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) such as a ship. The rotor sail (2) has a rotational axis (10) and comprises: an elongate cylindrical rotor body (9) having a peripheral wall rotatable about the rotational axis (10). The rotor sail (2) has an elongate aero-slat (18) mounted in spaced relation and substantially parallel to the rotor body (9) and which positionally adjustable relative to the rotor body (9) in two rotational degrees of freedom. In a specific proposal, the aero-slat (18) is positionally adjustable relative to the rotor body (9) by orbital movement (21) about said rotational axis (10). In a specific proposal, the aero-slat (18) has a longitudinal axis (23) in spaced relation to the rotor body (9) and substantially parallel to the rotational axis (10), and the aero-slat (18) is positionally adjustable relative to the rotor body (9) by pivotal movement (22) about said longitudinal axis (23).

Description

[0001] A ROTOR SAIL AND A METHOD OF OPERATING A ROTOR SAIL

[0002] Field of the Invention

[0003] 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.

[0004] Background

[0005] 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.

[0006] 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 to harness the wind’s energy for propulsion, for example in so-called Wind Assisted Ship Propulsion (WASP) systems which use wind propulsion to supplement the propulsive force produced by internal combustion engines in order to reduce fuel consumption and thereby improve economy and reduce emissions.

[0007] As will be understood by those of skill in the art, a conventional rotor sail comprises a large cylindrical rotor body which is powered 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.

[0008] 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.

[0009] As will be appreciated, it is advantageous to optimise the design of a rotor sail to increase the lift force produced by the Magnus effect.

[0010] The present inventors have observed that the resulting increased pressure region on the first side of a rotor sail and the decreased pressure region on the opposite second side both influence a large area extending outwardly from the rotor sail to produce a pressure field under the Magnus effect which extends outwardly from the rotor. It has been observed that this gives rise to a currently untapped (by prior art rotor sails) pressure differential extending some distance to windward of the rotor, albeit of smaller magnitude than that created across the rotor itself.

[0011] 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.

[0012] Summary of the Invention

[0013] 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 an elongate aero-slat, the aero-slat being mounted in spaced relation and substantially parallel to the rotor body and being positionally adjustable relative to the rotor body in two rotational degrees of freedom. It has been found that by providing a rotor sail with an adjustable aero-slat of the type noted above, the aero-slat can be positioned in use somewhat to windward of the rotor body in a region of air influenced by the Magnus effect arising from rotation of the rotor body, the aeroslat thereby being positionable upstream of the rotor body to produce a lift force to supplement the lift force produced by the rotor body itself, without adversely interfering with the airflow incident on the rotor body.

[0014] In some proposals, the aero-slat is positionally adjustable relative to the rotor body by orbital movement about the rotational axis of the rotor body. Such orbital movement may thus represent one of the aforementioned degrees of freedom, and may permit the aero-slat to be positioned in use in an optimal region of the pressure field arising from the Magnus effect of the rotor body. Orbital adjustment of the aero-slat about the rotational axis of the rotor (and thus effectively about the rotor itself) may permit the aero-slat to be adjusted in dependence on changes in ship direction and / or wind direction. It is proposed that the aero-slat may be moved orbitally around the rotational axis of the rotor body through a full 360° range of movement and held in any position throughout said range of movement for operation.

[0015] The aero-slat may have a longitudinal axis in spaced relation to the rotor body and substantially parallel to the rotational axis, and may be positionally adjustable relative to the rotor body by pivotal movement about said longitudinal axis. Such pivotal movement may thus represent the other said degree of freedom, and may permit the angle of attack of the aero-slat to be optimised for the apparent wind direction. It is proposed that the aero-slat may be moved pivotally about said longitudinal axis through a full 360° range of movement and held in any position throughout said range of movement for operation.

[0016] Said pivotal movement and said orbital movement may be independent of one another.

[0017] Adjustment of the position of the aero-slat in the degrees of freedom mentioned above may allow the position of the aero-slat to be optimised for all possible apparent wind directions incident on the rotor sail.

[0018] Optionally, the longitudinal axis about which the aero-slat is pivotally moveable is proximate a trailing edge of the aero-slat. This may allow for adjustment of the aero-slat’s angle of attack without affecting (significantly) its spacing from the rotor body.

[0019] Advantageously the aero-slat has a leading edge, a trailing edge, a notional chord line interconnecting said leading edge and said trailing edge in transverse cross-section, and a chord length c measured between said leading edge and said trailing edge in transverse cross-section.

[0020] In some embodiments, said chord length may be: less than R, where R denotes the radius of the rotor body. In other embodiments, the chord length may be between 0.3R and 0.9R. Embodiments are also envisaged in which the chord length is between 0.4R and 0.8R, or optionally between 0.5R and 0.7R.

[0021] In some proposals, the aero-slat may have a transverse cross-sectional profile which is substantially mirror symmetrical about the chord line. In other words, the aero-slat may take the form of a zero-camber aerofoil. This configuration allows the aero-slat to be positioned relative to apparent wind incident on the rotor sail in an optimal manner regardless of which side of the rotor sail is to windward and which is to leeward.

[0022] In some embodiments, the aero-slat may be asymmetrical relative to an axis passing through the midpoint of the chord line and perpendicular to the chord line. The midpoint of the chord line is the point on the chord line that is equidistant from the leading and trailing edges. In some examples, the trailing edge of the aero-slat may be differently shaped from the leading edge. For example, the leading edge may have a greater radius of curvature than the trailing edge. The aero-slat may be substantially teardrop-shaped, along the direction of the chord line. That is, the transverse cross-sectional profile of the aero-slat may be substantially teardrop-shaped. Conveniently, an aero-slat having such an asymmetrical shape can actively modify the pressure field around the rotor sail (i.e. around both the rotor body and the aeroslat).

[0023] In some embodiments, the transverse cross-sectional profile of the aero-slat is shaped such that a maximum transverse thickness t of said aero-slat has a magnitude of between 0.2c and 0.5c, where c denotes the chord length measured between said leading edge and said trailing edge in transverse cross-section. Embodiments are also envisaged in which the maximum transverse thickness t of said aero-slat has a magnitude of between 0.3c and 0.4c. For example, in some embodiments, the maximum transverse thickness t of said aero-slat may be approximately 0.35c.

[0024] In some embodiments, the maximum transverse thickness t may be positioned so as to be spaced from said leading edge by a distance (p) of between 0.1c and 0.4c, or between 0.2c and 0.3c. For example, embodiments are envisaged in which the maximum transverse thickness t is positioned so as to be spaced from the leading edge by a distance (p) of approximately 0.25c.

[0025] Optionally, and with reference to a notional wind direction (W) incident on and radial to the rotor body, the aero-slat may be positionable relative to the rotor body in an operational position in which it is spaced from the rotor body by: (i) a distance measured along the notional wind direction (W) of: between 0.4R and 0.8R (for example between 0.5R and 0.7R); and: (ii) a lateral distance measured perpendicular to the notional wind direction (W) of: between 0.5R and 0.9R (for example between 0.6R and 0.8R).

[0026] Advantageously, the operational position may be configured such that the aero-slat makes an angle of attack, measured between the chord line of the aero-slat and said notional wind direction (W), of between 15 degrees and 30 degrees.

[0027] According to a second aspect, there is provided a rotor vessel equipped with a rotor sail according to the first aspect.

[0028] 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 aspect in an upstanding position on the vessel such that said rotational axis is substantially orthogonal to a longitudinal axis of the vessel; continuously rotating the rotor body about said rotational axis; determining the direction (W) of apparent wind incident on the rotor sail; and positioning the aero-slat relative to the rotor body in dependence on said apparent wind direction (W).

[0029] The method advantageously comprises positioning the aero-slat such that its trailing edge is proximate the rotor body and its leading edge is distal to the rotor body.

[0030] It is proposed that the rotor body will be rotated in a clockwise direction in response to apparent wind being incident on the port side of the vessel (e.g. continuously rotated in a clockwise direction during periods of use in port-side wind conditions), and will be rotated in a counterclockwise direction in response to apparent wind being incident on the starboard side of the vessel (e.g. continuously rotated in a counterclockwise direction during periods of use in starboard-side wind conditions).

[0031] Advantageously, the method may comprise positioning said aero-slat such that it is spaced from said rotor body by: (i) an upwind distance measured radially along said direction (W) of apparent wind incident on the rotor sail of between 0.4R and 0.8R (e.g. between 0.5R and 0.7R); and: (ii) a lateral distance measured from and perpendicular to said notional radial line of between 0.5R and 0.9R (e.g. between 0.6R and 0.8R).

[0032] In some embodiments, the method may comprise positioning the aero-slat such that it makes an angle of attack, measured between the chord line and the direction (W) of apparent wind incident on the rotor sail, of between 15 degrees and 30 degrees.

[0033] The method of the invention may involve substantially continuously monitoring wind incident on the rotor sail during use and adjusting the position of the aero-slat (and optionally also the rotational speed of the rotor body) in response to a change in said apparent wind direction.

[0034] Summary of the Figures

[0035] 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:

[0036] Figure 1 is a perspective view illustrating a rotor ship equipped with four rotor sails;

[0037] Figure 2 is a contour map of airflow velocity in the region around a conventional rotor sail, during operation;

[0038] Figure 3 is a contour map of static air pressure in the region around a conventional rotor sail, during operation;

[0039] Figure 4 is a schematic perspective view of a rotor sail in accordance with an embodiment of the present invention;

[0040] Figure 5 is a schematic view from above of the rotor sail shown in Figure 4;

[0041] Figure 6 is a contour map of airflow velocity in the region around a rotor sail according to the invention, during operation and with the apparent wind direction being from the right-hand side of the rotor sail (in the orientation illustrated);

[0042] Figure 7 is a contour map of static air pressure in the region around a rotor sail according to the invention, during operation and with the apparent wind direction being from the right-hand side of the rotor sail (in the orientation illustrated); Figure 8 is a contour map similar to that of Figure 7 but with the apparent wind direction being from the left-hand side of the rotor sail (in the orientation illustrated)

[0043] Figure 9 is a schematic transverse cross-sectional view through a rotor sail according to the invention;

[0044] Figure 10 is a schematic transverse cross-sectional view through a slat component of the rotor sail;

[0045] Figure 11 is a view similar to that of Figure 9, showing further characteristics of the rotor sail, the rotor sail being shown configured for conditions in which the apparent wind is incident on the rotor sail from the starboard side;

[0046] Figure 12 is a view corresponding to that of Figure 11 , but showing the rotor sail configured for conditions in which the apparent wind is incident on the rotor sail from the port side;

[0047] Figure 13 is another view similar to that of Figure 9, showing further characteristics of the rotor sail;

[0048] Figure 14 is a schematic perspective view from below showing the upper region of the rotor sail and part of an adjustment mechanism for positioning the slat;

[0049] Figure 15 is an enlarged perspective view from below showing further detail of the adjustment mechanism shown in Figure 14;

[0050] Figure 16 is a schematic perspective view from above showing the upper region of the rotor sail and other aspects of the adjustment mechanism; and

[0051] Figure 17 is an enlarged perspective view from above showing further detail of the aspects of the adjustment mechanism shown in Figure 16.

[0052] Detailed Description of the Invention

[0053] 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.

[0054] 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. As illustrated, the rotor sails 2 are arranged at the corners of a notional rectangle and comprise a forward pair of rotor sails 2 located towards the bow 3 of the vessel 1 and an aft 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 6 of the vessel 2, or alternatively along an axis substantially parallel to the central longitudinal axis 6 of the vessel 2). It is also envisaged that a vessel 1 may be equipped with a single rotor sail 2.

[0055] As is conventional, each rotor sail 2 is mounted so as to extend generally vertically above the deck 7 of the vessel and terminates in an end plate 8 at its uppermost (free) end. As is also conventional, each rotor sail 2 comprises an elongate cylindrical rotor body 9 arranged for rotation about a respective rotational axis 10 (only one such rotational axis 10 being illustrated in Figure 1) oriented so as to be orthogonal to a longitudinal axis 6 of the vessel 1 , and thus substantially vertical in flat water conditions in which the deck 7 is substantially horizontal.

[0056] Turning now to consider Figure 2, there is shown an exemplary contour map of airflow velocity in the region of a conventional rotor sail (i.e. not embodying the present invention) during operation and specifically during rotation of the rotor sail’s rotor body 9 in a counterclockwise direction and with apparent wind (denoted by arrow W) incident on the rotor sail from the right-hand side of the rotor sail (in the orientation illustrated). Figure 2 may thus be considered to represent operation of the rotor sail in starboard beam wind conditions (in which the forward direction of the ship 1 along the longitudinal axis 6 is directed upwardly in the orientation illustrated). In such conditions, the skilled person will recognise that the rotor sail is operated by rotating the rotor body 9 in a counterclockwise direction. As those of skill in the art will also readily appreciate, Figure 2 thus shows a region 11 of reduced airflow speed on a first side of the rotor body 9, and a region 12 of increased airflow speed on the opposite side of the rotor body 9, arising from rotation of the rotor body 9 according to the Magnus effect.

[0057] Figure 3 shows a corresponding exemplary contour map of static air pressure in the region of the conventional rotor sail depicted in Figure 2, and thus shows pressure conditions arising from the airflow caused by rotation of the rotor body 9. As those of skill in the art will also readily appreciate, Figure 3 therefore shows a region 13 of increased air pressure on the first side of the rotor body 9 (i.e. corresponding to the region 11 of reduced airflow speed shown in Figure 2), and a region 14 of decreased air pressure on the opposite side of the rotor body 9 (i.e. corresponding to the region 12 of increased airflow speed shown in Figure 2), arising from rotation of the rotor body 9. This pressure difference across the rotor body 9 creates a lift force LR which is somewhat aligned with the longitudinal axis 6 of the ship and which thus has a significant component aligned with the forward direction of the ship so as to produce effective propulsive thrust.

[0058] Of significance to the invention disclosed in more detail hereinbelow, it will be observed with reference to Figure 3 that both the region 13 of increased air pressure and the region 14 of reduced pressure arising from the Magnus effect extend some distance outwardly from the rotor body 9 (albeit with reducing intensity). In particular, it may be observed that an effective pressure differential extends some distance to windward of the rotor body 9 into a region denoted 15 in Figure 3. Because this region 15 is located upwind of the rotor body 9, the airflow in this region is relatively stable and not unduly turbulent. The improved rotor sail 2 of the present invention seeks to exploit the pressure differential in this region upwind of the rotor body 9, to supplement the lift force LR produced by the rotor body 9 itself.

[0059] Turning now to consider Figures 4 and 5, there is shown a rotor sail 2 in accordance with the present invention. As will be observed, the rotor sail 2 comprises an elongate cylindrical rotor body 9 having a peripheral wall 16 which is rotatable about the rotational axis 10. The rotor body 9 is capped at its uppermost end by an upper end plate 8 and is capped at its lowermost end by a similar lower end plate 17. It is proposed that the two end plates 8, 17 will be arranged and configured to remain substantially static during operation of the rotor sail 2 and thus whilst the rotor body 9 spins about the rotational axis 10. The rotor body 9 is thus mounted for rotation relative to, and between, the two end plates 8, 17. As will be appreciated, and in common with conventional rotor sails, the rotor body 9 (and thus also its peripheral wall 16) will be rotated in use about the rotational axis 10, for example via the use of one or more electrical motors (not shown).

[0060] In addition to the rotor body 9, the rotor sail 2 also comprises an aerodynamic component in the form of a substantially rigid elongate slat 18 (hereinafter referred to as an aero-slat). As illustrated most clearly in Figure 4, the aero-slat 18 is mounted between the peripheral regions of the two end plates 8, 17 so as to extend between the end plates 8, 17 in substantially parallel and spaced relationship to the rotor body 9. In the embodiment illustrated, the aero-slat 18 is of substantially equal vertical height to the rotor body 9. As shown in Figures 4 and 5, the aero-slat 18 has a leading edge 19 and a trailing edge 20, both of which are substantially parallel to the rotational axis 10 of the rotor body 9. In the embodiment illustrated in Figures 4 and 5, it will be observed that the aero-slat 18 has uniform cross-sectional profile along its entire length, and takes the form of a zero camber aerofoil.

[0061] As will be explained in more detail below, the aero-slat 18 is arranged to be positionally adjustable relative to the rotor body 9 in two rotational degrees of freedom. The aero-slat 18 may thus be positioned relative to the rotor body in dependence on wind conditions, for example in dependence on the direction of apparent wind incident on the rotor sail 2.

[0062] Figure 6 shows an exemplary contour map of airflow velocity in the region of a rotor sail 2 of the general configuration described above, and Figure 7 shows a corresponding contour map of air static air pressure in the region of the same rotor sail. Figures 6 and 7 represent similar starboard beam wind conditions to those represented in Figures 2 and 3, and so Figures 6 and 7 may be compared to Figures 2 and 3 to understand the aerodynamic effect of the aero-slat 18. In particular, because of the radially outward spacing of the aero-slat 8 from the rotor body 9, and its positionally adjustable mounting, it will be noted that the aeroslat 18 may be positioned in use so as to be located generally within the above-mentioned region 15 upwind of the rotor body 9, into which the effective pressure differential arising from the Magnus effect of the rotor body 9 extends. In more detail, it will be noted that the aero-slat 18 may be positioned with its leading edge 19 directed generally upwind (and thus distal to the rotor body 9), and its trailing edge 20 directed generally downwind (and thus proximate the rotor body 9). As will therefore be understood, the aero-slat 18 may be positioned so as to be influenced by the pressure differential created by rotation of the rotor body 9, to create an additional lift force Ls which serves to supplement the primary lift force LF arising from the Magnus effect, thereby increasing the total lift force produced by the rotor sail 2.

[0063] Figure 8 shows a similar contour map of static air pressure in the region of the rotor sail in port beam wind conditions (i.e. showing the apparent wind in the opposite direction to that shown in Figure 7, and with the forward direction of the ship 1 along the longitudinal axis 6 again directed upwardly in the orientation illustrated). In such conditions, the skilled person will recognise that the rotor sail is operated by rotating the rotor body 9 in a clockwise direction. As will be understood, and as will be explained in more detail hereinbelow, the aero-slat 18 may be repositioned for such port side wind conditions by moving the aero-slat 18 relative to the position shown in Figure 7, so that it is again positioned upwind of the rotor body 9 (albeit now on the port side of the rotor body 9) and with its leading edge 19 directed upwind and its trailing edge 20 directed downwind. As will become clear, the aero-slat 18 may further be repositioned in any suitable intermediate position between the starboard side position shown in Figure 7 and the port side position shown in Figure 8, for optimisation of its performance in wind conditions in which the apparent wind direction W is between the starboard beam direction shown in Figure 7 and the port beam direction shown in Figure 8. Repositioning the aero-slat 18 in dependence on the apparent wind direction in this manner thereby allows the rotor sail 2 to be trimmed to the instant wind conditions, for example to account for changes in the apparent wind direction arising due to changes in true wind direction, changes in vessel heading, changes in true wind speed, and / or changes in vessel speed.

[0064] Turning now to consider Figure 9, the positional adjustment of the aero-slat 18 relative to the rotor body 9 will be described in more detail.

[0065] Firstly, the aero-slat 18 is mounted for orbital movement about the rotational axis 10 of the rotor body 9, as denoted by arrow 21. In some embodiments, the aero-slat 18 may be moved orbitally around the rotational axis 10 of the rotor body 9 through a full 360° range of movement and held in any position throughout said range of movement for operation.

[0066] Secondly, the aero-slat 18 is mounted for pivotal movement (denoted by arrow 22) about a longitudinal axis 23 which is radially spaced from the rotor body 9 and substantially parallel to the rotational axis 10 of the rotor body about which the aero-slat 18 is orbitally moveable. It is proposed that the longitudinal axis 23 may be positioned within the transverse cross- sectional profile of the aero-slat 18 so as to extend through the length of the aero-slat 18 and, as illustrated in Figure 9, in currently preferred embodiments may be located proximate the trailing edge 20 of the aero-slat 18. In some embodiments, the aero-slat 18 may be moved pivotally about its longitudinal axis 23 through a full 360° range of movement and held in any position throughout said range of movement for operation.

[0067] The above-described orbital movement and the above-described pivotal movement of the aero-slat 18 are independent of one another.

[0068] The aero-slat 18 may thus be trimmed to wind conditions by: i) orbital movement around the rotational axis 10 of the rotor body 9 in order to achieve a suitable orbital position (e.g. to position the aero-slat 18 to windward of the rotor body 9); and ii) pivotal movement about its longitudinal axis 23 (e.g. to position the aero-slat 18 at a suitable angle of attack relative to the apparent wind direction W).

[0069] Figure 10 is a transverse cross-sectional view through the aero-slat 18 and illustrates a proposed profile for the aero-slat 18. As noted above, the aero-slat 18 has a leading edge 19 and a trailing edge 20. In a conventional manner, a notional chord line 24 interconnects the leading edge 19 and the trailing edge 20. It is proposed that the above-noted longitudinal axis 23, about which the aero-slat 18 is mounted for pivotal movement, will be located on the notional chord line 24.

[0070] The aero-slat 18 illustrated in Figure 10 will be understood to have a transverse cross- sectional profile which is substantially mirror symmetrical about the chord line 24. The aeroslat 18 may thus be considered to take the form of a zero-camber aerofoil. In order to maximize the tolerance of the aero-slat 18 to changes in wind direction and gusts during operation, it is proposed that it may have a rounded leading edge 19 and shallow recovery profile, as illustrated in Figure 10.

[0071] The aero-slat 18 has a maximum thickness t measured orthogonal to the chord line 24, and a chord length c measured between the leading edge 19 and the trailing edge 20. It is proposed that the maximum thickness t may have a magnitude of be between 0.2c and 0.5c. In some examples, the maximum thickness t may be between 0.3c and 0.4c, with a value of approximately 0.35c currently being considered particularly advantageous. The position of the point of maximum thickness t along the chord line c may be defined by a distance p measured from the leading edge 19, as shown in Figure 10. It is proposed that the maximum transverse thickness t may be positioned such that the distance p is between 0.1c and 0.4c. In some examples, the distance p may be between 0.2c and 0.3c, with a value of approximately 0.25c currently being considered particularly advantageous.

[0072] It is proposed that the chord length c of the aero-slat 18 will be less than the radius (denoted R) of the rotor body 9. For example, the chord length c may be between 0.3R and 0.9R. In some embodiments the chord length c may be between 0.4R and 0.8R, for example between 0.5R and 0.7R.

[0073] Figures 11 and 12 illustrate schematically a proposed operational position of the aero-slat 18 relative to the rotor body 9. Figure 11 shows the rotor sail 2 in starboard beam wind conditions with the apparent wind incident on the rotor body 9 from the starboard side (in which conditions the rotor body 9 will be rotated in a counterclockwise direction), and Figure 12 shows the rotor sail 2 in port beam wind conditions with the apparent wind incident on the rotor body 9 from the port side (in which conditions the rotor body 9 will be rotated in a clockwise direction). In each figure, the apparent wind direction W is shown incident on and radial to the rotor body 9, along a notional radial line w. It is to be appreciated, that during operation on vessel 1 , the position of the aero-slat 18 may be substantially continuously adjusted relative to the rotor body (9) in response to changes in the apparent wind direction W.

[0074] As shown in both Figure 11 and Figure 12, the operational position of the aero-slat 18 is such that it is spaced from the rotor body 9 by: i) a distance 25 measured along the notional radial wind direction w; and ii) a lateral distance 26 measured perpendicular to the notional radial wind direction w. Embodiments are proposed in which the distance 25 measured along the notional wind direction w is between 0.4R and 0.8R, and optionally between 0.5R and 0.7R. The lateral distance 26 may be between 0.5R and 0.9R and may, for example, be between 0.6R and 0.8R in some embodiments.

[0075] During periods of counterclockwise rotation of the rotor body 9 (i.e. in starboard wind conditions similar to those illustrated in Figure 11 and Figure 7), in order for the aero-slat 18 to be located within the target region 15 shown in Figure 7, the aero-slat 18 should be positioned on the upwind side of the rotor body 9 and to the right of the notional radial line w aligned with the apparent wind direction W. Conversely, during periods of clockwise rotation of the rotor body 9 (i.e. in port wind conditions similar to those illustrated in Figure 12 and Figure 8), the aero-slat 18 should be positioned on the upwind side of the rotor body 9 and to the left of the notional radial line w aligned with the apparent wind direction W.

[0076] With reference to Figure 13, another parameter of relevance to the optimum positioning of the aero-slat 18 relative to the rotor body 9 in use is that of the aero-slat’s angle of attack (denoted A in Figure 13 and measured between the chord line 24 of the aero-slat 18 and the notional radial line w aligned with the apparent wind direction W). It is proposed that the angle of attack A should be between 15 degrees and 30 degrees.

[0077] Figures 14 and 15 show schematically a possible mechanism for pivotal adjustment of the aero-slat 18 about its longitudinal axis 23. The mechanism shown is provided at the uppermost end of the aero-slat 18, beneath the upper end plate 8. It is to be appreciated, however, that a similar mechanism will also be provided at the lowermost end of the aero-slat 18 at the lower end plate 17. In more detail, the aero-slat 18 is shown mounted at its upper end to an intermediate support plate 27, for pivotal movement 22 about its longitudinal axis 23. Pivotal movement 22 of the aero-slat 18 relative to the intermediate support plate 27 may be achieved by actuation of an electric motor (not shown) provided within the structure of the aero-slat 18, for example via a suitable internal gear arrangement 28 (shown schematically in Figure 15). It is proposed that the upper and lower pivotal adjustment mechanisms will be actuated in synchronism during operation of the rotor sail, to ensure that the aero-slat 18 remains in parallel relationship to the rotor body 9 as it is moved pivotally about its axis 23.

[0078] Figures 16 and 17 show schematically a possible mechanism for orbital adjustment of the aero-slat 18 about the rotational axis 10 of the rotor body 9. The mechanism shown is provided at the uppermost end of the aero-slat 18, at the upper end plate 8. It is to be appreciated, however, that a similar mechanism will also be provided at the lowermost end of the aero-slat 18 at the lower end plate 17. In the arrangement illustrated, the peripheral edge of the end plate 8 has a toothed profile in the manner of a gear wheel. A small pinion gear 29 is rotatably mounted to the intermediate support plate 27 and meshed with the toothed peripheral edge of the end plate 8. It is proposed that the pinion gear 29 may be driven by a suitable electric motor (not shown), to rotate relative to the intermediate support plate 27 and thereby move around the peripheral edge of the end plate 8. It is proposed that the upper and lower orbital adjustment mechanisms will be actuated in synchronism during operation of the rotor sail, to ensure that the aero-slat 18 remains in parallel relationship to the rotor body 9 as it is moved orbitally about the rotational axis of the rotor body 9.

[0079] It is to be noted that whilst Figures 16 and 17 show the pinion gear 29 of the orbital adjustment mechanism engaging a toothed peripheral edge of the end plate 8, it is proposed that the pinion gear 29 could instead engage a similar fixed gear wheel mounted below the end plate 8 and concentric therewith. In such an arrangement, the end plate 8 may thus be sized to extend over the end of the aero-slat 18 such that the aero-slat 18 may be positioned entirely beneath the end plate 8 regardless of its orbital or pivotal position relative to the rotor body 9. Of course, a similar arrangement may also be provided at the lower end plate 17.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0084] 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.

[0085] 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.

[0086] 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, + / - 10%.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

Claims

CLAIMS1. A rotor sail (2) having a rotational axis (10) and comprising: an elongate cylindrical rotor body (9) having a peripheral wall rotatable about the rotational axis (10); and an elongate aero-slat (18), the aero-slat (18) being mounted in spaced relation and substantially parallel to the rotor body (9) and being positionally adjustable relative to the rotor body (9) in two rotational degrees of freedom.

2. A rotor sail (2) according to claim 1 , wherein the aero-slat (18) is positionally adjustable relative to the rotor body (9) by orbital movement (21) about said rotational axis (10).

3. A rotor sail (2) according to claim 1 or claim 2, wherein the aero-slat (18) has a longitudinal axis (23) in spaced relation to the rotor body (9) and substantially parallel to the rotational axis (10), the aero-slat (18) being positionally adjustable relative to the rotor body(9) by pivotal movement (22) about said longitudinal axis (23).

4. A rotor sail (2) according to claim 2, wherein the aero-slat (18) has a longitudinal axis (23) in spaced relation to the rotor body (9) and substantially parallel to the rotational axis(10), the aero-slat (18) being positionally adjustable relative to the rotor body (9) by pivotal movement (22) about said longitudinal axis (23), said pivotal movement (22) and said orbital movement (21) being independent of one another.

5. A rotor sail (2) according to claim 3 or claim 4, wherein said longitudinal axis (23) about which said aero-slat (18) is pivotally moveable (22) is proximate a trailing edge (20) of the aero-slat (18).

6. A rotor sail (2) according to any preceding claim, wherein said aero-slat (18) has a leading edge (19), a trailing edge (20), and a chord length (c) measured between said leading edge (19) and said trailing edge (20) in transverse cross-section, said chord length (c) being: less than R; optionally between 0.3R and 0.9R; optionally between 0.4R and 0.8R; optionally between 0.5R and 0.7R; where R denotes the radius of the rotor body (9).

7. A rotor sail (2) according to any preceding claim, wherein said aero-slat (18) has a leading edge (19), a trailing edge (20), and a notional chord line (24) interconnecting said leading edge (19) and said trailing edge (20) in transverse cross-section, the aero-slat (18)having a transverse cross-sectional profile which is asymmetrical relative to an axis passing through a midpoint of the chord line (24) and perpendicular to the chord line (24).

8. A rotor sail (2) according to claim 7, wherein the leading edge (19) has a greater radius of curvature than the trailing edge (20).

9. A rotor sail (2) according to claim 8, wherein the transverse cross-sectional profile of the aero-slat (18) is substantially teardrop-shaped.

10. A rotor sail (2) according to any preceding claim, wherein said aero-slat (18) has a leading edge (19), a trailing edge (20), and a notional chord line (24) interconnecting said leading edge (19) and said trailing edge (20) in transverse cross-section, the aero-slat (18) having a transverse cross-sectional profile which is substantially mirror symmetrical about said chord line (24).

11. A rotor sail (2) according to claim 10, the aero-slat (18) having a chord length c measured between said leading edge (19) and said trailing edge (20) in transverse crosssection, wherein said transverse cross-sectional profile is shaped such that a maximum transverse thickness t of said aero-slat (18) has a magnitude of: between 0.2c and 0.5c; optionally between 0.3c and 0.4c; optionally approximately 0.35c.

12. A rotor sail (2) according to claim 11, wherein said maximum transverse thickness (t) is positioned so as to be spaced from said leading edge (19) by a distance (p) of: between 0.1c and 0.4c; optionally between 0.2c and 0.3c; optionally approximately 0.25c.

13. A rotor sail (2) according to any preceding claim, wherein, with reference to a notional wind direction (w) incident on and radial to the rotor body (9), said aero-slat (18) is positionable relative to said rotor body (9) in an operational position in which it is spaced from said rotor body (9) by: (i) a distance (25) measured along said notional wind direction (w) of: between 0.4R and 0.8R; optionally between 0.5R and 0.7R; and: (ii) a lateral distance (26) measured perpendicular to said notional wind direction (w) of: between 0.5R and 0.9R; optionally between 0.6R and 0.8R; where R denotes the radius of the rotor body (9).

14. A rotor sail (2) according to claim 13, wherein said aero-slat (18) has a leading edge (19), a trailing edge (20), and a notional chord line (24) interconnecting said leading edge (19) and said trailing edge (20) in transverse cross-section, and wherein said operationalposition is such that the aero-slat (18) makes an angle of attack, measured between said chord line (24) and said notional wind direction (w), of between 15 degrees and 30 degrees.

15. A rotor vessel (1) equipped with a rotor sail (2) according to any preceding claim.

16. 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 claims 1 to 11 in an upstanding position on the vessel (1) such that said rotational axis (10) is substantially orthogonal to a longitudinal axis (6) of the vessel (1); continuously rotating said rotor body (9) about said rotational axis (10); determining the direction (W) of apparent wind incident on the rotor sail (2); and positioning said aero-slat relative to said rotor body (9) in dependence on said apparent wind direction (W).

17. A method according to claim 16, wherein the aero-slat (18) has a leading edge (19) and a trailing edge (20); the method involving positioning the aero-slat (18) such that said trailing edge (20) is proximate said rotor body (9) and said leading edge (20) is distal to said rotor body (9).

18. A method according to claim 16 or claim 17, wherein said rotor body (9) 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).

19. A method according to claim 18, wherein: during periods of clockwise rotation of said rotor body (9), the aero-slat (18) is positioned on the upwind side of said rotor body (9) and to the left of a notional radial line (w) aligned with said direction (W) of apparent wind incident on the rotor sail (2); and during periods of counterclockwise rotation of said rotor body (9), the aero-slat (18) is positioned on the upwind side of said rotor body (9) and to the right of a notional radial line (w) aligned with said direction (W) of apparent wind incident on the rotor sail (2).

20. A method according to claim 19, wherein the rotor sail (2) is in accordance with claim 10, the method comprising positioning said aero-slat (18) such that it is spaced from said rotor body (9) by: (i) an upwind distance measured radially along said direction (W) of apparent wind incident on the rotor sail (2) of: between 0.4R and 0.8R; optionally between 0.5R and 0.7R; and: (ii) a lateral distance measured from and perpendicular to said notionalradial line (w) of: between 0.5R and 0.9R; optionally between 0.6R and 0.8R; where R denotes the radius of the rotor body (9).

21. A method according to claim 19 or claim 20, wherein the rotor sail (2) is in accordance with claim 11, the method comprising positioning said aero-slat (18) such that it makes an angle of attack, measured between said chord line (24) and said direction (W) of apparent wind incident on the rotor sail, of between 15 degrees and 30 degrees.

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