Boat paddle balancing

Abstract

A method of balancing a boat paddle having two blades 30, 40, which are connected together by a shaft 20 and which each have a centre of gravity offset from a longitudinal axis 21 of the shaft in a di

Description

The present invention relates to a method of balancing a boat paddle, in particular a double-bladed paddle, and to a paddle balanced by the method as well as a weight adapted for the purpose of balancing a boat paddle by the method. Double-bladed boat paddles, in particular paddles in which two blades are connected together by a shaft manipulated by a user, as a means of manual propulsion of smallsize watercraft such as kayaks, canoes and other such single-occupant or multi-occupant vessels, especially recreational and sporting boats, have been considerably developed from basic shapes in which the blades are merely flat and generally symmetrical in relation to the axis of the connecting shaft. Enhanced interaction of blade and water in conjunction with more efficient water entry and propulsion by the blade have been achieved by asymmetrical blade constructions in which the two blades are angled relative to one another about the shaft axis by, for example 30 to 90 degrees and in which each blade has an elongate, quasi-aerofoil shape with a concave drive face bounded at one longitudinal edge by a water entry edge. The blade shape eases water entry and exit and the concavity increases drive area at the blade and water interface, while the relative angle of the blades takes into account user rotation of the paddle about the shaft axis during transfer from propulsion by one of the blades to propulsion by the other blade. The asymmetrical aerofoil form typically has greater and weightier blade area on one side of the shaft axis than the other, with the result that each blade has a centre of gravity offset from the shaft axis in a direction away from the water entry edge. A biasing effect of this centre of gravity offset is further increased by a cranking of each blade relative to the shaft axis so that the longitudinal axis of the elongate blade intersects the shaft axis at a small angle in the region of 10 degrees, in particular cranking in such a way that with the concave drive face facing upwardly the blade is cranked downwardly. The effect of the blade centre of gravity location is that in an equilibrium state with the shaft axis horizontal the blade water entry edge tends towards a location spaced horizontally from the shaft axis. The consequence of a paddle design of that kind or a design with a similar blade centre of gravity offset is that the different forces produced by the eccentric location of the blade centres of gravity relative to shaft axis combine in use to rotate the paddle so that the blade water entry edges point towards the user, i.e. a rotation shifting the water entry edges into a horizontally rather than downwardly directed orientation. This rotation has to be counteracted by the user by applying and maintaining an oppositely directed rotation to orientate the paddle water entry edges downwardly in alternation, which over a period of prolonged paddling can lead to muscular fatigue and even, for example in competitive activities, a degree of repetitive strain injury. In general, the need for the user to constantly resist an undesired reoriention of the blades reduces the capability of fine motor control and compromises propulsion efficiency. It is therefore an object of the present invention to realise a method of balancing a paddle of the general kind described, i.e. a paddle with two blades which are connected by a shaft and have offset centres of gravity influencing the orientation of paddle water entry edges, so that the need for a user to combat an undesired rotation of the paddle during use is removed to at least a substantial extent. A further object of the invention is realisation of a double-bladed paddle balanced to provide a desired downward orientation of blade water entry edges Yet another object of the invention is provision of a paddle balancing weight able to be fitted to an existing paddle and to be set or adjusted to balance the paddle in a desired way. Other objects and advantages of the invention will be apparent from the following description. According to a first aspect of the present invention there is provided a method of balancing a boat paddle having two blades which are connected together by a shaft and which each have a centre of gravity offset from a longitudinal axis of the shaft in a direction away from a water entry edge of that blade, the method comprising the step of equipping the shaft between the blades with a weight acting eccentrically with respect to the shaft longitudinal axis to exert on the paddle when the axis is horizontal a turning moment about that axis, the weight being positioned in a rotational position with respect to the shaft longitudinal axis so that the turning moment acts in opposition to the gravitational effect of the centres of gravity of the blades to allow the paddle to assume a state of equilibrium in any rotational orientation of the paddle about the shaft longitudinal axis when the axis is horizontal. Through equipping the shaft with a balancing weight having a centre of gravity in an eccentric position relative to the blade connecting shaft axis and, in particular, in a selected rotational position producing the stated turning moment, the paddle can be balanced so as to adopt a state of equilibrium in which there is no rotational force working against the user when, especially, the paddle is in a desired use orientation, thus with the water entry edges of both blades facing downwardly. The user therefore does not have to constantly turn the blades into that orientation or resist turning of the blades out of that orientation. The exact rotational position of the weight can be predetermined, for example in the case of a paddle manufactured at the outset with such a weight as a partially or wholly integrated element, or can be determined empirically, for example in the case of equipping or retrofitting an existing, unbalanced paddle. In a paddle construction optimised for performance, for example in competitive kayaking, the blades may be angled relative to one another so that their centres of gravity are offset from the shaft longitudinal axis in respectively different directions, the weight then being positioned to exert a turning moment opposing and negating the mean gravitational effect of the centres of gravity. The positioning of the weight to provide a turning mount counteracting that produced by the blade centres of gravity makes it possible for a user to comfortably maintain optimal or any desired blade orientations relative to a water surface over which a boat is to be propelled by the paddle. Such an orientation can be, for example, one in which the water entry edge of each blade is directed downwardly between the vertical and an angle of up to 15 degrees relative thereto, for example substantially 10 degrees. The shaft is preferably equipped with the weight substantially centrally between the blades, which is a convenient location and ensures that the weight does not tend to tilt the shaft axis. The user typically grips the paddle by both hands on either side of the shaft centre, so that positioning the weight at the centre does not interfere with holding and manipulating the paddle. In a preferred example of the method the step of equipping comprises rotating the weight about the shaft longitudinal axis to influence the turning moment. If the weight has such an adjustment capability, which can be achieved in various ways depending on how the weight is constructed, it is possible to set the weight to realise coarse balancing of the paddle and then finely set the weight to provide exact balancing, whether achieved by visual assessment or measurement. Advantageously, the weight comprises a mass adjustable towards and away from the shaft longitudinal axis and the method comprises adjusting the mass to vary a spacing of the centre of gravity of the mass from the shaft longitudinal axis and thereby the exerted turning moment. This represents a further adjustment capability for achieving the desired equilibrium state of the paddle, in effect adjustment of the magnitude of the turning moment vector. In one example of the method the step of equipping the shaft with the weight comprises attaching the weight to the shaft of an already manufactured paddle, especially removably attaching the weight. Such a weight can be designed to be easily and quickly fitted and allow removal for fitting to another paddle. Alternatively, however, the step of equipping the weight can be carried out at least in part during manufacture of the paddle so that weight is at least partially integrated in the paddle at the outset. This can be achieved in various ways and might include moulding a mass into or onto the paddle shaft - which conventionally is tubular - at one side at a spacing from the shaft axis or securing a mass to, for example, a mount provided at or in the shaft. In these and other such possibilities, at least part of the weight may be formed on the shaft. According to second aspect of the present invention there is provided a boat paddle balanced by a method according to the first aspect of the invention, the paddle comprising a shaft and two blades connected together by the shaft and each having a centre gravity offset from a longitudinal axis of the shaft in a direction away from a water entry edge of that blade, wherein the shaft is equipped between the blades with a weight acting eccentrically with respect to the shaft longitudinal axis to exert on the paddle when the axis is horizontal a turning moment about that axis, the weight being positioned in a rotational position with respect to the shaft longitudinal axis so that the turning moment acts in opposition to the gravitational effect of the centres of gravity of the blades to allow the paddle to assume a state of equilibrium in any rotational orientation of the paddle about the shaft longitudinal axis when horizontal. A paddle embodying the invention, i.e. an appropriately balanced paddle, can thus be a paddle of known construction balanced by the method described in the foregoing. The balanced paddle has the already-mentioned desirable feature of being usable by a user without the need to reorientate the paddle blades by application of a significant rotational force to the paddle shaft during use or, at least, without the need to constantly hold the paddle against a restoring force seeking to rotate the shaft and thereby the blades into an undesired blade attitude or angle of attack. In a preferred embodiment of the paddle the blades are angled relative to one another so that the centres of gravity thereof are offset from the shaft longitudinal axis in respectively different directions, the weight being positioned to exert a turning moment opposing the mean gravitational effect of the centres of gravity so that a user can select and maintain a desired blade orientation without the need to apply a significant rotational force to the paddle shaft. Such a selected orientation is, for example, one in which the water entry edge of each blade in the equilibrium state of the paddle is directed downwardly between the vertical and an angle of up to 15 degrees relative thereto. The advantages attaching to these preferred features correspond with those mentioned above in connection with the method. In a particularly advantageous blade design each of the blades defines an elongate scoop having its water entry edge at a longitudinal edge of the scoop and its centre of gravity offset from the shaft longitudinal axis in a direction away from that longitudinal edge. Such a scoop results in an approximately aerofoil-shaped blade crosssection easing blade entry into a body of water, movement in submerged state through the body of water and thereafter exit from the body of water. The scoop also imparts to the blade a generally concave drive face of increased area, namely a face that moves towards the user during a propulsive stroke and produces a measure of water displacement under pressure due to any inertia of the vessel occupied by the user and, with the inertia overcome, movement of the vessel forwards in the water by reaction to the stroke, such forward movement becoming continuous under a sustained fast paddling action. The spacing of the centre of gravity of the scoop, thus the blade, from the shaft axis may arise from an asymmetrical blade profile with its longitudinal water entry edge closer to the shaft axis than its opposite longitudinal edge, which results in an unequal distribution of weight either side of the axis, and from a cranking of each blade away from the shaft axis so that, in particular, the axis largely runs at a distance from the concave face of the blade rather than actually through the blade except in the region of the junction of the blade root and the shaft. The cumulative imbalances arising from these asymmetries in blade design are corrected in their entirety or at least substantially mitigated by the weight through its selected rotational position on the shaft. The shaft is preferably equipped with the weight substantially centrally between the blades, which is the optimum position from the aspect of achieving balancing and which, as already mentioned, does not interfere with paddle use. As also mentioned in connection with a method exemplifying the invention, the weight can be rotatable about the shaft longitudinal axis to influence the turning moment and preferably comprises a mass adjustable towards and away from the shaft longitudinal axis to vary a spacing of the centre of gravity of the mass from the shaft longitudinal axis and thereby the exerted turning moment. The two different adjustment capabilities assist determination of a desired vector producing the blade equilibrium state in any rotational orientation of the paddle about the shaft axis, especially through an initial coarse setting and then a fine setting of that state. In a preferred construction of a paddle with the adjustable mass the weight comprises a projection extending away from the shaft and the mass is positionable on the projection at a selectable spacing from the shaft, the mass being fixable to the projection at the selected spacing. The projection, which can extend radially, chordally or tangentially of the shaft, provides a simple carrier for the mass and allows the mass to be set in a position providing a selectable level of lever arm action on the shaft to achieve the desired paddle balancing. For preference, the weight is removably attached to the shaft so that it can be fitted and removed as and when desired. Alternatively, and as previously mentioned, at least part of the weight can be formed on the shaft. According to a third aspect of the present invention there is provided a weight adapted for balancing a boat paddle by a method according to the first aspect of the invention, the paddle having two blades which are connected together by a shaft and which each have a centre of gravity offset from a longitudinal axis of the shaft in a direction away from a water entry edge of that blade, wherein the weight is attachable to the paddle shaft to act eccentrically with respect to the shaft longitudinal axis to exert on the paddle when the axis is horizontal a turning moment about that axis and wherein the weight comprises attaching means attachable to the shaft, a projection carried by the attaching means and arranged to project away from the shaft when the attaching means is attached to the shaft and a mass carried by the projection to be at a spacing from the shaft when the attaching means is attached thereto. Such a weight can be of simple and economic construction and adapted to use for balancing a shaft by, for example, the attaching means having a passage which in a fitted state of the weight has a cross-sectional shape and dimension, especially diameter, matched to a given shape and outside diameter of the paddle shaft. In unfitted state, the passage may have a larger diameter to facilitate fitting to the shaft, such as will be the case if the attaching means comprises, for example, a clamp tightenable around the shaft. For preference, the mass is variable positionable on the projection at a selectable spacing from the shaft, the mass being fixable to the projection at that spacing. This feature enables fine setting or adjustment of the turning movement exerted by the weight. In that case, the mass is preferably slidably engaged with the projection and positionable by sliding along the projection to be at the selectable spacing. Alternatively, the mass is threadedly engaged with the projection and positionable by screwing along the projection. Preferred examples of the method and preferred embodiments of the paddle and weight will now be more particularly described with reference to the accompanying drawings, in which: Fig. 1 is a schematic front elevation of a known double-bladed paddle, in particular two blades connected by a shaft, in a use orientation of the paddle; Fig. 2 is a diagrammatic cross-section, to an enlarged scale, of the paddle of Fig. 1 along the section line ll-ll in Fig. 1; Fig. 3 is a schematic plan view of the paddle of Fig. 1; Fig. 4 is a diagrammatic cross-section, similar to Fig. 2, of the paddle of Fig. 3 along the section line IV-IV in Fig. 3; Fig. 5 is a schematic front elevation of the paddle turned through a right angle from the orientation of Fig. 1; Fig. 6 is a diagrammatic cross-section, similar to Fig. 2, of the paddle of Fig. 5 along the section line VI-VI in Fig. 5; Fig. 7 is a diagrammatic cross-section, similar to Fig. 6, but to an enlarged scale and showing a paddle embodying the invention, thus the paddle of Fig. 1 equipped at the shaft with a balancing weight by a method exemplifying the invention; Fig. 8 is a cross-section corresponding with that of Fig. 7, but with change in the spacing of a centre of gravity of a mass of the weight from the shaft; Fig. 9 is a diagrammatic cross-section of the paddle, in an orientation similar to that of Fig. 2, showing the balancing effect of the weight; Fig. 10 is a schematic partly-sectioned end view, to an enlarged scale, of a balancing weight attachable to the paddle shaft to provide the embodiment of Fig. 7 or Fig. 8 and realise a method exemplifying the invention; and Fig. 11 is a schematic cross-sectional view of the paddle shaft equipped with a balancing weight in a variant in which the weight is in part integrated in the shaft. Referring now to the drawings, there is shown in Figs. 1 to 7 a known double-bladed paddle 10 formed by a straight shaft 20, which is centrally held and manipulated by a user, i.e. an occupant of a vessel to be propelled by appropriate interaction of the paddle double blades with water surrounding the vessel, and which has a longitudinal axis 21, and two blades 30 and 40 connected together by the shaft. Fig. 1 shows the paddle in the general orientation for use by the user, with the user in front of the paddle 10 in the viewing direction of Fig. 1. The blade 30 on the left in Fig. 1 is the lefthand blade from the viewpoint of the user and similarly the blade 40 on the right in Fig. 1 is the righthand blade for the user. The user achieves propulsion by alternating entry of the blades 30 and 40 into the water and pulling back on each entered and submerged blade in turn to provide forward thrust of the vessel against the resistance of the water. The alternation of the entries of the two blades 30 and 40 into the water normally includes a small intermediate rotation of the shaft 20 by the user to adjust blade angle relative to the water surface. Figs. 1 and 2 show a use orientation specific to an appropriate entry angle for just the blade 30. The shaft 20 is shown in broken-away form. Its total length is typically approximately twice the length of each blade 30 and 40 and can vary according to user size and type of vessel to be propelled by the paddle as well as other factors. The paddle 10, which is, for example, of carbonfibre composite construction, shown in Figs. 1 to 6 has a relatively complex blade shape and blade relationship to the connecting shaft 20, in particular to the shaft longitudinal axis 21. The present invention confers a significant benefit in relation to a paddle with the blade shape and blade-to-shaft relationship as set out in more detail below, but the invention is equally applicable to blades with different shapes, indeed any paddle with blades that may, due to paddle imbalance, tend to adopt an undesired orientation departing from that required by the intended use for propulsion, specifically an undesired orientation with inappropriate water entry angles of the blades. In the illustrated embodiment, each blade 30 and 40 has the form of an elongate scoop with an asymmetrical profile and a compound curvature, namely a shallow curvature in the general direction of the shaft axis 21 and a shallow curvature in a direction transversely thereto, exemplified in the latter case by the cross-section along the section line ll-ll of Fig. 1 as shown to enlarged-scale in the cross-sectional diagram of Fig. 2. In order to more clearly illustrate blade orientation, the transverse cross-section of the blade 30 is schematically represented in Fig. 2 by a median solid line and that of the blade 40 by a median dashed line, both depicted in relation to a horizontal plane H and a vertical plane V intersecting at the shaft axis 21. The scoop is optimised for entry into, movement through and exit from a body of water and in broad terms has a quasi-aerofoil form. The concave side of each blade 30 or 40, i.e. the side facing the user in the use orientation of Fig. 1, defines a drive face 31 or 41 for propulsive interaction with the water and the convex side, i.e. the side away from the user, represents a back face 32 or 42 where ventilation or underpressure may occur. Each blade 30 and 40 has a water entry edge 33 and 43, respectively, at one longitudinal edge of the scoop, namely the lower edge in the use orientation of the paddle. The opposite longitudinal edge 34 or 44, which here is at the top and has a more pronounced curvature in profile than the edge 33 or 43, has a slight lip which projects on the concave side of the blade and acts as a restraint to excessive water spillage from the blade. At the blade end fixed to the shaft 20, i.e. blade root, the longitudinal edges converge to form a smooth or continuous transition to the shaft. Conversely, at the free blade end the blade is bounded by a relatively wide edge so as to maximise water engagement area over the scoop length. A schematic plan view of the paddle 10 in the use orientation of Fig. 1 is shown in Fig. 3 and a diagrammatic cross-section, analogous to Fig. 2, in Fig. 4. Apart from the asymmetrical profile resulting from the differently curved longitudinal edges 33 and 34 or 43 and 44 and different widths of the ends, each blade 30 and 40 individually has an asymmetrical relationship in two different respects to the shaft 20. In addition, the two blades together have an asymmetrical relationship to the shaft in a third respect. In a first aspect of the individual asymmetrical blade-to-shaft relationship, each blade is positioned relative to the axis 21 of the shaft 20 so that, as can be seen on the left in Fig. 1 in the case of the blade 30, a greater area and hence weight of the blade scoop lies above the axis than below when the paddle 10 is in the use orientation of Fig. 1. As a result, the blade 30 has an eccentric centre of gravity 35 approximately in the position shown in Fig. 1 above the axis 21, thus offset from the axis 21 in a direction away from the blade water entry edge 33, and in that position the gravitational effect of the centre of gravity causes the paddle in a rest or equilibrium state to adopt an orientation approximately at right angles to that of Fig. 1. The adopted orientation is that shown in the front elevation of Fig. 5, which corresponds with an inversion of the plan view of Fig. 3 albeit with a different viewing direction (elevation as opposed to plan). The blade 40 has a corresponding eccentric centre of gravity 45. In this rest state and as shown in the cross-section of Fig. 6, the drive face 31 of the blade 30 is approximately horizontally disposed so that the water entry edge 33 is directed horizontally towards the user and consequently the blade is approximately parallel to the surface of water in which the vessel occupied by the user floats. The paddle has to be rotated back to the use orientation of Fig. 1 to allow the water entry edge 33 of firstly the blade 30 and subsequently the water entry edge 43 of the blade 40 to enter the water. As mentioned in the introduction, the need to apply such rotation or, more specifically, to resist rotation out of the use orientation to the undesired rest orientation generates fatigue for the user and has a detrimental effect overall on the action of paddling. In a second aspect of the individual asymmetrical blade-to-shaft relationship, which is evident particularly from the rest state orientation of Figs. 5 and 6, each blade 30 and 40 is constructed and attached to the shaft 20 to be cranked relative thereto in such a way as to be offset downwardly from the shaft axis 21 in the rest state orientation. As a consequence almost the entire weight of the blade lies below the shaft in the rest state orientation. This substantially increases the inertia of the paddle in that orientation and, as a consequence, resistance to return to the use orientation of Figs. 1 and 2. In the mentioned third aspect of asymmetry of the two blades - as considered together rather than individually - in relation to the shaft, the blades 30 and 40 are themselves angled relative to one another so that their centres of gravity 35 and 45 are offset from the shaft axis 21 in respectively different directions, as will be evident from the different cross-sectional diagrams of Figs. 2, 4 and 6. The angular spacing of the two blades is, for example, anywhere in a range of 0 to 90 degrees, thus an included angle of that amount, depending on user choice and intended paddle use, for example a discipline in kayak racing. The combined force deriving from the gravitational effect of the two blade centres of gravity may produce a variation in the blade equilibrium state in the sense of tipping the water entry edge 33 of the blade 30 upwardly and that of the blade 40 downwardly, as shown in the blade relationship of Fig. 6. Figs. 7 to 9 are enlarged-scale diagrammatic cross-sections based on those of Figs. 2, 4 and 6, but showing a procedure for balancing the paddle 10 by a method exemplifying the invention and, as a result of the method, a balanced paddle embodying the invention. In that connection, it should be noted that the balancing method is equally applicable to a double-bladed paddle with a blade shape and blade orientation differing from those of Figs. 1 to 6, but subject to a blade centre-of-gravity offset or eccentricity producing a same or similar paddle equilibrium state in which the blades are undesirably rotated out of appropriate water entry orientations. Such a differently shaped paddle, which has been appropriately balanced, accordingly represents another embodiment of the invention. In brief, the method entails equipping the paddle 10, more specifically the shaft 20 thereof, with a weight 50 disposed to act eccentrically with respect to the shaft longitudinal axis 21 to exert on the paddle a turning moment about that axis, the weight being located in a selected rotational position - which can be determined by trial and error or predetermined by a manufacturer or by reference to a scale or similar - about the axis 21 so that the vector of the moment opposes the gravitational effect of the centres of gravity 35 and 45 of the blades 30 and 40 to an extent, which is readily determined by setting or adjusting the weight to provide the required vector, sufficient to cause the paddle to adopt an equilibrium state in any rotational orientation of the paddle about the shaft longitudinal axis when, as a reference, the axis is horizontal. The user can therefore maintain a desired blade use orientation, in particular with the water entry edges 33 and 43 facing downwardly, without having to counteract an imbalance force lending to rotate the blades out of their use orientations. The weight 50 is thus a body or other element positioned on the shaft 20, but with its centre of gravity eccentric to the shaft axis 21. Such a weight can be provided at the paddle at the outset, thus at the time of manufacture, or be applied subsequent to manufacture and in that case, for example, a retrospective fit to an existing paddle. If the weight is provided at the time of manufacture it can be formed to be partly or wholly integrated in the shaft 20. Paddle balancing can then be undertaken in optimal manner by the manufacturer, particularly if the weight is provided in such a way that it is finely adjustable to achieve the appropriate vector. An at least partly integrated weight might, however, adversely affect economic production of the shaft 20, which is typically a tube of wound carbonfibre or other light, but strong synthetic material. A separately constructed weight which is fitted to a finished shaft has the advantage that it can be designed to be adjustable relative to the shaft to achieve the required vector, thus direction and magnitude of the turning movement necessary to achieve downward orientation of the blade entry edges 33 and 43 in the paddle rest state. Fig. 7 shows, as a sectional diagram similar to Figs. 2, 4 and 6, an embodiment of the invention and in addition illustrates a method exemplifying the invention, in which the paddle shaft 20 is equipped centrally between the blades 30, 40 with the eccentrically positioned weight 50 when the paddle is in the orientation depicted in Fig. 6, that is to say with the paddle undesirably rotated about the shaft axis 21 by the eccentric centres of gravity 35 and 45 of the blades 30 and 40 out of the use orientation of Figs. 1 and 2 and into the equilibrium state orientation of Figs. 5 and 6. The weight 50 here comprises a mass 51 attached to the shaft 20 by a projection 52, which in this instance extends approximately tangentially of the shaft so that the combined weight of the mass 51 and projection 52 generate a clockwise turning moment as indicated by the arrow 53. The projection 52 could equally well extend chordally or radially of the shaft, since the spacing of the centre of gravity, which here lies within the mass 51, of the weight 50 from the shaft axis 21 provides the eccentricity required for a balancing rotation of the shaft. In this example, the weight 50 is fitted to the shaft 25 in the equilibrium state shown in Figs. 5 and 6, thus with the blade 30 lying generally parallel to the horizontal plane H. The rotational position of the weight 50 is approximately as shown in Fig. 7. However, the weight is preferably constructed and fitted to the shaft 20 to be adjustable - in or counter to the direction of the arrow 53 - in its rotational position so as to precisely set the vector of the turning moment until the turning moment generated by the centres of gravity 35, 45 of the blades 30, 40 is effectively neutralised and the paddle is balanced so as to adopt an equilibrium or rest state in any rotational orientation about the shaft axis 21. Such an equilibrium state orientation includes the intended use orientation in which the blade water entry edges 33, 43 are directed downwardly, whereby the paddle in the use orientation is relieved of any intrinsic force seeking to shift the paddle out of the use orientation. Setting of the vector can be achieved by simple visual observation of the orientations adopted by the blades 30 and 40 after rotation of the paddle by the weight and adoption by the paddle of an equilibrium state including that of Figs. 1 and 2 rather than solely that of Figs. 5 and 6. Fig. 8 shows a development, in which not only the weight 50 as a whole is adjustable rotationally about the shaft axis 21, but also the mass 51 is adjustable in position on the projection 52 to vary the spacing of the centre of gravity of the mass from the shaft or shaft axis and thus the magnitude of the vector of the turning moment. The lever arm exerted by the weight to turn the shaft can be increased or decreased by increase or decrease, respectively, in that spacing. The dimensioning of the mass 51 and projection 52 are such that the mass 51 can be positioned to achieve the desired balancing of the paddle without being spaced so far from the shaft axis 21 as to interfere with paddle use or excessively increase the paddle profile. The result of application of the weight 50 to the paddle 10 for balancing purposes is illustrated in Fig. 9, which shows a user-selected equilibrium state of the paddle corresponding with a use orientation of the blades 30 and 40 in which both are directed downwardly - water entry edges 33 and 34 lowermost - towards a water surface W. The downwardly directed orientation of the blades 30 and 40 in such a selected equilibrium state means that the user is relieved of expenditure of energy necessary to overcome the tendency of the unbalanced paddle 10 to adopt the equilibrium state of Figs. 5 and 6, that is to say a state in which the blade orientation is at about 90 degrees to a desired use orientation. Fig. 10 illustrates, by way of example, a preferred embodiment of the weight 50 for attaching to a paddle 10 to achieve balancing in accordance with the diagram of Fig. 9. The weight 50 comprises attaching means in the form of a clamp 54 attachable to the shaft 20 of the paddle 10 approximately centrally between the blades 30 and 40 and positionable in a selected rotational position on the shaft, specifically a position producing the corrective turning moment counteracting and neutralising the turning moment produced by the eccentrically located centres of gravity 35, 45 of the paddle blades. When the weight is in the finally selected rotational position, the clamp 54 is then tightened on the shaft 21 by way of a clamping screw 54a of the clamp. The clamp, which embraces the periphery of the shaft, itself makes no or minimal contribution to the balancing action. The weight 50 further comprises the projection 52 here in the form of a rod 55, which has a threaded end portion firmly screwed into the clamp 54, and the mass 51, which is slidably engaged on the rod 55 and can be moved along the rod in either one of two opposite directions. As can be seen in Fig. 10, the rod 55 projects substantially tangentially away from the shaft 10 when the weight 50 is attached thereto and forms, together with the mass 51, the effective part of the weight 50, that is to say the part responsible for the balancing action. By virtue of the extension of the rod 55 away from the shaft 20 and the positioning of the mass 51 on the rod, in particular at a spacing from the shaft, the centre of gravity of the weight, which is primarily the weight of the mass 51 supplemented by that of the rod 55, is located at a spacing from the axis 21 of the shaft so as to be able to generate the turning moment indicated by the arrow 53. The weight 50 can be altered in its rotational position on the shaft 20 by loosening and retightening the clamping screw 54a of the clamp 54 and the mass 51 can be adjusted along the rod 55 until the user, for example by observation, is satisfied that the paddle can assume a state of equilibrium in any selectable rotational orientation about the shaft axis 21, thus is rotationally balanced. During this procedure, the paddle shaft can be supported near its end on suitable supports that allow the paddle to rotate and do not obstruct access to the fitted weight 50. When the paddle 10 has been balanced and the final position of the mass 51 on the rod 55 determined, the former can be locked against movement relative to the latter by a grub screw 56 provided at the mass. The mass itself can be of any desired form, here a cylindrical aluminium alloy body. The weight 50 increases the overall mass of the paddle, but to an acceptable degree set against the benefit gained from balancing the paddle. The procedure for balancing the paddle 10 or a differently shaped double-bladed paddle with paddles having eccentrically located or offset - relative to the shaft longitudinal axis 21 - centres of gravity producing undesired blade orientations in the paddle rest or equilibrium state is self-evident from the foregoing description. With the unbalanced paddle 10 supported in its equilibrium state (corresponding or generally corresponding with Figs. 5 and 6) by its shaft 20 on appropriate supports or rests, the weight 50 is fitted to the shaft, preferably so that the mass 51 is intersected by the vertical plane V, and is adjusted as outlined in the preceding paragraph so that the turning moment exerted by the weight negates the turning moment about the shaft axis 21 generated by the centres of gravity 35 and 45 of the two blades 30 and 40. The paddle 10 is thus balanced in the sense of adopting an equilibrium state in any rotational orientation about the shaft axis. Accordingly, the paddle will be in a state of equilibrium when in an intended use orientation, i.e. an orientation in which the blades 30 and 40 are positioned with the blade entry edges 33 and 43 facing downwardly. As mentioned further above, the paddle 10 can be manufactured at the outset with a wholly or partially integrated balancing weight arranged to achieve the desired balancing effect, although the embodiment of the weight 50 described with reference to and as schematically illustrated in Fig. 10 advantageously provides an attachment which can be easily fitted to new or older paddles and transferred between paddles or to replacement paddles. Fig. 11 is a schematic cross-sectional view of a variant embodiment of a balanced paddle having a tubular shaft 120 connecting together two blades (not shown) corresponding with the blades 30 and 40 in construction and in arrangement relative to the shaft longitudinal axis 21 of the shaft 20. In the variant embodiment, the paddle is equipped with a weight 150 at the time of manufacture. For this purpose the shaft 120, which is tubular and has an axis 121, is internally provided with a short cylindrical aluminium bush 122 with a diametrally extending threaded bore having a predetermined orientation relative to the paddle blades and communicating with an opening in the tube wall of the shaft 120. The bush 122 is fixed in the shaft 120 to be secure against axial movement and rotation relative thereto. A projection in the form of a threaded rod 155 is threaded into the bore to be firmly, but removably seated in the bush and carries a mass 151 in the form of a cylindrical body, the rod 155 being threadedly engaged in an eccentric bore (not shown) in the body. The mass can be adjusted along the rod 155 in opposite directions by screwing. In addition, the centre of gravity (marked by a cross) of the mass can be varied in its rotational position about the shaft by rotation of the mass on the rod; rotation of the mass through 180 degrees moves the mass between the limits of its rotational adjustment range. In Fig. 11, the mass is shown by solid lines in one of the rotational limit positions and by dashed lines in the other one of those positions, the mass being fixable in those and any other such positions by fixing means (not shown). In addition, the solid-line and dashed-line depictions show the mass 151 into two different selectable and fixable positions along the rod 155. The partly integrated weight 150 of the Fig. 11 embodiment thus offers adjustments similar to those offered by the weight 50 of Fig. 10. In the case of Fig. 11, however, the orientation of the threaded bore in the bush 122 as predetermined by the paddle manufacturer establishes at least a coarse rotational position of the weight. Adjustment by way of rotational and / or linear movement of the mass 151 on the rod 155 allows fine change in that position, if such should be desirable for precise setting. If expedient to do so, the weight, i.e. rod 155 and mass 151, can be supplied separately from the paddle 100 and fitted by the user. The construction shown in Fig. 11 is merely an example of a partly integrated weight for balancing purposes and other constructions achieving the desired result are equally usable. Similarly, it will be readily apparent that the construction of the applied weight 50 of Fig. 10 can be changed in various ways without departing from the object of exerting a desired turning moment on the paddle shaft by eccentric positioning of the weight.

Claims

1. A method of balancing a boat paddle having two blades which are connected together by a shaft and which each have a centre of gravity offset from a longitudinal axis of the shaft in a direction away from a water entry edge of that blade, the method comprising the step ofequipping the shaft between the blades with a weight acting eccentrically with respect to the shaft longitudinal axis to exert on the paddle when the axis is horizontal a turning moment about that axis,the weight being positioned in a rotational position with respect to the shaft longitudinal axis so that the turning moment acts in opposition to the gravitational effect of the centres of gravity of the blades to allow the paddle to assume a state of equilibrium in any rotational orientation of the paddle about the shaft longitudinal axis when the axis is horizontal.

2. A method according to claim 1, wherein the blades are angled relative to one another so that the centres of gravity thereof are offset from the shaft longitudinal axis in respectively different directions, the weight being positioned to exert a turning moment opposing the mean gravitational effect of the centres of gravity.

3. A method according to claim 1 or claim 2, wherein the shaft is equipped with the weight substantially centrally between the blades.

4. A method according to any one of the preceding claims, wherein the step of equipping comprises rotating the weight about the shaft longitudinal axis to influence the turning moment.

5. A method according to any one of the preceding claims, wherein the weight comprises a mass adjustable towards and away from the shaft longitudinal axis and the method comprises adjusting the mass to vary a spacing of the centre of gravity of the mass from the shaft longitudinal axis and thereby the exerted turning moment.6.A method according to any one of the preceding claims, wherein the step ofequipping the shaft with the weight comprises attaching the weight to the shaft of an already manufactured paddle.

7. A method according to claim 6, wherein the weight is removably attached to the shaft.

8. A method according to any one of claims 1 to 5, wherein the step of equipping the weight is carried out at least in part during manufacture of the paddle.

9. A method according to claim 8, wherein at least part of the weight is formed on the shaft.

10. A boat paddle balanced by a method as claimed in any one of the preceding claims, the paddle comprising a shaft and two blades connected together by the shaft and each having a centre gravity offset from a longitudinal axis of the shaft in a direction away from a water entry edge of that blade, wherein the shaft is equipped between the blades with a weight acting eccentrically with respect to the shaft longitudinal axis to exert on the paddle when the axis is horizontal a turning moment about that axis, the weight being positioned in a rotational position with respect to the shaft longitudinal axis so that the turning moment acts in opposition to the gravitational effect of the centres of gravity of the blades to allow the paddle to assume a state of equilibrium in any rotational orientation of the paddle about the shaft longitudinal axis when the axis is horizontal.

11. A paddle according to claim 10, wherein the blades are angled relative to one another so that the centres of gravity thereof are offset from the shaft longitudinal axis in respectively different directions, the weight being positioned to exert a turning moment opposing the mean gravitational effect of the centres of gravity.

12. A paddle according to claim 10 or claim 11, wherein each of the blades defines an elongate scoop having its water entry edge at a longitudinal edge of the scoop and its centre of gravity offset from the shaft longitudinal axis in a direction away from that longitudinal edge.

13. A paddle according to any one of claims 10 to 12, wherein the shaft is equipped with the weight substantially centrally between the blades.21 09 2314. A paddle according to any one of claims 10 to 13, wherein the weight is rotatable about the shaft longitudinal axis to influence the turning moment.

15. A paddle according to any one of claims 10 to 14, wherein the weight comprises a mass adjustable towards and away from the shaft longitudinal axis to vary a spacing of the centre of gravity of the mass from the shaft longitudinal axis and thereby the exerted turning moment.

16. A paddle according to claim 15, wherein the weight comprises a projection extending away from the shaft and the mass is positionable on the projection at a selectable spacing from the shaft, the mass being fixable to the projection at that spacing.

17. A paddle according to any one of claims 10 to 15, wherein the weight is removably attached to the shaft.

18. A paddle according to any one of claims 10 to 16, wherein at least part of the weight is formed on the shaft.

19. A weight adapted for balancing a boat paddle by a method as claimed in any one of claims 1 to 10, the paddle having two blades which are connected together by a shaft and which each have a centre of gravity offset from a longitudinal axis of the shaft in the direction away from a water entry edge of that blade, wherein the weight is attachable to the paddle shaft to act eccentrically with respect to the shaft longitudinal axis to exert on the paddle when the axis is horizontal a turning moment about that axis and wherein the weight comprises attaching means attachable to the shaft, a projection carried by the attaching means and arranged to project away from the shaft when the attaching means is attached to the shaft and a mass carried by the projection to be at a spacing from the shaft when the attaching means is attached thereto.

20. A weight according to claim 19, wherein the mass is variably positionable on the projection so as to be at a selectable spacing from the shaft and is fixable to the projection at that spacing.

21. A weight according to claim 20, wherein the mass is slidably engaged with the projection and positionable by sliding along the projection to be at the selectable spacing.

22. A weight according to claim 20, wherein the mass is threadedly engaged with the projection and positionable by screwing along the projection to be at the selectable spacing.

23. A weight according to any one of claims 19 to 22, wherein the attaching means comprises a releasable clamp tightenable around the shaft.

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