between 5° and 25°. In some embodiments the angle 0 may be between 10° and 20° such as, for example, 15° As shown in Figure 3, the above-noted operational configuration achievable by the flaps 11a, 11b may be further characterized in some embodiments in terms of the angle of convergence (denoted C in Figure 3) of the chord lines 15a, 15b of the flaps 11a, 11b. For example, in some embodiments the chord lines 15a, 15b of the flaps 11a, 11b may converge at an angle C of between 35° and 55°, optionally between 40° and 50°. In some particular embodiments, the chord lines 15a, 15b of the flaps 11a, 11b may converge at an angle C of approximately 45° With reference to Figure 4, it will be understood that the trailing edges 13a, 13b of the flaps 11a, 11b are positioned on a notional circle 17 centred on the rotational axis 9 of the rotor body 8 and having a radius d which is greater than the radius R of the rotor body 8. The radius d of the notional circle 17 thus denotes the distance by which the trailing edges 13a, 13b are each spaced from the rotational axis 9. In some embodiments it is proposed that the flaps 11a, 11b may be configured such that their trailing edges 13a, 13b are spaced from the rotational axis 9 by a distance d of between 1.8R and 2.2R. For example, embodiments are envisaged in which the trailing edges 13a, 13b of the flaps 11a, 11b may be spaced from the rotational axis 9 by a distance d of approximately 2R. Figure 5 shows the region of the leading edge 12b of one of the flaps 11b in enlarged transverse cross-sectional view. As indicated by notional tangent line 18, the region of the flap’s aerosurface 14b proximate the leading edge 12b of the flap 11b lies substantially tangential to the peripheral wall 10 of the rotor body 8. Although not illustrated in Figure 5, it will be understood that the other flap 11a has a similarly configured aerosurface 14a tangential to the peripheral wall 10 proximate its leading edge 12a. As may be understood from Figures 3 and 4, however, the aerosurface 14a, 14b of each flap 11a, 11b further comprises a region proximate the respective trailing edge 13a, 13b which lies approximately radial to the peripheral wall 10 of the rotor body 8. As shown most clearly in Figure 5, each flap 11a, 11b further comprises a radially inwardly directed concave surface 19 of constant radius which is arranged to face the peripheral wall 10 of the rotor body 8 in equispaced relation thereto. As will therefore be appreciated, each flap 11a, 11b is thus spaced from the peripheral wall 10 of the rotor body 8 by a small gap 20 defined by the spacing between the radially inwardly directed surface 19 and the peripheral wall 10. In some embodiments it is proposed that the gap 20 may have a radial dimension of approximately 0.01 R to 0.02R, where R again denotes the radius of the rotor body 8. As also shown in Figure 5, the leading edge 12a, 12b of each flap 11a, 11b may have a curved profile. In some embodiments it is proposed that the curved profile of each leading edge 12a, 12b may have a radius (denoted r in Figure 5) of approximately 0.01 R to 0.02R, where R once more denotes the radius of the rotor body 8. Whilst the rotor sail 2 of the present proposal has been thus-far described with reference to the adoption of a specific operational configuration in which the flaps (11a, 11b) are positioned with their concave aerosurfaces (14a, 14b) oppositely and outwardly directed relative to one another and their chord lines (15a, 15b) convergent in a direction (16) away from the rotor body (8), it is to be appreciated that embodiments are envisaged in which the range of possible flap positions additionally includes alternate operational configurations in which such conditions are not necessarily satisfied. For example, Figure 6 illustrates one such alternate configuration, which may be considered a drag configuration. Comparing the configuration shown in Figure 6 with that shown in Figure 3, it will be observed that the flaps 11a, 11b have both been pivotally adjusted about the rotational axis 9 of the rotor body 8, relative to their positions in Figure 3, so as to adopt alternate respective positions which are approximately diametrically opposed across the rotor body 8. More particularly, it will be observed that in the alternate configuration illustrated in Figure 6, the flaps 11a, 11b are positioned such that their chord lines 15a, 15b are divergent in a direction away from the rotor body 8. Furthermore, it will be noted that in this configuration the trailing edges 13a, 13b of the flaps 11a, 11b subtend a significantly larger angle 0 from the rotational axis 9 than is the case for the configuration illustrated in Figure 3. In some embodiments, in a configuration similar to that shown in Figure 6 (in which the chord lines 15a, 15b are divergent away from the rotor body 8), it envisaged that the angle 0 may be between 140° and 180°; for example approximately 160°. As will be explained in more detail below, it is envisaged that the configuration illustrated in Figure 6 may be used in tail-wind conditions in which the apparent wind direction (denoted W in Figure 6) is substantially aligned with the course direction of the vessel 1. As will be appreciated, in the configuration illustrated in Figure 6, each flap 11a, 11b presents a respective rear surface 21a, 21b (opposite to the respective flap’s aerosurface 14a, 14b) to the wind, thereby increasing the total projected area of the rotor sail 2 presented to the wind in such conditions. The divergent-flap configuration illustrated in Figure 6 may therefore be considered a ‘drag’ or ‘parachute’ configuration particularly suited to tail wind conditions. It is proposed that when configured in this manner the rotor body 8 will not be rotated about its rotational axis, and indeed may be prevented from rotating by a suitable locking mechanism. Alternatively, however, it is proposed that the rotor body 8 may be rotated at low speed in this configuration in tail wind conditions. Whilst Figure 6 shows an arrangement in which the flaps 11a, 11b are configured such that their respective rear surfaces 21a, 21b (opposite to the aerosurfaces 14a, 14b) extend substantially radially relative to the rotor 8, other surface profiles are also possible such as, for example, denoted by the dashed line 22 in Figure 6. Such alternative rear surface profiles may further increase the drag provided by each flap 11a, 11b in the ‘drag’ or ‘parachute’ configuration. Figure 7 illustrates another type of alternative flap configuration, and shows only a single flap 11 b for simplicity, it being understood that the other flap 11a will have a similar configuration. In this proposal, the aerosurface 14b of the flap 11b is defined by a relatively thin wall such that the reverse side of the wall, opposite the aerosurface 14b, defines the rear surface 21b. As will therefore be appreciated, in this proposal the rear surface 21b will be convex, opposite to the concave aerosurface 14a. Additionally, due to the relatively thin nature of the wall defining the aerosurface 14a and the rear surface 21b of the flap 11b, the flap 11b further comprises a plurality of spaced-apart reinforcing ribs 23b on its rear side, which can further assist in ‘capturing’ wind and increasing drag in the ‘drag’ or ‘parachute’ configuration of the rotor sail. Figure 8 illustrates (on respective flaps 11a, 11b) two alternative types offlap which may be used in some embodiments of the rotor sail 2. As will be noted, in the arrangement illustrated in Figure 8, each flap 11a, 11b has a different configuration. However, it should be appreciated that in practical embodiments, both flaps 11a, 11b of the rotor sail will have substantially identical configurations. Considering initially the right-hand flap 11a in Figure 8, it will be noted that the flap 11a comprises an extension piece 24a hingedly connected to the main part of the flap 11a along the flap’s trailing edge 13a. The extension piece 24a extends the full height of the flap 11a and is shown in Figure 8 in an extended position in which it extends somewhat radially outwardly from the trailing edge 13a defining the edge of the flap’s aerosurface 14a so as to increase the projected area of the flap 11a presented to the wind in the ‘drag’ or ‘parachute’ configuration of the rotor sail. It is envisaged that the extension piece 24a will only be deployed in the extended position illustrated in tail wind conditions, and will at all other times be folded inwardly so as to adopt a stowed position in which it lies proximate or against the rear surface 21a of the main part of the flap 11a. The extension piece 24a may be actuated between its stowed position and its extended position via a plurality of (for example telescopic) actuators 25. Turning now to consider the left-hand flap 11 b in Figure 8, it will be noted that the flap 11b comprises another form of extension piece 24b. In this case the extension piece 24b may be slidingly mounted to the rear side of the main part of the flap 11 b so to be slidingly actuable (as denoted by arrow 26 in Figure 8) between an extended position as shown in which it extends generally radially outwardly from the trailing edge 13b defining the edge of the flap’s aerosurface 14b, and a stowed configuration (not shown) in which will lie proximate or adjacent the rear surface 21b of the main part of the flap 11b. Again, it is envisaged that the extension piece 24b will only be deployed in the extended position illustrated in tail wind conditions, and will at all other times be slid inwardly so as to adopt its stowed position. Turning now to consider the end plate 7 of the rotor sail 2, it is possible for the end plate 7 to take various configurations. For example, in its simplest form, the end plate 7 may take a generally conventional circular form (not illustrated) centred on the rotational axis 9 of the rotor body 8. In such embodiments the end plate 7 may either be mounted directly to the rotor body 8 for rotation therewith, or may be mounted to the supporting structure of the rotor sail so as to remain static in operation whilst the rotor body 8 rotates therebelow. In either case, it is proposed that the end plate 7 may have a diameter sized such that the vertical trailing edges 13a, 13b of the two flaps 11a, 11b are (at least approximately) radially aligned with the peripheral edge of the end plate 2, such that no part (or only a small degree) of either flap 11a, 11b will radially project beyond the peripheral edge of the end plate 7, regardless of their position. Figures 9 to 11 illustrate an alternative proposed configuration of the rotor sail’s end plate 7, the peripheral edge of which is denoted in dashed lines in Figures 9 and 11. For the sake of clarity, the end plate 7 is shown in Figures 9 and 11 with a degree of transparency so that the rotor body 8 and the flaps 11a, 11 b therebelow can be seen. In this proposal it is important that the end plate 7 should be mounted independently of the rotor body 8 so that the end plate 7 will remain static during operation of the rotor sail with the rotor body 8 spinning therebelow. The vessel’s forward direction (i.e., towards the bow 3) and aft direction (i.e. towards the stern 4) are both indicated in each of Figures 9 to 11 by way of a frame of reference for the proposed installation orientation of the rotor sail 2 and its end plate 7. Figure 9 (plan view from above) and Figure 10 (side elevational view from the vessel’s port side) both illustrate the rotor sail 2 in a feathered configuration (which will be described in more detail below) suitable for headwind conditions, in which the notional chord lines 15a, 15b of the flaps 11a, 11b are convergent in a direction away from the rotor body. Figure 11 (plan view from above) illustrates the rotor sail in a ‘drag’ or ‘parachute’ configuration similar to that described above in which the flaps 11a, 11b are divergent with their trailing edges 13a, 13b subtending an angle of approximately 180° from the rotational axis 9 of the rotor body 8. Whilst it may be possible to position the flaps 11a, 11b more closely together than illustrated in the feathered configuration of Figure 9, it envisaged that in many embodiments the feathered configuration illustrated in Figure 9 and the ‘drag’ or ‘parachute’ configuration illustrated in Figure 11 will represent respective end points of the range of flap positions possible in use of the rotor sail. As shown in Figures 9 and 11, the end plate 7 is asymmetrical in a forward-aft sense, such that its peripheral edge has a different profile forward of the rotational axis 9 of the rotor body 8 to its profile aft of the rotational axis 9. As shown most clearly in Figure 9, the aft region of the end plate 7 is shaped such that its peripheral edge follows a path of constant radius centred on the rotational axis 9. It is proposed that the aft region of the end plate’s peripheral edge may be approximately aligned with the vertical trailing edges 13a, 13b of the flaps 11a, 11b, regardless of what position each flap 11a, 11b may adopt within the range of possible flap positions. In contrast, and as shown in Figure 11, the forward region of the end plate 7 is shaped such that its peripheral edge follows a path matching the concave aerosurfaces 14a, 14b of the flaps 11a, 11b, and the peripheral wall 10 of the rotor body 8 therebetween, when the flaps 11a, 11b are positioned in the ‘drag’ or ‘parachute’ configuration. Turning now to consider Figures 12 to 16, there will be described in simple terms a possible mechanism for rotation of the rotor body 8 and actuation of the flaps 11a, 11b within their range of possible flap positions. As shown most clearly in Figures 12 and 13, the rotor body 8 is provided in the form of a closed-ended cylinder which is journalled at its upper and lower ends to an internal and axially extending supporting mast 27 for rotation therearound. The mast 27 may be anchored to the deck structure 6 of the vessel 1 so as to extend orthogonally therefrom. The top plate 7 is securely attached to the upper end of the mast so as to remain static at all times. The lower region of the internal mast 27 may be supported by a reinforcing structure 28 (for example comprising a plurality of radial ribs as illustrated), the upper region of which also supports an array of electric motors 29 arranged to drive the rotor body 8 in rotation about the rotational axis 9 by frictional engagement with the inner surface of the peripheral wall 10. As illustrated most clearly in Figure 15, the upper end of each flap 11a, 11b has a respective radially inwardly directed pivot arm 30a, 30b. The two pivot arms 30a, 30b are independently journalled to the internal mast between the upper end of the rotor body 8 and the end plate 7 thereabove, as shown in Figure 12, so as to pivotally support the upper ends of the flaps 11a, 11b relative to the mast 29 and thus also the rotational axis 9 of the rotor body 8. As also shown in Figures 12 and 15, the upper end of each flap 11a, 11b is also provided with a respective upstanding spigot 31a, 31b. Both spigots 31a, 31b are arranged for sliding movement within an arcuate groove 32 provided in the underside of the end plate 7 and which has a constant radius centred on the rotational axis 9. As illustrated most clearly in Figure 16, the lower end of each flap 11a, 11b is fixedly connected to a respective bearing ring 33a, 33b. As may be understood from Figure 12, each bearing ring 33a, 33b may be arranged to rotate about a bearing plate 34 formed at the bottom of the internal reinforcing structure, for example such that the bearing plate 34 and each respective bearing ring 33a, 33b form cooperating bearing races arranged for rotational movement relative to one another. The pivotal position of each flap 11a, 11b may be adjusted by controlled actuation of a respective electric motor 35a, 35b (for example servo motors) arranged to rotate each bearing ring 33a, 33b about the rotational axis 9. Turning now to consider Figures 17 to 31, a proposed operating regime for the rotor sail 2 proposed above will now be described across a range of apparent wind angles. The term ‘apparent wind angle’ is used herein to refer to the angle between the direction of apparent wind incident on the rotor sail (as might be determined by a wind instrument mounted on top of the rotor sail’s end plate 7, for example) and a dead-ahead direction of the vessel 1 coincident with the longitudinal axis of the vessel 1. It is to be appreciated, therefore, that the term ‘apparent wind angle’ may be used to refer to the angle between the vessel’s heading and apparent wind direction regardless of whether the wind is incident from the port or starboard side of the vessel 1. A vessel 1 subject to apparent wind from the starboard beam may thus be said to experience an apparent wind angle of 90°, and a vessel 1 subject to apparent wind from the port beam may also thus be said to experience an apparent wind angle of 90° (as opposed to 270° as might be understood in terms of polar coordinates). In proposed installations comprising a plurality of rotor sails 2, such as that illustrated in Figure 1, it is proposed that the rotor sails 2 may be provided as part of an automated (e.g., computer controlled) trimming system. The trimming system may comprise a plurality of individual wind instruments, each wind instrument being associated with a respective rotor sail 2 (for example mounted to the end plate 7 thereof) and configured to determine an instant apparent wind angle value for each rotor sail 2 and monitor changes to the apparent wind angle in real time. The rotational direction and rotational speed of the rotor body 8, and the positions of the flaps 11a, 11b of each rotor sail 2 may be controlled in dependence on the respective apparent wind angle value, and actively adjusted in response to changes in apparent wind experienced by each rotor sail 2. It is to be appreciated, however, that single rotor sail installations are also possible, in which case the single rotor sail 2 will be actively controlled in the same manner in response to apparent wind measurements. Figure 17 illustrates schematically a vessel 1 equipped with a single rotor sail 2 mounted in an upstanding position such that its rotational axis 9 is orthogonal to the longitudinal axis of the vessel 1. The vessel 1 is illustrated in headwind conditions in which it is moving forwards directly into the wind. As illustrated in Figure 17, the rotor sail 2 is therefore subject to an apparent wind angle of 0°. However, according to the operating regime proposed herein, headwind conditions may be considered to include apparent wind angle values of between 0° and 3°. In such conditions it is proposed that the flaps 11a, 11b of the rotor sail 2 should be positioned on the leeward side of the rotor body 8 in a feathered configuration similar to that mentioned briefly above, and that rotation of the rotor body 8 may be prevented. Alternatively, however, it is proposed that the rotor body 8 may be rotated at low speed in this configuration in headwind conditions. Figures 18a and 18b both illustrate the proposed feathered configuration of the flaps 11a, 11 b in more detail. As will be observed, the flaps 11a, 11b are positioned such that their concave aerosurfaces 14a, 14b are oppositely and outwardly directed relative to one another, and such that their chord lines 15a, 15b are convergent in a substantially downwind direction away from the rotor body 8. In some embodiments it is proposed that the flaps 11a, 11b may be positioned such that their chord lines 15a, 15b each make an equal angle c to the direction of apparent wind incident on the rotor sail 2. The angle c may be between 5° and 15°, such that the chord lines 15a, 15b converge at an angle C of between 10° and 30°. In some embodiments, the angle c may be approximately 10°, such that the chord lines 15a, 15b converge at an angle C of approximately 20°. As illustrated in Figure 18a, it is proposed that in the feathered configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 substantially parallel with the apparent wind direction. In some embodiments it is proposed that in the feathered configuration the trailing edges 13a, 13b may subtend an angle 0 from the rotational axis 9 of between 30° and 50°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 40°. Turning now to consider Figure 19, the vessel 1 is illustrated in conditions in which it is moving forwards generally into the wind but at a small acute angle thereto rather than directly into the wind as in the headwind conditions of Figure 17. As illustrated in Figure 19, the rotor sail 2 may therefore be considered subject to an apparent wind angle of between 4° and 12°, in which conditions it is proposed that the flaps 11a, 11b will be positioned in a first operational configuration as illustrated in Figures 20a and 20b, and the rotor body 8 will be rotated about its rotational axis 9 at a first rotational speed. Regarding the rotation of the rotor body 8, it will be noted that as illustrated, with the apparent wind incident on the port side of the vessel 1, the rotor body 8 will be rotated in a clockwise direction. As will be appreciated by those of skill in the art, were the apparent wind to be incident on the starboard side of the vessel at an angle of between 4° and 12°, then the rotor body 8 should be rotated in a counterclockwise direction at the first rotational speed. In some embodiments, it is proposed that the first rotational speed may be approximately 30 revolutions per minute. With reference to Figures 20a and 20b, it is proposed that the first operational configuration adopted in response to determination of an apparent wind angle value of between 4° and 12° may be configured such that the concave aerosurfaces 14a, 14b of the flaps 11a, 11b are oppositely and outwardly directed relative to one another and the chord lines 15a, 15b are convergent in a direction away from the rotor body 8. In more detail, the chord line 15b of the windwardmost flap 11b may make an angle CbOf between 10° and 20° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a may make an angle ca of between 25° and 35° to the direction of apparent wind incident on the rotor sail 2. As will be appreciated, therefore, the flaps 11a, 11b may be positioned such that their chord lines 15a, 15b converge at an angle C of between 35° and 55°. In some embodiments, the flaps 11a, 11b may be positioned in response to determination of an apparent wind angle value of between 4° and 12° such that the windwardmost flap 11b makes an angle CbOf approximately 15° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a makes an angle ca of approximately 30° to the direction of apparent wind incident on the rotor sail 2, in which case the chord lines 15a, 15b will converge at an angle C of approximately 45°. As illustrated in Figure 20a, it is furthermore proposed that in the first operational configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 making an angle a relative to the direction of apparent wind incident on the rotor sail 2. In some embodiments it is proposed that the angle a may be between 5° and 10°. In some embodiments it is proposed that the angle a may be between 7° and 8°, for example approximately 7.5°. The first operational configuration adopted in response to determination of an apparent wind angle value of between 4° and 12° may be further configured such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 5° and 25°. In some embodiments, the first operational configuration may be such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 10° and 20°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 15°. Turning now to consider Figure 21, the vessel 1 is illustrated in conditions in which it is moving forwards generally into the wind at a larger acute angle thereto than in the conditions illustrated in Figure 19. As illustrated in Figure 21, the rotor sail 2 may therefore be considered subject to an apparent wind angle of between 13° and 24°, in which conditions it is proposed that the flaps 11a, 11b will be positioned in a second operational configuration as illustrated in Figures 22a and 22b, and the rotor body 8 will be rotated about its rotational axis 9 at a second rotational speed. Regarding the rotation of the rotor body 8, it will be noted that as illustrated, with the apparent wind incident on the port side of the vessel 1, the rotor body 8 will be rotated in a clockwise direction. As will be appreciated by those of skill in the art, were the apparent wind to be incident on the starboard side of the vessel at an angle of between 13° and 24°, then the rotor body 8 should be rotated in a counterclockwise direction at the second rotational speed. In some embodiments, it is proposed that the second rotational speed may be approximately 60 revolutions per minute. With reference to Figures 22a and 22b, it is proposed that the second operational configuration adopted in response to determination of an apparent wind angle value of between 13° and 24° may be configured such that the concave aerosurfaces 14a, 14b of the flaps 11a, 11b are oppositely and outwardly directed relative to one another and the chord lines 15a, 15b are convergent in a direction away from the rotor body 8. In more detail, the chord line 15b of the windwardmost flap 11b may make an angle Cb of between 10° and 20° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a may make an angle ca of between 55° and 65° to the direction of apparent wind incident on the rotor sail 2. As will be appreciated, therefore, the flaps 11a, 11b may be positioned such that their chord lines 15a, 15b converge at an angle C of between 35° and 55°. In some embodiments, the flaps 11a, 11b may be positioned in response to determination of an apparent wind angle value of between 13° and 24° such that the windwardmost flap 11b makes an angle Cb of approximately 15° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a makes an angle ca of approximately 60° to the direction of apparent wind incident on the rotor sail 2, in which case the chord lines 15a, 15b will converge at an angle C of approximately 45°. As illustrated in Figure 20a, it is furthermore proposed that in the second operational configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 making an angle a relative to the direction of apparent wind incident on the rotor sail 2. In some embodiments it is proposed that the angle a may be between 35° and 40°. In some embodiments it is proposed that the angle a may be between 37° and 38°, for example approximately 37.5°. The second operational configuration adopted in response to determination of an apparent wind angle value of between 13° and 24° may be further configured such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 5° and 25°. In some embodiments, the second operational configuration may be such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 10° and 20°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 15°. Turning now to consider Figure 23, the vessel 1 is illustrated in conditions in which it is moving forwards generally into the wind at a larger acute angle thereto than in the conditions illustrated in Figure 21. As illustrated in Figure 23, the rotor sail 2 may therefore be considered subject to an apparent wind angle of between 25° and 37°, in which conditions it is proposed that the flaps 11a, 11b will be positioned in a third operational configuration as illustrated in Figures 24a and 24b, and the rotor body 8 will be rotated about its rotational axis 9 at a third rotational speed. Regarding the rotation of the rotor body 8, it will be noted that as illustrated, with the apparent wind incident on the port side of the vessel 1, the rotor body 8 will be rotated in a clockwise direction. As will be appreciated by those of skill in the art, were the apparent wind to be incident on the starboard side of the vessel at an angle of between 25° and 37°, then the rotor body 8 should be rotated in a counterclockwise direction at the third rotational speed. In some embodiments, it is proposed that the third rotational speed may be approximately 90 revolutions per minute. With reference to Figures 24a and 24b, it is proposed that the third operational configuration adopted in response to determination of an apparent wind angle value of between 25° and 37° may be configured such that the concave aerosurfaces 14a, 14b of the flaps 11a, 11b are oppositely and outwardly directed relative to one another and the chord lines 15a, 15b are convergent in a direction away from the rotor body 8. In more detail, the chord line 15b of the windwardmost flap 11b may make an angle Cb of between 25° and 35° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a may make an angle ca of between 70° and 80° to the direction of apparent wind incident on the rotor sail 2. As will be appreciated, therefore, the flaps 11a, 11b may be positioned such that their chord lines 15a, 15b converge at an angle C of between 35° and 55°. In some embodiments, the flaps 11a, 11b may be positioned in response to determination of an apparent wind angle value of between 25° and 37° such that the windwardmost flap 11b makes an angle CbOf approximately 30° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a makes an angle ca of approximately 75° to the direction of apparent wind incident on the rotor sail 2, in which case the chord lines 15a, 15b will converge at an angle C of approximately 45°. As illustrated in Figure 20a, it is furthermore proposed that in the third operational configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 making an angle a relative to the direction of apparent wind incident on the rotor sail 2. In some embodiments it is proposed that the angle a may be between 50° and 55°. In some embodiments it is proposed that the angle a may be between 52° and 53°, for example approximately 52.5°. The third operational configuration adopted in response to determination of an apparent wind angle value of between 25° and 37° may be further configured such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 5° and 25°. In some embodiments, the third operational configuration may be such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 10° and 20°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 15°. Turning now to consider Figure 25, the vessel 1 is illustrated in conditions in which it is moving forwards generally into and across the wind at a larger acute angle thereto than in the conditions illustrated in Figure 23. As illustrated in Figure 25, the rotor sail 2 may therefore be considered subject to an apparent wind angle of between 38° and 60°, in which conditions it is proposed that the flaps 11a, 11b will be positioned in a fourth operational configuration as illustrated in Figures 26a and 26b, and the rotor body 8 will be rotated about its rotational axis 9 at a fourth rotational speed. Regarding the rotation of the rotor body 8, it will be noted that as illustrated, with the apparent wind incident on the port side of the vessel 1, the rotor body 8 will be rotated in a clockwise direction. As will be appreciated by those of skill in the art, were the apparent wind to be incident on the starboard side of the vessel at an angle of between 38° and 60°, then the rotor body 8 should be rotated in a counterclockwise direction at the fourth rotational speed. In some embodiments, it is proposed that the fourth rotational speed may be approximately 120 revolutions per minute. With reference to Figures 26a and 26b, it is proposed that the fourth operational configuration adopted in response to determination of an apparent wind angle value of between 38° and 60° may be configured such that the concave aerosurfaces 14a, 14b of the flaps 11a, 11b are oppositely and outwardly directed relative to one another and the chord lines 15a, 15b are convergent in a direction away from the rotor body 8. In more detail, the chord line 15b of the windwardmost flap 11b may make an angle Cb of between 40° and 50° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a may make an angle ca of between 85° and 95° to the direction of apparent wind incident on the rotor sail 2. As will be appreciated, therefore, the flaps 11a, 11b may be positioned such that their chord lines 15a, 15b converge at an angle C of between 35° and 55°. In some embodiments, the flaps 11a, 11b may be positioned in response to determination of an apparent wind angle value of between 38° and 60° such that the windwardmost flap 11b makes an angle Cb of approximately 45° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a makes an angle ca of approximately 90° to the direction of apparent wind incident on the rotor sail 2, in which case the chord lines 15a, 15b will converge at an angle C of approximately 45°. As illustrated in Figure 20a, it is furthermore proposed that in the fourth operational configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 making an angle a relative to the direction of apparent wind incident on the rotor sail 2. In some embodiments it is proposed that the angle a may be between 65° and 70°. In some embodiments it is proposed that the angle a may be between 67° and 68°, for example approximately 67.5°. The fourth operational configuration adopted in response to determination of an apparent wind angle value of between 38° and 60° may be further configured such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 5° and 25°. In some embodiments, the fourth operational configuration may be such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 10° and 20°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 15°. Turning now to consider Figure 27a, the vessel 1 is illustrated in beam wind conditions in which it is moving forwards generally across the wind at an angle of approximately 90° to the apparent wind angle. Figures 27b and 27c both show the vessel in wind conditions in which the vessel is moving generally across and with the wind at obtuse angles to the apparent wind angle. In Figure 27b the apparent wind angle is approximately 120° and in Figure 27c the apparent wind angle is approximately 150°. Figures 27a, 27b and 27c are used here to show three discrete exemplary apparent wind angles falling within a wide range of between 61° and 178° within which it is proposed that the flaps 11a, 11b will be positioned in a fifth operational configuration as illustrated in Figures 28a and 28b, and the rotor body 8 will be rotated about its rotational axis 9 at a fifth rotational speed. Regarding the rotation of the rotor body 8, it will be noted that as illustrated, with the apparent wind incident on the port side of the vessel 1, the rotor body 8 will be rotated in a clockwise direction. As will be appreciated by those of skill in the art, were the apparent wind to be incident on the starboard side of the vessel at an angle of between 61° and 178°, then the rotor body 8 should be rotated in a counterclockwise direction at the fifth rotational speed. In some embodiments, it is proposed that the fifth rotational speed may be approximately 120 revolutions per minute. With reference to Figures 28a and 28b, it is proposed that the fifth operational configuration adopted in response to determination of an apparent wind angle value of between 61° and 178° may be configured such that the concave aerosurfaces 14a, 14b of the flaps 11a, 11b are oppositely and outwardly directed relative to one another and the chord lines 15a, 15b are convergent in a direction away from the rotor body 8. In more detail, the chord line 15b of the windwardmost flap 11b may make an angle Cb of between 55° and 65° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a may make an angle ca of between 100° and 110° to the direction of apparent wind incident on the rotor sail 2. As will be appreciated, therefore, the flaps 11a, 11b may be positioned such that their chord lines 15a, 15b converge at an angle C of between 35° and 55°. In some embodiments, the flaps 11a, 11b may be positioned in response to determination of an apparent wind angle value of between 61° and 178° such that the windwardmost flap 11b makes an angle CbOf approximately 60° to the direction of apparent wind incident on the rotor sail 2, and the chord line 15a of the other leewardmost flap 11a makes an angle ca of approximately 105° to the direction of apparent wind incident on the rotor sail 2, in which case the chord lines 15a, 15b will converge at an angle C of approximately 45°. As illustrated in Figure 20a, it is furthermore proposed that in the fifth operational configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 making an angle a relative to the direction of apparent wind incident on the rotor sail 2. In some embodiments it is proposed that the angle a may be between 80° and 85°. In some embodiments it is proposed that the angle a may be between 82° and 83°, for example approximately 82.5°. The fifth operational configuration adopted in response to determination of an apparent wind angle value of between 61° and 178° may be further configured such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 5° and 25°. In some embodiments, the fifth operational configuration may be such that the trailing edges 13a, 13b subtend an angle 0 from the rotational axis 9 of between 10° and 20°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 15°. Figure 29 illustrates the vessel in tailwind conditions in which it is running directly with the wind. As illustrated in Figure 29, the rotor sail 2 is therefore subject to an apparent wind angle of 180°. However, according to the operating regime proposed herein, tailwind conditions may be considered to include apparent wind angle values between 179° and 180°. In such conditions it is proposed that rotation of the rotor body 8 should be prevented (or rotated only at low speed) and that the flaps 11a, 11 b of the rotor sail 2 should be positioned in a drag configuration (similar to those previously described) as illustrated more clearly in Figures 30a and 30b in which the flaps 11a, 11b are positioned substantially diametrically opposite one another across the rotor body 8 with their concave aerosurfaces 14a, 14b directed substantially downwind and thus generally away from the apparent wind incident on the rotor sail 2. In such a position, each flap 11a, 11b therefore presents its respective rear surface 21a, 21b (i.e., opposite its respective concave aerosurface 14a, 14b) to the apparent wind. As shown most clearly in Figure 30b, the drag configuration adopted in response to determination of an apparent wind angle value of between 179° and 180° may be configured such that the chord lines 15a, 15b of the flaps 11a, 11b are divergent in an upwind direction away from the rotor body 8. As illustrated in Figure 30a, it is proposed that in the drag configuration the trailing edges 13a, 13b of the flaps 11a, 11b may be equispaced from a notional midline 37 substantially parallel with the apparent wind direction. In some embodiments it is proposed that in the drag configuration the trailing edges 13a, 13b may subtend an angle 0 from the rotational axis 9 of between 140° and 180°. For example, the flaps 11a, 11b may be positioned such that the angle 0 is approximately 160°. As will be appreciated, as a vessel 1 equipped with one or more of the rotor sails 2 proposed herein navigates a voyage in changing wind conditions, the apparent wind angle will vary considerably (and perhaps even continuously) throughout the voyage. Under the control of computer system programmed to implement the above-described operating regime for each rotor sail 2, each rotor sail 2 may be substantially continuously adjusted or trimmed to suit the instant wind conditions and may, for example, be transitioned between the feathered configuration, the first to fifth operational configurations, and the drag configuration in dependence on the instant apparent wind angle value. With reference to Figure 31, it may be appreciated that the operating regime proposed above and described with reference to Figures 17 to 31 therefore involves effectively dividing a range of apparent wind angles from 0° to 180° into a plurality of apparent wind angle sectors 38-44. The rotor sail 2 is configurable into a plurality of distinct flap configurations selected from a range of possible flap positions. The method therefore involves monitoring the apparent wind angle value, determining in which of the sectors 38-44 the determined apparent wind angle falls (illustrated as falling within sector 43 in Figure 31), and positioning the flaps 11a, 11b in the flap configuration corresponding to that sector. The method may also involve adjusting the rotational speed of the rotor body 8 about its rotational axis 9 in dependence on which of the sectors 38-44 the determined apparent wind angle falls, and optionally also in dependence on measured apparent wind speed. Whilst the method has been described with reference to a particular regime in which the range of apparent wind angles from 0° to 180° is divided into seven apparent wind angle sectors 38-44, with there being seven corresponding flap configurations into which the flaps 11a, 11b are positioned, it is to be appreciated that in alternative implementations the full range of apparent wind angles could be divided into more or fewer apparent wind angle sectors with there being corresponding more or fewer corresponding flap configurations into which the flaps may be positioned. For example, implementations are envisaged which may be based on considerably more apparent wind angle sectors and corresponding flap configurations, to provide more granularity to the trim of the rotor sail 2 during use. It is also to be appreciated that the specific apparent wind angle values for the various sectors 38-44 described above are purely exemplary, and other apparent wind angle ranges for each sector are also possible. Whilst the invention has been described above with reference to particular embodiments and examples, it is to be appreciated that various modifications or changes can be made to the arrangements illustrated and described, without departing from the scope of the invention. For example, Figure 32 illustrates a side view of a rotor sail 2 having a flap arrangement 11 (e.g., comprising a pair of discrete flaps 11a, 11b) of a slightly different configuration in which the trailing edges 13a, 13b do not lie parallel to the rotational axis 9 of the rotor body 8. Instead, in the arrangement of Figure 32 it will be observed that the flap arrangement 11 has a region 45 along its height which has a larger chord width than at its upper and lower ends, so as to have a somewhat upwardly and downwardly tapering shape. In this arrangement, the flap arrangement 11 may be configured such that the trailing edges 13a, 13b each lie in a respective plane passing through the rotational axis 9, but even this is not considered essential in some implementations. Figure 33 illustrates an alternative form of flap arrangement which comprises a single flap 11 defining both aerosurfaces 14a, 14b, both leading edges 12a, 12b and both trailing edges 13a, 13b. The single flap 11 may be pivotally adjustable about the rotational axis 9 of the rotor body 8 in a similar manner and may be configured such that its aerosurfaces 14a, 14b, trailing edges 13a, 13b and its two corresponding notional chord lines 15a, 15b have many of the same characteristics described above in connection with the operational configuration of the discrete flaps 11a, 11b of the previous embodiments. It will therefore be observed that the single flap 11 is configured such that its trailing edges 13a, 13b are circumferentially spaced-apart about the rotational axis 9 and equispaced from a notional midline 37 about which the flap 11 is mirror symmetrical. The trailing edges 13a, 13b may each be spaced from the notional midline 37 by a distance x of between 0.2R and 0.8R, where R denotes the radius of the rotor body 8, and may each be spaced from the rotational axis 9 of the rotor body by a distance d of between 1.8R and 2.2R (for example 2R. It will be noted that the aerosurfaces 14a, 14b are again oppositely and outwardly directed, and that the notional chord lines 15a, 15b are convergent in a direction extending away from the rotor body 9. The chord lines 15a, 15b of the flap 11 may converge at an angle C of between 35° and 55°, optionally between 40° and 50°. In some particular embodiments, the chord lines 15a, 15b may converge at an angle C of approximately 45°. It is proposed that the single flap 11 may be configured such that the trailing edges 13a, 13b subtend an angle (0) of up to 40° from the rotational axis 9 of the rotor body 8. For example, configurations are proposed in which the angle (0) may be between 5° and 25°, or between 10° and 20°. For example, particular embodiments may be configured such the trailing edges 13a, 13b subtend an angle (0) of approximately 15° from the rotational axis 9. Because the single flap 11 shown in Figure 33 has a pair of discrete and spaced-apart trailing edges 13a, 13b, it may be configured to define a recess 45 between the trailing edges 13a, 13 which is open in a direction away from the rotational axis 9. The recess 45 thus defines a large open space between the trailing edges 13a, 13b which is able to ‘capture’ wind when presented to the apparent wind direction. It is proposed that the single flap rotor sail 2 shown in Figure 33 may be operated in a somewhat similar manner to that proposed in connection with the previously described twin-flap proposal, via adjustment of the position of the flap 11 about the rotational axis 9 of the rotor 8 in dependence on the apparent wind angle. Due to its configuration, the single flap 11 may thus be positioned in various operational configurations, relative to the apparent wind direction, which are equivalent to (or at least which approximate to) the operational configurations described above with reference to the twin-flap proposal. In tailwind conditions, the single flap 11 may be positioned such that the notional midline 37 (about which the flap 11 is mirror-symmetrical) is directed into the apparent wind. In this configuration, it will therefore be appreciated that the recess 45 defined between the trailing edges 13a, 13b will be presented to the apparent wind and will thus ‘capture’ wind in a drag configuration. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Unless otherwise defined herein, scientific and technical terms used in connection with the presently disclosed inventive concept(s) shall have the meanings that are commonly understood by those of ordinary skill in the art. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps than those listed in a claim. Throughout this specification, including the claims which follow, unless the context requires otherwise, the words “have”, “comprise”, and “include”, and variations such as “having”, “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” or “approximately” one particular value, and / or to “about” or “approximately” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedents “about” or “approximately” it will be understood that the particular value forms another embodiment. The terms “about” or “approximately” in relation to a numerical value are optional and mean, for example, +1- 10%. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements or such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. As may be used herein, any reference to “one embodiment,” “an embodiment,” “some embodiments,” “one example,” “for example,” or “an example” means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearance of the phrase “in some embodiments” or “one example” in various places in the specification is not necessarily all referring to the same embodiment, for example. Further, all references to one or more embodiments or examples are to be construed as non-limiting to the claims. As used in this specification, any formulation used of the style “at least one of A, B or C”, and the formulation “at least one of A, B and C” means that those formulations comprise any and all joint and several permutations of A, B, C, that is, A alone, B alone, C alone, A and B in any order, A and C in any order, B and C in any order and A, B, C in any order. There may be more or less than three features used in such formulations. The term “or combinations thereof’ As may be used herein refers to all permutations and combinations of the listed items preceding the term. For example, “A, B, C, or combinations thereof” is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AAB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context. As may be used herein, the term "substantially" means that the subsequently described event or circumstance completely occurs or that the subsequently described event or circumstance occurs to a great extent or degree. For example, when associated with a particular event or circumstance, the term "substantially" means that the subsequently described event or circumstance occurs at least 80% of the time, or at least 85% of the time, or at least 90% of the time, or at least 95% of the time. For example, the term "substantially adjacent" may mean that two items are 100% adjacent to one another, or that the two items are within close proximity to one another but not 100% adjacent to one another, or that a portion of one of the two items is not 100% adjacent to the other item but is within close proximity to the other item. The words "preferred" and "preferably" are used herein refer to embodiments of the invention that may provide certain benefits under some circumstances. It is to be appreciated, however, that other embodiments may also be preferred under the same or different circumstances. The recitation of one or more preferred embodiments therefore does not mean or imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the disclosure, or from the scope of the claims. Features of embodiments of the invention are set out in the following paragraphs: Clause 1: A rotor sail (2) having a rotational axis (9) and comprising: an elongate cylindrical rotor body (8) having a peripheral wall (10) rotatable about the rotational axis (9); and a flap arrangement (11) adjacent the rotor body (8), the flap arrangement (11) having: a pair of leading edges (12a, 12b) proximate the peripheral wall (10); a pair of trailing edges (13a, 13b) distal to the peripheral wall (10); a pair of aerosurfaces (14a, 14b), each aerosurface (14a, 14b) interconnecting a respective said leading edge and a respective said trailing edge; and a pair of notional chord lines (15a, 15b), each chord line (15a, 15b) interconnecting a respective said leading edge and a respective said trailing edge in transverse cross-section; the flap arrangement (11) being configured or configurable such that: said aerosurfaces (14a, 14b) are oppositely and outwardly directed relative to one another and said chord lines (15a, 15b) are convergent in a direction (16) away from the rotor body (8). Clause 2: A rotor sail (2) according to clause 1, wherein said aerosurfaces (14a, 14b) are substantially concave. Clause 3: A rotor sail (2) according to clause 1 or clause 2, wherein said flap arrangement (11) is concentrically adjustable relative to the rotational axis (9) within a range of positions about the rotor body (8). Clause 4: A rotor sail (2) according to any one of clauses 1 to 4, wherein said flap arrangement (11) comprises a pair of discrete flaps (11a, 11b), each said flap (11a, 11b) having a respective said leading edge (12a, 12b), a respective said trailing edge (13a, 13b), a respective said aerosurface (14a, 14a), and a respective said chord line (15a, 15b), wherein the flaps (11a, 11b) are independently concentrically adjustable relative to the rotational axis (9) within a range of flap positions about the rotor body (8), said range of flap positions including at least one operational configuration in which said flaps (11a, 11b) are positioned such that their aerosurfaces (14a, 14b) are oppositely and outwardly directed relative to one another and their chord lines (15a, 15b) are convergent in a direction (16) away from the rotor body (8). Clause 5: A rotor sail (2) according to any one of clauses 1 to 3, wherein said flap arrangement comprises a single flap (11) defining both leading edges (12a, 12b) and both trailing edges (13a, 13b), said single flap (11) being configured such that said trailing edges (13a, 13b) are circumferentially spaced-apart about the rotational axis (9). Clause 6: A rotor sail (2) according to clause 4, wherein each flap (11a, 11b) is mounted for pivotal adjustment about said rotational axis (9) through a range of flap positions in which the trailing edge (13a, 13b) of the flap is centred on said rotational axis (9). Clause 7: A rotor sail (2) according to any one of clauses 1 to 6, wherein each trailing edge (13a, 13b) is spaced from the rotational axis (9) of the rotor body (8) by a distance (d) of between 1.8R and 2.2R, where R denotes the radius of the rotor body (8). Clause 8: A rotor sail (2) according to clause 7, wherein each trailing edge (13a, 13b) is spaced from the rotational axis (9) of the rotor body (8) by a distance (d) of approximately 2R. Clause 9: A rotor sail (2) according to any one of clauses 1 to 8, wherein each aerosurface (14a, 14b) has a region proximate a respective leading edge (12a, 12b) which lies substantially tangential to the peripheral wall (10) of the rotor body (8). Clause 10: A rotor sail (2) according to any one of clauses 1 to 9, wherein each aerosurface (14a, 14b) comprises a region proximate a respective trailing edge (13a, 13b) which lies approximately radial to the peripheral wall (8) of the rotor body (8). Clause 11: A rotor sail (2) according to any one of clauses 1 to 10, wherein the flap arrangement (11) is spaced from the peripheral wall (10) of the rotor body (8) by a gap (20), said gap (20) having a radial dimension of approximately 0.01 R to 0.02R, where R denotes the radius of the rotor body (8). Clause 12: A rotor sail (2) according to any one of clauses 1 to 11, wherein each leading edge (12a, 12b) has a curved profile having a radius (r) of approximately 0.01 R to 0.02R, where R denotes the radius of the rotor body (8). Clause 13: A rotor sail according to any one of clauses 1 to 12, wherein said flap arrangement (11) is configured or configurable such that said trailing edges (13a, 13b) subtend an angle (0) of between 5° and 25° from the rotational axis (9) of the rotor body (8). Clause 14: A rotor sail according to clause 4, wherein said operational configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of between 5° and 25° from the rotational axis (9) of the rotor body (8). Clause 15: A rotor sail according to clause 13, wherein said flap arrangement (11) is configured or configurable such that said trailing edges (13a, 13b) subtend an angle (0) of between 10° and 20° from the rotational axis (9) of the rotor body (8). Clause 16: A rotor sail according to clause 14, wherein said operational configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of between 10° and 20° from the rotational axis (9) of the rotor body (8). Clause 17: A rotor sail according to clause 15, wherein said flap arrangement (11) is configured or configurable such that said trailing edges (13a, 13b) subtend and angle (0) of approximately 15° from the rotational axis (9) of the rotor body (8). Clause 18: A rotor sail according to clause 16, wherein said operational configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 15° from the rotational axis (9) of the rotor body (8). Clause 19: A rotor sail (2) according to any one of clauses 1 to 18, wherein said flap arrangement (11) is configured or configurable such that said chord lines (15a, 15b) converge at an angle (C) of between 35° and 55°. Clause 20: A rotor sail according to clause 4, wherein said operational configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of between 35° and 55°. Clause 21: A rotor sail (2) according to clause 19, wherein said flap arrangement (11) is configured or configurable such that said chord lines (15a, 15b) converge at an angle (C) of between 40° and 50°. Clause 22: A rotor sail according to clause 20, wherein said operational configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of between 40° and 50°. Clause 23: A rotor sail (2) according to clause 21, wherein said flap arrangement (11) is configured or configurable such that said chord lines (15a, 15b) converge at an angle (C) of approximately 45°. Clause 24: A rotor sail according to clause 22, wherein said operational configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of approximately 45°. Clause 25: A rotor sail (2) according to clause 4, wherein said range of flap positions further includes configurations in which the flaps (11a, 11b) are positioned such that their chord lines (15a, 15b) are divergent in a direction away from the rotor body (8). Clause 26: A rotor sail (2) according to clause 25, wherein said range of flap positions includes an alternate operational configuration in which the flaps (11a, 11b) are positioned such that their trailing edges (13a, 13b) subtend an angle (0) of between 140° and 180° from the rotational axis (9) of the rotor body (8). Clause 27: A rotor sail (2) according to clause 26, wherein said alternate operational configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 160° from the rotational axis (9) of the rotor body (8). Clause 28: A rotor sail (2) according to clause 4, wherein said range of flap positions includes a feathered configuration in which the flaps (11a, 11b) are positioned such that their trailing edges (13a, 13b) subtend an angle (0) of between 30° and 50° from the rotational axis (9) of the rotor body (8). Clause 29: A rotor sail (2) according to clause 28, wherein said feathered configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 40° from the rotational axis (9) of the rotor body (8). Clause 30: A rotor sail (2) according to clause 28 or clause 29, wherein said feathered configuration is such that said aerosurfaces (14a, 14b) are oppositely and outwardly directed relative to one another and the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8). Clause 31: A rotor sail (2) according to clause 30, wherein said feathered configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of between 10° and 30°. Clause 32: A rotor sail (2) according to clause 30 or clause 31, wherein said feathered configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of approximately 20°. Clause 33: A method of operating a rotor sail (2) on a vessel (1), the method comprising: providing a rotor sail (2) according to any one of clauses 1 to 32 in an upstanding position on the vessel (1) such that said rotational axis (9) is substantially orthogonal to a longitudinal axis (36) of the vessel (1); determining an apparent wind angle value representative of the angle between the direction of apparent wind incident on the rotor sail (9) and a dead-ahead direction of the vessel (1) coincident with said longitudinal axis (36); and positioning said flap arrangement (11) relative to said rotor body (8) in dependence on said apparent wind angle value. Clause 34: A method according to clause 33, wherein a range of apparent wind angles from 0° to 180° is divided into a plurality of apparent wind angle sectors, and a plurality of distinct flap configurations are defined within said range of flap positions, each said distinct flap configuration corresponding to a respective said sector, wherein the method involves monitoring said apparent wind angle value, determining in which of said sectors the determined apparent wind angle falls, and positioning the flap arrangement (11) in the corresponding flap configuration. Clause 35: A method according to clause 34, wherein said range of apparent wind angles is divided into seven said wind angle sectors, and wherein the method involves positioning said flap arrangement (11) in one of seven respective distinct flap configurations. Clause 36: A method according to any one of clauses 33 to 35, comprising providing the rotor sail (2) in a form according to clause 4, wherein said step of positioning the flap arrangement (11) involves positioning said discrete flaps (11a, 11b) relative to said rotor body (8) in dependence on said apparent wind angle value. Clause 37: A method according to clause 36, wherein: in response to determination of an apparent wind angle value of between 0° and 3° said flaps (11a, 11b) are positioned in a feathered configuration; wherein said feathered configuration is such that said aerosurfaces (14a, 14b) of the flaps (11a, 11b) are oppositely and outwardly directed relative to one another and the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a substantially downwind direction away from the rotor body (8). Clause 38: A method according to clause 37, wherein said feathered configuration is such that said chord lines (15a, 15b) of the flaps (11a, 11b) each make a substantially equal angle of between 5° and 15° to said direction of apparent wind incident on the rotor sail (2). Clause 39: A method according to clause 38, wherein said feathered configuration is such that said chord lines (15a, 15b) of the flaps (11a, 11b) each make an angle of approximately 10° to said direction of apparent wind incident on the rotor sail (2). Clause 40: A method according to any one of clauses 37 to 39, wherein said feathered configuration is such that the trailing edges (13a, 13b) of said flaps (11a, 11b) are equispaced from a notional midline (37) substantially parallel with said direction of apparent wind incident on the rotor sail (2). Clause 41: A method according to any one of clauses 36 to 40, wherein: in response to determination of an apparent wind angle value of between 4° and 12° said flaps (11a, 11b) are positioned in a first operational configuration and the rotor body (8) is rotated about the rotational axis (9) at a first rotational speed, wherein said first operational configuration is such that: said aerosurfaces (14a, 14b) of the flaps (11a, 11b) are oppositely and outwardly directed relative to one another; the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8); the chord line (15b) of a first windwardmost said flap (11b) makes an angle (Cb) of between 10° and 20° to said direction of apparent wind incident on the rotor sail (2); and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of between 25° and 35° to said direction of apparent wind incident on the rotor sail (2). Clause 42: A method according to clause 41, wherein said first operational configuration is such that the chord line (15b) of said first windwardmost flap (11b) makes an angle (Cb) of approximately 15° to said direction of apparent wind incident on the rotor sail (2), and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of approximately 30° to said direction of apparent wind incident on the rotor sail (2). Clause 43: A method according to clause 41 or clause 42, wherein said first rotational speed is approximately 30 revolutions per minute. Clause 44: A method according to any one of clauses 41 to 43, wherein said first operational configuration is such that the trailing edges (13a, 13b) of said flaps (11a, 11b) are equispaced from a notional midline 37 making an angle (a) of: i) between 5° and 10°; or ii) between 7° and 8°; or iii) 7.5°, relative to said direction of apparent wind incident on the rotor sail (2). Clause 45: A method according to any one of clauses 36 to 44, wherein: in response to determination of an apparent wind angle value of between 13° and 24° said flaps (11a, 11b) are positioned in a second operational configuration and the rotor body (8) is rotated about the rotational axis (9) at a second rotational speed, wherein said second operational configuration is such that: said aerosurfaces (14a, 14b) of the flaps (11a, 11b) are oppositely and outwardly directed relative to one another; the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8); the chord line (15a, 15b) of a first windwardmost flap (11b) makes an angle (Cb) of between 10° and 20° to said direction of apparent wind incident on the rotor sail (2); and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of between 55° and 65° to said direction of apparent wind incident on the rotor sail (2). Clause 46: A method according to clause 45, wherein: said second operational configuration is such that the chord line (15b) said first windwardmost flap (11b) makes an angle (Cb) of approximately 15° to said direction of apparent wind incident on the rotor sail (2), and the chord line (11a) of the other leewardmost flap (11a) makes an angle (ca) of approximately 60° to said direction of apparent wind incident on the rotor sail. Clause 47: A method according to clause 45 or clause 46, wherein said second rotational speed is approximately 60 revolutions per minute. Clause 48: A method according to any one of clauses 45 to 47, wherein said second operational configuration is such that the trailing edges (13a, 13b) of said flaps (11a, 11b) are equispaced from a notional midline (37) making an angle (a) of: i) between 35° and 40°; or ii) between 37° and 38°; or iii) 37.5°, relative to said direction of apparent wind incident on the rotor sail (2). Clause 49: A method according to any one of clauses 36 to 48, wherein: in response to determination of an apparent wind angle value of between 25° and 37° said flaps (11a, 11b) are positioned in a third operational configuration and the rotor body (8) is rotated about the rotational axis (9) at a third rotational speed, wherein said third operational configuration is such that: said aerosurfaces (14a, 14b) of the flaps (11a, 11b) are oppositely and outwardly directed relative to one another; the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8); the chord line (15b) of a first windwardmost flap (11b) makes an angle (Cb) of between 25° and 35° to said direction of apparent wind incident on the rotor sail (2); and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of between 70° and 80° to said direction of apparent wind incident on the rotor sail (2). Clause 50: A method according to clause 49, wherein: said third operational configuration is such that the chord line (15b) said first windwardmost flap (11b) makes an angle (Cb) of approximately 30° to said direction of apparent wind incident on the rotor sail (2), and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of approximately 75° to said direction of apparent wind incident on the rotor sail (2). Clause 51: A method according to clause 49 or clause 50, wherein said third rotational speed is approximately 90 revolutions per minute. Clause 52: A method according to any one of clauses 49 to 51, wherein said third operational configuration is such that the trailing edges (13a, 13b) of said flaps (11a, 11b) are equispaced from a notional midline (37) making an angle (a) of: i) between 50° and 55°; or ii) between 52° and 53°; or iii) 52.5°, relative to said direction of apparent wind incident on the rotor sail (2). Clause 53: A method according to any one of clauses 36 to 52, wherein: in response to determination of an apparent wind angle value of between 38° and 60° said flaps (11a, 11b) are positioned in a fourth operational configuration and the rotor body (8) is rotated about the rotational axis (9) at a fourth rotational speed, wherein said fourth operational configuration is such that: said aerosurfaces (14a, 14b) of the flaps (11a, 11b) are oppositely and outwardly directed relative to one another; the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8); the chord line (15b) of a first windwardmost flap (11b) makes an angle (Cb) of between 40° and 50° to said direction of apparent wind incident on the rotor sail (2); and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of between 85° and 95° to said direction of apparent wind incident on the rotor sail (2). Clause 54: A method according to clause 53, wherein: said fourth operational configuration is such that the chord line (15b) of said first windwardmost flap (11b) makes an angle (Cb) of approximately 45° to said direction of apparent wind incident on the rotor sail (2), and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of approximately 90° to said direction of apparent wind incident on the rotor sail (2). Clause 55: A method according to clause 53 or clause 54, wherein said fourth rotational speed is approximately 120 revolutions per minute. Clause 56: A method according to any one of clauses 53 to 55, wherein said fourth operational configuration is such that the trailing edges (13a, 13b) of said flaps (11a, 11b) are equispaced from a notional midline (37) making an angle (a) of: i) between 65° and 70°; or ii) between 67° and 68°; or iii) 67.5°, relative to said direction of apparent wind incident on the rotor sail (2). Clause 57: A method according to any one of clauses 36 to 56, wherein: in response to determination of an apparent wind angle value of between 61° and 178° said flaps (11a, 11b) are positioned in a fifth operational configuration and the rotor body (8) is rotated about the rotational axis (9) at a fifth rotational speed, wherein said fifth operational configuration is such that: said aerosurfaces (14a, 14b) of the flaps (11a, 11b) are oppositely and outwardly directed relative to one another; the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8); the chord line (15b) of a first windwardmost flap (11b) makes an angle (Cb) of between 55° and 65° to said direction of apparent wind incident on the rotor sail (2); and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of between 100° and 110° to said direction of apparent wind incident on the rotor sail (2). Clause 58: A method according to clause 57, wherein: said fifth operational configuration is such that the chord line (15a) of said first windwardmost flap (11a) makes an angle (Cb) of approximately 60° to said direction of apparent wind incident on the rotor sail (2), and the chord line (15a) of the other leewardmost flap (11a) makes an angle (ca) of approximately 105° to said direction of apparent wind incident on the rotor sail (2). Clause 59: A method according to clause 57 or clause 58, wherein said fifth rotational speed is approximately 120 revolutions per minute. Clause 60: A method according to any one of clauses 57 to 59, wherein said fifth operational configuration is such that the trailing edges (13a, 13b) of said flaps (11a, 11b) are equispaced from a notional midline (37) making an angle (a) of: i) between 80° and 85°; or ii) between 82° and 83°; or iii) 82.5°, relative to said direction of apparent wind incident on the rotor sail (2). Clause 61: A method according to any one of clauses 33 to 60, wherein said rotor body (8) is rotated in a clockwise direction in response to apparent wind being incident on the port side of the vessel (1), and is rotated in a counterclockwise direction in response to apparent wind being incident on the starboard side of the vessel (1). Clause 62: A method according to any one of clauses 36 to 61, wherein: wherein: in response to determination of an apparent wind angle value of between 179° and 180° said flaps (11a, 11b) are positioned in a drag configuration and rotation of the rotor body (8) about the rotational axis (9) is substantially prevented; wherein said drag configuration is such that said flaps (11a, 11b) are positioned substantially diametrically opposite one another across the rotor body (8) with their aerosurfaces (14a, 14b) directed substantially downwind and thus generally away from the apparent wind incident on the rotor sail (2), each flap (11a, 11b) thereby presenting a surface (21a, 21b) opposite its respective concave aerosurface (14a, 14b) to the apparent wind. Clause 63: A method according to clause 62, wherein said drag configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) are divergent in an upwind direction away 5 from the rotor body (8). Clause 64: A method according to clause 62 or clause 63, wherein said drag configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of between 140° and 180° from the rotational axis (9) of the rotor body (8). Clause 65: A method according to clause 64, wherein said drag configuration is such that io the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 160° from the rotational axis (9) of the rotor body (8). Clause 66: A method according to any one of clauses 62 to 65, wherein said drag configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) are equispaced from a notional midline (37) substantially parallel with said direction of apparent wind incident is on the rotor sail (2).

Claims

1. A rotor sail (2) having a rotational axis (9) and comprising: an elongate cylindrical rotor body (8) having a peripheral wall (10) rotatable about the rotational axis (9); and a flap arrangement (11) adjacent the rotor body (8), the flap arrangement (11) having:10a pair of leading edges (12a, 12b) proximate the peripheral wall (10); a pair of trailing edges (13a, 13b) distal to the peripheral wall (10); a pair of aerosurfaces (14a, 14b), each aerosurface (14a, 14b) interconnecting a respective said leading edge and a respective said trailing edge; and a pair of notional chord lines (15a, 15b), each chord line (15a, 15b) interconnecting a respective said leading edge and a respective said trailing edge in transverse cross-section; the flap arrangement (11) being configured or configurable such that: said aerosurfaces (14a, 14b) are oppositely and outwardly directed relative to one another and said chord lines (15a, 15b) are convergent in a direction (16) away from the rotor body (8), from the respective leading edges to the respective trailing edges.

2. A rotor sail (2) according to claim 1, wherein said aerosurfaces (14a, 14b) are substantially concave.

3. A rotor sail (2) according to claim 1 or claim 2, wherein said flap arrangement (11) is concentrically adjustable relative to the rotational axis (9) within a range of positions about the rotor body (8).2 0 4. A rotor sail (2) according to any preceding claim, wherein said flap arrangement (11)comprises a pair of discrete flaps (11a, 11b), each said flap (11a, 11b) having a respective said leading edge (12a, 12b), a respective said trailing edge (13a, 13b), a respective said aerosurface (14a, 14a), and a respective said chord line (15a, 15b), wherein the flaps (11a, 11b) are independently concentrically adjustable relative to25 the rotational axis (9) within a range of flap positions about the rotor body (8), saidrange of flap positions including at least one operational configuration in which said flaps (11a, 11b) are positioned such that their aerosurfaces (14a, 14b) are oppositely and outwardly directed relative to one another and their chord lines (15a, 15b) are convergent in a direction (16) away from the rotor body (8).3 0 5. A rotor sail (2) according to any one of claims 1 to 3, wherein said flap arrangementcomprises a single flap (11) defining both leading edges (12a, 12b) and both trailing21 08 24edges (13a, 13b), said single flap (11) being configured such that said trailing edges (13a, 13b) are circumferentially spaced-apart about the rotational axis (9).

6. A rotor sail (2) according to claim 4, wherein each flap (11a, 11b) is mounted for pivotal adjustment about said rotational axis (9) through a range of flap positions in 5 which the trailing edge (13a, 13b) of the flap is centred on said rotational axis (9).

7. A rotor sail (2) according to any preceding claim, wherein each trailing edge (13a, 13b) is spaced from the rotational axis (9) of the rotor body (8) by a distance (d) of between 1.8R and 2.2R, where R denotes the radius of the rotor body (8).

8. A rotor sail (2) according to claim 7, wherein each trailing edge (13a, 13b) is spaced io from the rotational axis (9) of the rotor body (8) by a distance (d) of approximately 2R.

9. A rotor sail (2) according to any preceding claim, wherein each aerosurface (14a, 14b) has a region proximate a respective leading edge (12a, 12b) which lies substantially tangential to the peripheral wall (10) of the rotor body (8).

10. A rotor sail (2) according to any preceding claim, wherein each aerosurface (14a, is 14b) comprises a region proximate a respective trailing edge (13a, 13b) which liesapproximately radial to the peripheral wall (8) of the rotor body (8).

11. A rotor sail (2) according to any preceding claim, wherein the flap arrangement (11) is spaced from the peripheral wall (10) of the rotor body (8) by a gap (20), said gap (20) having a radial dimension of approximately 0.01 R to 0.02R, where R denotes the2 o radius of the rotor body (8).

12. A rotor sail (2) according to any preceding claim, wherein each leading edge (12a, 12b) has a curved profile having a radius (r) of approximately 0.01 R to 0.02R, where R denotes the radius of the rotor body (8).

13. A rotor sail according to claim 4, wherein said operational configuration is such that 25 the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of between5° and 25° from the rotational axis (9) of the rotor body (8).

14. A rotor sail according to claim 13, wherein said operational configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of between 10° and 20° from the rotational axis (9) of the rotor body (8).21 08 2415. A rotor sail according to claim 14, wherein said operational configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 15° from the rotational axis (9) of the rotor body (8).

16. A rotor sail according to claim 4, wherein said operational configuration is such that5 the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of between35° and 55°17. A rotor sail according to claim 16, wherein said operational configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of between 40° and 50°io 18. A rotor sail according to claim 17, wherein said operational configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of approximately 45°.

19. A rotor sail (2) according to claim 4, wherein said range offlap positions further includes configurations in which the flaps (11a, 11b) are positioned such that their is chord lines (15a, 15b) are divergent in a direction away from the rotor body (8).

20. A rotor sail (2) according to claim 19, wherein said range of flap positions includes an alternate operational configuration in which the flaps (11a, 11b) are positioned such that their trailing edges (13a, 13b) subtend an angle (0) of between 140° and 180° from the rotational axis (9) of the rotor body (8).2 0 21. A rotor sail (2) according to claim 20, wherein said alternate operational configurationis such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 160° from the rotational axis (9) of the rotor body (8).

22. A rotor sail (2) according to claim 4, wherein said range of flap positions includes a feathered configuration in which the flaps (11a, 11b) are positioned such that their 25 trailing edges (13a, 13b) subtend an angle (0) of between 30° and 50° from therotational axis (9) of the rotor body (8).

23. A rotor sail (2) according to claim 22, wherein said feathered configuration is such that the trailing edges (13a, 13b) of the flaps (11a, 11b) subtend an angle (0) of approximately 40° from the rotational axis (9) of the rotor body (8).21 08 2424. A rotor sail (2) according to claim 22 or claim 23, wherein said feathered configuration is such that said aerosurfaces (14a, 14b) are oppositely and outwardly directed relative to one another and the chord lines (15a, 15b) of the flaps (11a, 11b) are convergent in a direction away from the rotor body (8).5 25. A rotor sail (2) according to claim 24, wherein said feathered configuration is suchthat the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle (C) of between 10° and 30°.

26. A rotor sail (2) according to claim 24 or claim 25, wherein said feathered configuration is such that the chord lines (15a, 15b) of the flaps (11a, 11b) converge at an angle io (C) of approximately 20°.

27. A method of operating a rotor sail (2) on a vessel (1), the method comprising:providing a rotor sail (2) according to any preceding claim in an upstanding position on the vessel (1) such that said rotational axis (9) is substantially orthogonal to a longitudinal axis (36) of the vessel (1);is determining an apparent wind angle value representative of the angle between thedirection of apparent wind incident on the rotor sail (9) and a dead-ahead direction of the vessel (1) coincident with said longitudinal axis (36); andpositioning said flap arrangement (11) relative to said rotor body (8) in dependence on said apparent wind angle value.2 0 28. A method according to claim 27 comprising providing the rotor sail (2) in a formaccording to claim 4, wherein said step of positioning the flap arrangement (11) involves positioning said discrete flaps (11a, 11b) relative to said rotor body (8) in dependence on said apparent wind angle value.

29. A method according to claim 27 of claim 28, wherein a range of apparent wind angles 25 from 0° to 180° is divided into a plurality of apparent wind angle sectors, and aplurality of distinct flap configurations are defined within said range of flap positions, each said distinct flap configuration corresponding to a respective said sector, wherein the method involves monitoring said apparent wind angle value, determiningin which of said sectors the determined apparent wind angle falls, and positioning the flap arrangement (11) in the corresponding flap configuration.21 08 24

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