WO1999058397A1

WO1999058397A1 - Rowing oar

Info

Publication number: WO1999058397A1
Application number: PCT/AU1999/000337
Authority: WO
Inventors: John Young Harrison; Stuart Andrew Maxwell
Original assignee: Bishop Innovation Limited
Priority date: 1998-05-08
Filing date: 1999-05-07
Publication date: 1999-11-18

Abstract

An oar for rowing a boat, the oar comprising a shaft with a longitudinal axis and a blade fixed at one end of the shaft. In use the blade is adapted to engage water in a substantially vertically aligned thrust position. The geometry of the blade in the thrust position comprises at least one horizontal cross sectional form, which is an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extend of its chord line. The chord line is substantially angularly offset from the longitudinal axis of the shaft. The aerofoil section is defined by an upper and lower surface, where the curvature of the upper surface is greater than the lower surface. The longitudinal axis of the shaft intersects the lower surface of the aerofoil section.

Description

Rowing Oar

Field of Invention

This invention relates to the design, from the hydrodynamic aspect rather than the structural aspect, of rowing oars, with emphasis on oars used in racing rowing boats or shells.

Background

Rowing and paddling have been used over centuries to propel various classes of water craft, making use only of human muscle power. Each of these activities require the use of one or more "paddles" in the case of canoeing or kayaking and two or more "oars" in the case of rowing. Paddles may be single ended or single bladed, as used in the Canadian canoeing style, or double ended as used in kayaks. Racing oars are generally referred to as "sculling oars" or "sculls" when the rower uses two oars, one on either side of the boat, and controls each with one hand; they are referred to as sweep oars or "sweeps" when the rower controls only one oar, using both hands. Clearly to preserve a reasonable degree of directional stability an even number of rowers must be boated when using sweep oars, the conventional numbers being two, four, and eight. In sculling the conventional numbers are one, two and four.

Single ended paddles and oars have much in common as far as shape is concerned; each consists of an essentially straight shaft or loom at one end of which is a handle and at the other end of which is a blade. These three elements can loosely be considered to lie in a single plane. The length of the blade (measured parallel to the shaft axis) is almost universally greater than its width, generally several times greater, and the width is many times greater than its thickness, typically more than ten times. The blade areas of paddles and sculling oars are not greatly different, despite the very different techniques employed in their use.

In paddling the paddler sits or kneels in the craft, facing forwards, and the only forces on the paddle are the water reaction force on the blade, and the paddler's hand force on the handle. He thus has complete control over the motion of the paddle, generally aiming to keep the blade as vertical as possible, and at right angles to the craft longitudinal axis, and close to the side of the craft to minimise directional deviation. With this technique the blade acts as a "drag device", the relative motion between water and blade being effectively normal to the blade surface. This motion, while creating force on the blade, causes much turbulence and energy loss.

Though this traditional technique is still in vogue, a modified, more energy consuming technique, was developed during the mid 1980's in which the paddle was urged outwards as well as rearward causing the paddle to act as a hydrofoil or wing, enhancing its effectiveness.

In contrast to the paddler, who faces forwards, the rower sits facing the stern of the boat or craft. Also the oar is supported in a "rowlock" at a point along the oar shaft about one quarter to one third of the overall length of the oar from the handle. The rowlock, which is located on the craft's gunwale or on a "rigger" which is a lateral extension of the gunwale, allows rotation of the oar about a vertical axis, rotation about a horizontal axis normal to the longitudinal shaft axis, and rotation about the longitudinal shaft axis. These axes intersect effectively at the centre of the rowlock, at which point no translational motion of the shaft, relative to the craft, can occur. The kinematics of the oar motion is thus more restricted than it is in the case of the paddle.

The complete rowing cycle is termed the "stroke" which consists of a "power phase" and a "recovery". The power phase is initiated by the "catch" when the blade is accelerated and dropped quickly into the water, force then continually being applied with the blade in the "thrust position" until the "finish", where the blade is extracted, and the recovery begins. Immediately after extraction the blade is "feathered" i.e. placed almost parallel to the water, and the recovery continues until the catch position is nearly reached when the blade is "squared" i.e. turned normal to the water surface, preparatory to the next catch.

Typically, the total angular rotation of the oar during the power phase, about a vertical axis, is about 90° to 100°; 60° from the catch to where the shaft is at 90° to the craft longitudinal axis, i.e. at the "square-off" position, and 30° to 40° beyond this square- off position to the finish. These are present day figures for competitive rowing, with boats fitted with sliding seats. The figures would be less prior to the introduction of sliding seats.

Before rowing as a sport became seriously competitive, the crafts used were comparatively slow and heavy, and for the major portion of the stroke the relative motion between the water and the blade was effectively normal to the blade and the blade acted as a "drag device" as has already been described in relation to the paddle blade behaviour.

The force exerted on a flat plate when immersed in water and moved in a direction normal to the plate surface is not greatly influenced by the shape of the plate, but is dependant on its area.

Accordingly, early blade shapes were only required to have adequate area rather than any particular shape. Since, in the early days, oars were shaped from a single piece of wood, it would have been wasteful to make oars with other than narrow, but long, blades.

However, as boat speeds increased along with the total angular excursion of the oar (about a vertical axis), the flow pattern associated with the blade changed quite markedly. The water velocity relative to the blade became no longer predominantly normal, this normal flow occurring only in the region of the power phase where the oar was close to the square-off position. Earlier in the power phase of the stroke, especially immediately after the catch, the water velocity was at quite a small angle to the blade surface. To borrow from naval architecture or aeronautical engineering technology, the "angle of attack" varied from only a few degrees immediately after the catch rising to 90° at the square-off position of the oar.

As a consequence of this type of flow, the blade behaves as a crude hydrofoil or wing in the early part of the stroke, the flow stalling with the advent of excessive angle of attack, effectively relegating the blade to being no more than a drag device. A flat plate or flat blade is not nearly as effective as a cambered or curved one for producing large amounts of "lift", the camber causing the blade to behave much more like a conventional aircraft wing. (In the case of a blade the "wing" lies in a substantially vertical rather than a horizontal plane.)

Camber consequently found its way into oars, culminating in the "Macon" blade design which is still used in competitive rowing today. To some extent this philosophy has also found its way into paddles. No oar manufacturers seem to have developed the wing concept further, relying on a fairly uniform thickness blade, with varying amounts of camber.

However over the last 50 years oar blades have gradually increased in width, and got shorter in length. About 50 years ago the blade aspect ratio (that is blade width divided by blade length) was approximately 0.2, and is approximately 0.3 on the latest Macon blade designs. Such Macon blades are symmetrical about the longitudinal shaft axis, measured in the general plane of the blade. However, in late 1991 Concept II introduced the "Big Blade" which was asymmetrical and had an aspect ratio of approximately 0.5. The blade was designed so that there was more area below than above the shaft axis when the oar blade was immersed in the thrust position during the power phase. The reason for doing this was to reduce the immersion of the shaft, with its consequent drag during the power phase, which immersion increased for increasing blade width.

In 1985 East German Patent 216 907 (Buchmann and Kuhnhard) is disclosed a concept for a bent shaft, the bend being such that a straight line from the handle cross section, extended through the shaft cross section at the rowlock, would pass through the centre of pressure of the blade. At the same time this design allows the junction of the shaft and blade to be close to the top edge of the blade. However bent shafts have been discouraged by the world rowing body FISA, presumably on the grounds of the additional manufacturing cost of shafts and resulting increased price of oars.

Also in 1985, German Patent 35 34 466 (Fleming) disclosed a "rowing apparatus" claiming to improve performance over current levels, its novelty residing in two main features: Firstly, a unique rowlock assembly which restricted the motion of the oar shaft, particularly in that the oar could not rotate about its longitudinal axis (thereby preventing feathering of the blade), and in that the rower could not slide the oar lengthwise in the rowlock; and secondly, a blade having a high aspect ratio.

Since the rowlock proposed in this prior art patent could itself resist any torque about the oar shaft longitudinal axis generated by the water force on the blade, the shaft could be straight and mounted at the uppermost edge of the blade, without the need for the rower to apply any resistive torque at the oar handle. Such a rowlock would be non-standard and expensive, and would not be compatible with current designs of oars. The action of the new oar associated with this rowlock (ie. non-feathering) would no doubt be considered counter to the general spirit of the traditional rowing action. The patent specification dismisses the additional air resistance on the non-feathering blades as insignificant, but at present boat speeds and oar blade areas, this resistance can be shown to be as much as 20% of the water resistance of the hull. From this point of view the invention would not appear to improve performance over current levels. The second main feature concerning a high aspect ratio oar blade will be enlarged upon later.

Though long ago the blades of oars and paddles were both effectively drag devices, this similarity gradually diminished as oar blades became foil-like in their action, until the advent in the 1980's of the modified paddling technique described earlier. The kinds of water flow regimes associated with both paddle blades and oar blades then became almost identical.

Many paddlers were quick to appreciate the benefits accruing from an aircraft wing section as opposed to a cambered flat plate section, and to realise such benefits applied equally to oars.

US Patent 4,737,126 (Lindeberg et al.) discloses such a winged blade section, and the inventors state that their "invention is not restricted to canoe paddles, but can also be applied in conjunction with oars". The modified paddling technique is well described in this patent, along with a discussion of some test results. Australian Patent Application 26211/92 (Jackson) discloses a Macon style blade of part aerofoil cross section, and states that the "invention relates to paddles, which expression includes oars and similar propulsion devices."

Netherlands Patent 9100967 (Gelling et al.) discloses a blade for oars, the blade having an aerodynamic profile in longitudinal cross sections parallel to the shaft axis and normal to the general plane of the blade. One embodiment includes a "slot" at the leading edge of the blade, as found in the leading edge of wings in some aircraft. In aircraft so equipped it is usual to have the most forward wing portion retractable rearwards to close the slot, returning the wing cross section to a more conventional shape. The use of slots in aircraft wings is to provide greater lift when taking off or landing at low speeds. In general however, the "drag", or resistance to forward motion, is increased, and in the case of an oar blade, it is possible that the lift may be increased, but at the same time the efficiency is not.

If this is so, any gains which might be expected become illusory. In the three prior art patents cited, the novelty resides in the use of a true or a part aerofoil shape, rather than a cambered flat plate. In the associated descriptions and drawings, it appears that the claims in all three cases are restricted to prismatic shapes, and in no case has the possibility of a shape with continuously varying cross sections, or a shape with a twist in it, either aerodynamic or geometric, been exploited. Such characteristics along with other configuration arrangements can be used to increase the "efficiency" of oars.

Returning now back to the second main feature of German Patent 35 34466 (Fleming et al.), it is clear that there is an awareness of the benefits accruing from increasing the aspect ratio of blades well above those values in current use, and the employment of non-prismatic blade shapes. However this prior art patent does not appear to consider any additional benefits to be achieved by utilising a more effective blade section than a circular arc or an ovoid, nor does it attempt to quantify the sort of performance increments to be anticipated from the use of such a rowing apparatus.

Up to the present time, the design of oar blade shapes (including Macon and Big Blades) has been much more subjective rather than quantitative, and the emphasis has been on achieving high loads on the blade, rather than concentrating on the

"efficiency" of the blade and oar combination, which requires drag forces as well as lift forces to be taken into account. The mechanical efficiency of the oar is paramount, as it determines what percentage of the rower's power input to the oar handle is used for propelling the boat.

There also seems to have been little effort put into investigating the effect of the "aspect ratio" of the blade or the effect of the degree to which the blade "chord line" (the line joining the leading and trailing edges of an aerofoil section or a cambered section) is angularly offset to the longitudinal axis of the shaft. Such an angular offset is disclosed in East German Patent Application No. 148207 (Elschner et al.) for a conventional Macon oar blade with a low aspect ratio.

Aim of Invention

The present invention seeks to improve the efficiency of oars over that currently obtainable. A quantitative methodology is taught for determining the three dimensional shape of the blade and blade shaft interface according to the present invention which takes into account all the pertinent variables governing blade efficiency, including aspect ratio and the angular inclination of its chord line of the blade cross section from the shaft longitudinal axis.

At the same time it is necessary to recognise the imperative that the oar must be ergonomically acceptable, otherwise the maximum possible power cannot be delivered by the rower. To this end, according to the present invention, solutions to potential practical problems will be also be addressed, for example those arising from excessive flotation of the blade when in the thrust position, excessive width of the blade (making the catch and finish of the power phase "untidy"), and excessive angular inclination of the chord line of the blade cross section from the shaft longitudinal axis (making the recovery awkward). Summary of Invention

In a first aspect, the present invention consists in an oar for rowing a boat, the oar comprising a shaft with a longitudinal axis and a blade fixed at one end of the shaft, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, characterised in that the at least one horizontal cross sectional form is an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the chord line being substantially angularly offset from the longitudinal axis of the shaft, the aerofoil section defined by an upper and lower surface, the curvature of the upper surface being greater than the iower surface, the longitudinal axis of the shaft intersecting the chord line of the aerofoil section.

It is preferred that the upper surface is convex and the lower surface is concave.

It is preferred that a tangent to the camber line is substantially angularly offset from the longitudinal axis of the shaft at the extremity of the camber line remote from the shaft.

It is preferred that the aspect ratio of the blade is greater than 0.7.

It is preferred that the aspect ratio of the blade is greater than 1.0.

It is preferred that the blade geometry comprises at least two aerofoil sections, the angular offset of the chord lines of the at least two aerofoil sections being different, resulting in the blade being twisted in shape.

It is preferred that the blade geometry comprises at least two aerofoil sections, the thickness profile of at least two aerofoil sections being different, resulting in the blade being non-prismatic. In one embodiment it is preferred that the shaft is substantially circular in cross section and the longitudinal axis of the shaft passes through the centre of each circular shaft cross section.

In another embodiment it is preferred that the shaft is non-circular in at least one cross section and the longitudinal axis of the shaft passes through the centroid of the at least one non-circular cross section.

It is preferred that the at least one non-circular cross section of the shaft is an elliptical or tear-drop shaped aerofoil section.

In a further embodiment it is preferred that the longitudinal axis of the shaft is straight when the shaft is unloaded.

In yet another embodiment it is preferred that the longitudinal axis of the shaft is curved when the shaft is unloaded.

In yet another embodiment it is preferred that the longitudinal axis of the shaft comprises two substantially straight portions which are mutually angularly offset when the shaft is unloaded.

It is preferred that the oar further comprises a handle at the opposite end of the shaft to the blade.

In yet another embodiment it is preferred that the longitudinal axis of the shaft is curved in an S-shape over a portion of its length adjacent to the handle when the shaft is unloaded.

It is preferred that the centroid of a cross section of the handle is offset from the longitudinal axis of the shaft in the vicinity of the handle.

It is preferred that the offset is a substantially vertical offset when the blade is in the thrust position. It is preferred that the cross section of the handle is substantially circular and has a centre corresponding to the centroid.

It is preferred that a sleeve surrounds the shaft in the region of the shaft between the blade and the handle, the sleeve comprising a substantially vertical reaction surface when the blade is in the thrust position, and a straight line extending from the top surface of the handle to the centre of pressure of the blade in the thrust position lies substantially within the vertical extent of the vertical reaction surface of the sleeve.

It is preferred that the chord line of the at least one aerofoil section is angularly offset from the longitudinal axis of the shaft by an angle greater than 10 degrees.

It is preferred that the chord line of the at least one aerofoil section is angularly offset from the longitudinal axis of the shaft by an angle greater than 15 degrees.

It is preferred that the shaft is faired into the lower surface of the aerofoil section of the blade, adjacent to the top edge of the blade when the blade is in the thrust position.

It is preferred that the longitudinal axis of the shaft intersects the chord line of the at least one aerofoil section at an intersection point which has a distance of greater than one quarter of the length of the chord line from the leading edge of the aerofoil section.

It is preferred that the longitudinal axis of the shaft intersects the chord line of the at least one aerofoil section at an intersection point which has a distance of greater than one third of the length of the chord line from the leading edge of the aerofoil section.

It is preferred that the blade and shaft material comprises of at least one of carbon fibre, boron fibre, fibre glass, plastic or timber.

It is preferred that the handle material comprises of at least one of carbon fibre, boron fibre, fibre glass, plastic or timber. In a second aspect, the present invention consists in a blade for an oar for rowing a boat, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, characterised in that the at least one horizontal cross sectional form is an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the aerofoil section defined by an upper and lower surface, and the curvature of the upper surface being greater than the lower surface.

It is preferred that the blade is adapted for fixing to one end of a shaft, such that the chord line is substantially angularly offset from the longitudinal axis of the shaft, and the longitudinal axis of the shaft intersects the chord line of the aerofoil section.

It is preferred that a tangent to the curved camber line is substantially angularly offset from the longitudinal axis of the shaft at the extremity of the curved camber line remote from the shaft.

It is preferred that the aspect ratio of the blade is greater than 0.7.

It is preferred that the aspect ratio of the blade is greater than 1.0.

It is preferred that the blade geometry comprises of at least two aerofoil sections, the thickness profiles of the at least two aerofoil sections being different, resulting in the blade being non-prismatic.

In one embodiment it is preferred that the shaft is substantially circular in cross section and the longitudinal axis of the shaft passes through the centre of each circular shaft cross section. In another embodiment it is preferred that the shaft is non-circular in at least one cross section and the longitudinal axis of the shaft passes through the centroid of the at least one non-circular cross section.

It is preferred that the blade material comprises of at least one of carbon fibre, boron fibre, fibre glass, plastic or timber.

In a third aspect the present invention consists in an oar for rowing a boat, the oar comprising a shaft with a longitudinal axis, a blade and a handle, the blade fixed at one end of the shaft and the handle fixed at the opposite end of the shaft, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, the at least one horizontal cross sectional form being an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the chord line being substantially angularly offset from the longitudinal axis of the shaft, the aerofoil section defined by an upper and lower surface, the curvature of the upper surface being greater than the lower surface, a sleeve surrounding the shaft in the region of the shaft between the blade and the handle, the sleeve comprising a substantially vertical reaction surface when the blade is in the thrust position, characterised in that a straight line extending from the top surface of the handle to the centre of pressure of the blade in the thrust position lies substantially within the vertical extent of the vertical reaction surface of the sleeve.

In a fourth aspect the present invention consists in an oar for rowing a boat, the oar comprising a shaft with a longitudinal axis, a blade and a handle, the blade fixed at one end of the shaft and the handle fixed at the opposite end of the shaft, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, the at least one horizontal cross sectional form being an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the chord line being substantially angularly offset from the longitudinal axis of the shaft, the aerofoil section defined by an upper and lower surface, the curvature of the upper surface being greater than the lower surface, characterised in that the longitudinal axis of the shaft is curved in an S-shape over a portion of its length adjacent to the handle when the shaft is unloaded.

Brief Description of Drawings

The present invention will now be described by way of example with reference to the following drawings.

Fig. 1 shows a representative aerofoil cross section, along with its mean line or camber line, its chord line, the lift force and drag force directions at a representative angle of attack α'; Fig. 2 is a representative graph showing the variation of lift coefficient CL with angle of attack α'. This graph also shows the variation of the ratio CD/CL with angle of attack α'. C_D being the drag coefficient. These graphs are representative for a blade with an aspect ratio <1 ;

Fig. 3 is a similar graph to that in Fig. 2, but representative for a blade of aspect ratio

>1 , the ratio being about three times greater than that of the blade represented in Fig.

2;

Fig. 4 is a "configuration diagram" showing the disposition of the longitudinal oar shaft axis relative to the boat longitudinal axis, the angular disposition of the blade chord line direction with respect to the longitudinal oar shaft axis, and the direction of water flow relative to the chord line of the blade. The acute angle between these latter directions is the angle of attack α'. The lengths of the lines in this figure are arbitrary;

Fig. 5 is a "velocity vector diagram" corresponding to the configuration diagram in Fig.

4. It shows to some unspecified scale, the velocity vectors of the rowlock and of the junction between the chord line and the shaft axis relative to a stationary point on the water surface. It also shows the velocity of the aforementioned junction relative to the rowlock;

Fig. 6 shows the path taken by a point at the mid length of a blade relative to a stationary point on the water surface, during the power phase of the stroke. The directions of the oar shaft at the catch and the finish are shown as well as the direction in which the boat is moving.

Fig. 7 shows the oar in side elevation when in the thrust position according to the present invention;

Fig. 8 shows the oar in Fig. 7 in plan view;

Fig. 9 shows an enlarged view of the blade, in elevation and plan, along with representative cross sections;

Figs. 10 - 11 show two possible arrangements of the junction of the handle and the shaft;

Fig. 12 shows a second embodiment of the oar where the shaft longitudinal axis is curved over most of its length;

Fig. 13 shows a third embodiment of the oar where the shaft longitudinal axis is curved over a portion of its length adjacent to the handle;

Fig. 14 shows a fourth embodiment of the oar where the shaft longitudinal axis comprises two substantially straight portions which are mutually angularly offset; and Fig. 15 shows an enlarged view of an alternative embodiment of the blade, in elevation and plan, along with representative cross sections.

Mode of Carrying Out Invention

In Fig. 1 the typical aerofoil section shown is characterised by curved mean line or camber line 1 , its leading edge 2, trailing edge 3, lower surface 4 (which may be convex or concave according to aeronautical engineering practice), upper surface 5 and chord line 6. Camber line 1 and chord line 6 both extend between leading edge 2 and trailing edge 3. The direction of fluid flow relative to the section is indicated by arrowed line 7, and the angle of attack α' is the acute angle between line 7 and chord line 6. Lift force 8 and drag force 9 are respectively normal to and parallel to line 7 and intersect at centre of pressure 10.

Lower and upper surfaces 4 and 5 are so disposed that at any point on the camber line they are equidistant from it. Since camber line 1 is curved in an upwardly convex fashion, the curvature of upper surface 5 is greater than the curvature of lower surface 4. Moreover upper surface 5 is convex and lower surface 4 is concave.

The amount of camber is generally expressed as a percentage of the chord length, and is the maximum deviation of the camber line from the chord line. Similarly maximum wing thickness is generally expressed as a percentage of chord length, and is referred to as the "wing thickness ratio".

If the magnitude of the lift force is denoted by FL and of the drag force by F_D, the density of the fluid by p, the relative velocity by V, and the wing area by A, then F = CL x Vz pAV² and F_D= Co x ^Λh pAV² , where CL and Co are experimentally obtained coefficients which vary with the angle of attack α' and with the "aspect ratio". For a prismatic wing of rectangular planform the aspect ratio is simply the wing length divided by its chord length. For other more complex planforms it is the wing length squared divided by the planform area. In the case of the oar blade the blade width corresponds to wing length, and the blade length to the wing width or chord length. Thus a prismatic blade of 200mm width and 400mm length has an aspect ratio of 0.5.

It is clear that the ratio F_D/F_L simply reduces to the ratio CD CL.. The way in which both C and CD/C vary with angle of attack α' is shown in Figs. 2 and 3, curve 11 representing the C_L variation with α', and curve 30 representing the CD/C variation with α'.

Both sets of curves are based on an existing well known aerofoil section having a 4% camber and a wing thickness ratio of 15%. However Fig. 2 corresponds to a wing with an aspect ratio of 0.5, which matches very closely with that in current Big Blade oar designs. The curves in Fig. 3 correspond to a wing with an aspect ratio of 1.5.

Since the load or force on a blade with a given area, aspect ratio, and velocity, fixes the required value of C_L, this in turn fixes the angle of attack α\ For any given angle of attack for such a blade there exists a particular value of the ratio CD/CL, and it is found that the smaller this value, the more efficient is the blade and, accordingly, the oar of which it forms part.

It can be seen from Figs. 2 and 3 that for a given value of CL the value of the ratio C_D/C_L is much smaller in Fig. 3 than in Fig. 2. Hence a blade with a relatively high aspect ratio will be more efficient than one with a relatively low aspect ratio, other things being equal.

In the configuration diagram shown in Fig. 4 point 13 is the rowlock and arrowed line 14 is the direction of motion of both boat and rowlock relative to a stationary point in or on the surface of the water. Line 6 is the chord line or a line parallel to the chord line of the blade passing through centre of pressure 23. It is convenient for the purposes of this analysis to assume that the junction of the shaft and blade is such that longitudinal axis 12 passes through centre of pressure 23. During the power phase of the stroke, the distance from the leading edge of the blade to the centre of pressure may vary, but not to any significant extent. The chord line as shown in the diagram is angularly disposed to shaft longitudinal axis 12 by the amount α. Arrowed lines 15 and 16 are the directions of the lift force F_L and of the drag force F_D respectively, both of which pass through centre of pressure 23. Since the drag force is parallel to the water flow direction relative to the chord line, it follows that the angle α' between lines 6 and 16 is the angle of attack pertaining for this particular configuration. Lift force direction line 15 is perpendicular to line 16.

Since line 14 gives the direction of the boat motion, it is parallel to the boat's longitudinal axis and subtends angle θ with oar shaft longitudinal axis 12. The acute angle between direction 14 and arrowed line 16 (showing the direction of water flow relative to chord line 6) is denoted by φ as shown.

In the vector diagram shown in Fig. 5 point 17 is a point of zero velocity and line 17- 31 represents, to some scale, the velocity vector of rowlock 13. The line 31 -32 likewise represents the velocity vector of centre of pressure 23 relative to rowlock 13. The direction from 31 to 32 implies, that at the instant under consideration, angle θ is increasing i.e. the oar shaft axis 12 is moving towards the "square-off" position in the power phase.

Line 17-32 represents the velocity vector of centre of pressure 23 of the blade relative to the fixed body of water. It can be construed as the "velocity of attack" in conjunction with the angle of attack '. Point 32 is established by virtue of the fact that 17-32 is parallel to 16 and 31 -32 is normal to 12, since point 23 is rotating about rowlock 13, and distance 13-23 is fixed.

Line 17-58 represents the velocity vector of a plate blade, with no angular offset relative to the shaft, and moving freely in the water i.e. both α and α' would be zero.

In this case 17-58 in Fig. 5 is parallel to 13-23 in Fig. 4, and angle 31-17-58 is equal to θ. Also angle 32-17-58 is equal to α + α' = β.

Inspection of Fig. 5 shows that by having a large "offset angle" α, the velocity of attack 17-32 is increased since β is simultaneously increased. This in turn means a slightly smaller angle of attack, which leads to better efficiency. Also, as can be seen from Fig. 4, by increasing α the lift force direction 15 becomes more parallel to the boat, reducing the tendency of the oar to "pinch" the boat.

An advantage of quite a separate kind arises with large α values, especially at the catch where values of the angle θ are smallest. The velocity 31-32 of centre of pressure 23 increases with increase in α. Since this velocity is directly related to the angular velocity of the oar about a vertical axis through rowlock 13, the rower does not find the catch so heavy and dead, and the risk of either muscular or skeletal damage is diminished, particularly in the case of young rowers, both male and female.

Apart from the more obvious variables influencing the oar efficiency such as blade profile area, blade load to be sustained, the speed of the boat and the blade surface finish, it is clear from the foregoing that other variables must be taken into account as well, including the aerodynamic shape of the blade, its aspect ratio, and the angular offset α of its chord line from the longitudinal axis of the shaft.

Blade surface finish affects the magnitude of the drag coefficient, and a rough surface finish can increase Co by as much as 10%. It is assumed in what follows, that all blades are smoothly finished, and that in comparisons the surface finish effects are neutral.

By making use of the information contained in Figs. 4 and 5 it can be shown that the efficiency of the oar expressed as the ratio of the power used to propel the boat to the power input to the oar by the rower, is given by:

(sin θ + cos θ tan β) (cos β + μ sin β)

where η = efficiency μ = CD/CL At first glance it appears that in the above equation, several of the variables cited earlier are not represented e.g. blade area, blade load and blade velocity. However these variables have to be utilised to find the value of the angle of attack which angle determines the value of β, and in turn of φ.

Assuming for the present, that a range of blade lift and drag coefficients as a function of α' is available, each for a range of aspect ratios, it is possible to solve the above equation for η, as it is easy to assign numerical values to the remaining variables involved. Boat speeds are well known for the various classes of racing boats.

Blade loads are fairly well known from strain gauge measurements carried out on oars used by rowers in semi racing situations. Such blade force measurements are variable, simply because the rowers vary in their physical make up, and it is in order to use average values.

Blade area can be assumed, as can the amount of chord line angular offset . Finally some particular value of aspect ratio must also be assumed. The efficiency of the oar at various angular positions relative to the boat centre line, indicated by the angle θ, can now be calculated.

A trial and error approach quickly leads to appropriate values of α', β and C_L. From CLand Co graphs appropπate to the aspect ratio assumed, a value of C_D/C_L corresponding to the value of α' just determined can be found i.e. the value of μ.

All quantities in the expression for the efficiency η have now been assigned numerical values, and a value for η follows.

If, for a given oar, this process is performed at say 5° or 10° intervals for θ, starting at the catch position, the variation of efficiency during the stroke becomes established. When θ is in the 70° to 90° range, it is not always found possible to assign any value of α' which leads to a sufficiently large value of CL to meet the blade load requirement. This is due to the very low values of the "velocity of attack" which occur when the oar is approaching the square off position. At this point the blade "stalls" becoming simply a drag device, where blade area, rather than aerodynamic shape is really all that matters.

Calculations show that, for reasonably representative values of the variables in the efficiency equation, the oar is found to be most efficient when the angle θ is about 50°, but the efficiency drops off only a few percentage points in the range 30° < θ < 70°.

It is also found that, other things remaining the same, that variations in the chord line offset angle α are beneficial in the range 10° to 40°. The benefit is found to be a maximum in the range 15° to 30° with an increased efficiency of about 2% over that obtained with α = 0°. A further benefit is that a larger value of α enables the oar to get closer to the square off position before the blade stalls, thereby extending the range of efficient operation.

The effect of increasing the aspect ratio is also significant, the increase in efficiency being about 2% for an increase in aspect ratio from 0.5 to 1.0, and about 3% for an increase from 0.5 to 1.5. In practice it has been found that a measurable improvement in efficiency over current oar blades is achieved for aspect ratios greater than 0.7, with most improvement in efficiency being found for aspect ratios greater than 1.0. These increases in efficiency are due mainly to the smaller values of C_D/C_L associated with the higher values of aspect ratio (refer to Figs. 2 and 3).

The efficiency increases due to large values of α and due to use of relatively high aspect ratio are cumulative, i.e. an oar with a blade of given aerodynamic cross section having an aspect ratio of 0.5 and zero chord line offset angle α can be up to 5% less efficient than one with an aspect ratio of 1.5 and a chord line offset angle α of about 20°. In this comparison blade areas and blade loads are assumed equal. Though the efficiencies vary by up to 5% during the early to middle position of the power phase, after stall the efficiencies will be equal, as blade areas have been assumed equal. For blade areas in use at present, the efficiency at the square-off position is approximately 75%-80%, for both sculling and sweep oars, and maximum efficiency values earlier in the stroke are in the vicinity of 85% again for both sculling and sweep oars. In the calculations so far performed, the aerofoil characteristics used have been based on those of aerofoils with known characteristics, determined in wind tunnel tests, but with the C and CD values transformed to account for each of the differing aspect ratios assumed for the blade.

This logic would be in order if the path of the blade in the water were straight, to mimic the straight line motion of the air in the wind tunnel relative to the aerofoil. However, referring to Fig. 6, it can be seen that the path of the blade is quite curved, particularly near the square off position. The oar shaft in the catch position is denoted by 33 and at the finish by 34. Line 26 is the path of a point situated about the mid length of the blade, and arrowed line 25 shows the path taken by the rowlock during this period.

Any variations in the values of C_L and C_D (based on wind tunnel tests) due to this difference in the motion paths, would have little effect on the relative values of the differing efficiencies cited earlier, but would affect their absolute values.

In order to obtain more relevant values of C_L and C_D for a range of blade shapes, a fully instrumented water tank testing rig has been constructed to permit the determination of C and CD values for any prescribed shape of blade, when moving in a range of circular paths of differing radius.

A further advantage accruing from the experimental determination of the CL and C_D values, is that it automatically takes account of the blade upper edge being very close to the water surface.

In a particular embodiment an oar with a tapered blade with a fair degree of camber, and having a small thickness to chord ratio, an aspect ratio between one and two, and a small amount of twist to reduce vortex shedding may be suitable. Figs. 7 and 8 show an oar with a blade as just described and having a substantial chord line offset angle α relative to shaft longitudinal axis 12. The oar comprises blade 18, shaft 33, sleeve and collar assembly 20 and handle 21. Line 22 represents the water surface, and the relative positions of the blade, the shaft, and the water surface are as might be expected with the oar in the thrust position during the power phase of a stroke. The blade 18 has more camber in regions close to its upper edge 29 than in the region near the lower edge 37, as can be seen in reference to Fig. 9. Sections A-A and B-B of this figure also show that the chord lines 35 and 36 of aerofoil sections A- A and B-B suffer an anticlockwise twist relative to chord line 34. Chord line 34 corresponds to aerofoil section C-C (not shown in a sectional view) adjacent to upper edge 29 of blade 18, but prior to the partial corruption of aerofoil section C-C by the junction of blade 18 with shaft 33. In general the above mentioned twist increases gradually from the upper edge 29 to the lower edge 37 of the blade. In addition to camber along the length of the blade, there is a slight curvature across the width of the blade, the convexity being on the upper surface of the blade. This curvature is evidenced by bulged line 39 in Figs. 8 and 9.

All cross sections of the blade are seen to have different aerofoil shapes, and different chord lengths, and there is a twist in the blade about an axis parallel to the leading edge 2 in Fig. 9. Further, and quite clearly, the blade is non-prismatic and has an aspect ratio of approximately 1.5.

It is important that the amount of camber, together with the magnitude of the thickness to chord ratio in the blade section is sufficient to ensure that a tangent 28 (Fig. 8) at the trailing edge and on the upper surface and parallel to the water surface, appears to approach and intersect the shaft longitudinal axis 12 when extended towards the handle.

If this preferred condition is not met, there is danger of the trailing edge catching and diving in the water during the recovery.

Also from a practical point of view, the blade volume is kept reasonably small, as the buoyancy effect, if excessive, interferes with the comfortable handling of the oar on the rowers part. Since for a given thickness to chord ratio for a blade section, the area varies as the square of the chord length, short length blades reduce this effect.

For a required blade area, if the length is shortened, the width must be increased to compensate, and the aspect ratio increases, which is desirable for high efficiency. It is necessary to preserve a reasonable blade area to maintain adequate efficiency levels when the oar is at or near the "squared off" position, and the blade is acting as a drag device, with blade area almost exclusively determining efficiency.

It can be seen from Figs. 8 and 9 that the junction between shaft 33 and blade 18 occurs on concave lower surface 4 of aerofoil section C-C of blade 18 and adjacent to its upper edge 29 when the blade is in the thrust position. It should be noted that throughout this description the "front face" and "rear face" of the blade in the thrust position are termed respectively the "lower surface" and "upper surface" in accordance with aeronautical engineering terminology. For the rowing oar blade according to the present invention, it has been found that a relatively large degree of blade camber is advantageous, resulting in lower surface 4 being concave and lower surface 5 being convex. Moreover it has been found advantageous if longitudinal axis 12 of shaft 33 intersects chord line 34 of adjacent aerofoil section C-C at an intersection point 19 which has a distance greater than one quarter of the length of chord 34 from leading edge 2 of the aerofoil section. Greatest advantage has been found where intersection point 19 has a distance of greater than one third of the length of chord 34 from leading edge 2. Shaft 33 is "faired" into the lower surface 4 of blade 18 via use of smooth fairing 55, fairing 55 extending towards both leading edge 2 and trailing edge 50. Depending on the location of the junction between shaft 33 and blade 18 in a chord-wise direction and the actual shape of fairing 55, gap 27 may range from being prominent to being totally obliterated .

By locating fairing 55 of shaft 33 and blade 18 as just described, convex upper surface 5 of blade 18, which provides the greater proportion of the total lift, is free of any flow disturbing projections which might otherwise limit the lift obtainable. The shaft, with its location relative to the blade as above, suffers negligible drag due to its almost lack of submergence in the water. Moreover fairing 55 acts to direct water flow under the shaft, rather than over the top of the blade, during the power phase of the stroke, further maximising blade lift. However this junction of shaft 33 and blade 18 is above the centre of pressure 23 of blade 18, and accordingly a torque about longitudinal axis 12 of the shaft is generated, which would normally need to be resisted by the rower. As discussed earlier, East German Patent 216 907 (Buchmann and Kuhnhard) discloses a means of eliminating such a torque by the use of a bent shaft. Such an approach has been discouraged for international competition due to the increased manufacturing cost of the shafts.

In one embodiment of the present invention shaft 33 is straight, but axis 38 of handle 21 is substantially vertically offset relative to shaft longitudinal axis 12 during the power phase, as shown in Figs. 7, 10 and 11. Many different cross sectional shapes of shaft and handle are possible, and one combination of such cross sections is shown in Fig. 10, where both cross sections are non-circular. One preferred cross sectional shape for the shaft is a horizontally aligned ellipical or tear-drop aerofoil section (not shown). However the most preferred embodiment from a manufacturing point of view is shown in Fig. 11 , where both shaft and handle cross sections are circular. Nevertheless, in both figures, the centroidal axis 38 of the handle cross section is substantially vertically offset above the centroidal axis 12 of the shaft cross section when the oar is in the power phase.

Due to the way in which the fingers grip the handle, the resultant hand force is much closer to the handle surface in the region where it is uppermost during the power phase than it is to the handle axis. The sleeve portion of sleeve and collar assembly 20 is D shaped in cross section with the flat reaction surface of the D substantially vertical in the power phase and bearing against a vertical surface in the rowlock gate. Line 24 extended from the top surface 41 of handle 21 to centre of pressure 23 of blade 18 is arranged to lie within the vertical extent of the flat, substantially vertical, reaction surface 42 of sleeve 20, (see Fig. 7), resulting in the blade force at centre of pressure 23, the rowlock force on the reaction surface of the sleeve portion of sleeve and collar assembly 20, and the hand force on the handle being substantially collinear, relieving the need for any torque to be consciously resisted by the rower.

Though the handle offset is relatively small (typically 20 to 50 mm) it is adequate to bring about the desired outcome. This approach does not require shaft 33 to be bent.

Sweep oars and also sculling oars currently vary relatively little in length, such variations as do occur generally being accounted for by choice on the part of the rower, depending either on physical build e.g. whether light or heavyweight, male or female, or on the class of boat, the faster boats tending to have longer oars than the slower ones.

Due to the significant chord angle offset α in the present invention resulting in a reduced hand input force for a given blade load, it is anticipated the optimum length of the oar, from an ergonomic point of view, will be from zero to 8% greater in length than those now in use.

The oar depicted in Figs. 7 and 8 would be for use on the left-hand or port side of the boat. Obviously the oar for use on the right-hand or starboard side of the boat would be manufactured to the opposite hand.

The present invention has been described in reference to oar shafts which have a longitudinal axis 12 which is essentially straight in the elastically undeflected (ie. unloaded) state. As mentioned earlier, the use of curved or bent shafts is discouraged for international competitions, moreover the cost of such shafts is higher than for straight shafts. However it should be realised that application of the present invention is not precluded for the case of shafts with a curved or bent longitudinal axis. If a certain degree of concave-upwards bend in the shaft is employed, the required degree of vertical offset between the shaft and handle axes is reduced and, if a sufficient degree of curvature or bend is employed, this offset could be set to zero or even negative (ie. the axis of the handle is offset a below the axis of the shaft) in certain circumstances.

Fig. 12 shows a second embodiment of the oar according to the present invention where the shaft longitudinal axis has a concave-upwards curvature over most of its length and therefore zero relative offset of the handle centroid is possible from a practical viewpoint. In this second embodiment, just as in the previous embodiment shown in Fig. 7, it is seen that a straight line 43 extending from the top surface 44 of handle 45 to centre of pressure 23 of blade 18 lies substantially within the vertical extent of the vertical reaction surface 46 of sleeve 47. Fig. 13 shows a third embodiment of the oar according to the present invention where shaft longitudinal axis 12 is curved in an S-shape over a portion of its length 51 adjacent to handle 45. Fig.14 shows a fourth embodiment of the oar according to the present invention where the shaft longitudinal axis comprises two substantially straight portions 52 and 53 which are mutually angularly offset by an angle ε (typically 2° to 3°) in a region adjacent to sleeve 47. Again, in both these third and fourth embodiments, it is seen that a straight line 43 extending from the top surface 44 of handle 45 to centre of pressure 23 of blade 18 is arranged to lie substantially within the vertical extent of the vertical reaction surface 46 of sleeve 47. The S-shape of the shaft in the case of the third embodiment and the angular offset (or bend) of the shaft in the case of the fourth embodiment results in zero relative offset of the handle centroid being again possible from a practical viewpoint.

Fig. 15 shows an alternative embodiment of the blade. For clarity, the same reference numerals have been employed as in the earlier embodiment shown in Fig. 9. However, in this embodiment, the aerofoil sections of the blade have a lesser thickness than that shown in Fig. 9. The intersection point 19 between the longitudinal axis 12 of shaft 33 and chord line 34 of aerofoil section D-D of blade 18 is shown to occur much further from the leading edge 2 of the aerofoil section (in this case approximately 85% of the length of chord 34 from leading edge 2). The choice of the position of intersection point 19 on the chord line of the relevant aerofoil section is dependent on many factors relating to the practical "rowability" of the oar and its hydrodynamic efficiency during the power phase of the stroke. In both blade embodiments shown in Figs. 9 and 15, fairing 55 at the junction of shaft 33 and blade 18 (where shaft 33 is faired into lower surface 4 of blade 18) is designed to direct water flow under shaft 33 rather than over the top of the blade during the power phase of the stroke. It can be seen in Fig.15 that fairing 55 commences only about halfway along the blade from leading edge 2. The more "rearward" positioning of the shaft-blade junction means that there is no gap similar to gap 27 as shown in Fig. 9.

It should be obvious to those skilled in the art that numerous variations and modifications could be made to the oar without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. An oar for rowing a boat, the oar comprising a shaft with a longitudinal axis and a blade fixed at one end of the shaft, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, characterised in that the at least one horizontal cross sectional form is an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the chord line being substantially angularly offset from the longitudinal axis of the shaft, the aerofoil section defined by an upper and lower surface, the curvature of the upper surface being greater than the lower surface, the longitudinal axis of the shaft intersecting the chord line of the aerofoil section.

2. An oar as claimed in claim 1 , wherein the upper surface is convex and the lower surface is concave.

3. An oar as claimed in claim 1 , wherein a tangent to the camber line is substantially angularly offset from the longitudinal axis of the shaft at the extremity of the camber line remote from the shaft.

4. An oar as claimed in claim 1 , wherein the aspect ratio of the blade is greater than 0.7.

5. An oar as claimed in claim 1 , wherein the aspect ratio of the blade is greater than 1.0.

6. An oar as claimed in claim 1 , wherein the blade geometry comprises at least two aerofoil sections, the angular offset of the chord lines of the at least two aerofoil sections being different, resulting in the blade being twisted in shape.

7. An oar as claimed in claim 1 , wherein the blade geometry comprises at least two aerofoil sections, the thickness profile of the at least two aerofoil sections being different, resulting in the blade being non-prismatic.

8. An oar as claimed in claim 1 , wherein the shaft is substantially circular in cross section and the longitudinal axis of the shaft passes through the centre of each circular shaft cross section.

9. An oar as claimed in claim 1 , wherein the shaft is non-circular in at least one cross section and the longitudinal axis of the shaft passes through the centroid of the at least one non-circular cross section.

10. An oar as claimed in claim 9, wherein the at least one non-circular cross section of the shaft is an elliptical or tear-drop shaped aerofoil section.

11. An oar as claimed in claim 1 , wherein the longitudinal axis of the shaft is straight when the shaft is unloaded.

12. An oar as claimed in claim 1 , wherein the longitudinal axis of the shaft is curved when the shaft is unloaded.

13. An oar as claimed in claim 1 , wherein the longitudinal axis of the shaft comprises two substantially straight portions which are mutually angularly offset when the shaft is unloaded.

14. An oar as claimed in claim 1 , wherein the oar further comprises a handle at the opposite end of the shaft to the blade.

15. An oar as claimed in claim 14, wherein the longitudinal axis of the shaft is curved in an S-shape over a portion of its length adjacent to the handle when the shaft is unloaded.

16. An oar as claimed in claim 14, wherein the centroid of a cross section of the handle is offset from the longitudinal axis of the shaft in the vicinity of the handle.

17. An oar as claimed in claim 16, wherein the offset is a substantially vertical offset when the blade is in the thrust position.

18. An oar as claimed in claim 16, wherein the cross section of the handle is substantially circular and has a centre corresponding to the centroid.

19. An oar as claimed in claim 14, wherein a sleeve surrounds the shaft in the region of the shaft between the blade and the handle, the sleeve comprising a substantially vertical reaction surface when the blade is in the thrust position, and a straight line extending from the top surface of the handle to the centre of pressure of the blade in the thrust position lies substantially within the vertical extent of the vertical reaction surface of the sleeve.

20. An oar as claimed in claim 1 , wherein the chord line of the at least one aerofoil section is angularly offset from the longitudinal axis of the shaft by an angle greater than 10 degrees.

21. An oar as claimed in claim 1 , wherein the chord line of the at least one aerofoil section is angularly offset from the longitudinal axis of the shaft by an angle greater than 15 degrees.

22. An oar as claimed in claim 1 , wherein the shaft is faired into the lower surface of the aerofoil section of the blade, adjacent to the top edge of the blade when the blade is in the thrust position.

23. An oar as claimed in claim 1 , wherein the longitudinal axis of the shaft intersects the chord line of the at least one aerofoil section at an intersection point which has a distance of greater than one quarter of the length of the chord line from the leading edge of the aerofoil section.

24. An oar as claimed in claim 1 , wherein the longitudinal axis of the shaft intersects the chord line of the at least one aerofoil section at an intersection point which has a distance of greater than one third of the length of the chord line from the leading edge of the aerofoil section.

25. An oar as claimed in claim 1 , wherein the blade and shaft material comprises of at least one of carbon fibre, boron fibre, fibre glass, plastic or timber.

26. An oar as claimed in claim 14, wherein the handle material comprises of at least one of carbon fibre, boron fibre, fibre glass, a plastic or timber.

27. A blade for an oar for rowing a boat, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, characterised in that the at least one horizontal cross sectional form is an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the aerofoil section defined by an upper and lower surface, and the curvature of the upper surface being greater than the lower surface.

28. A blade for an oar as claimed in claim 27, wherein the upper surface is convex and the lower surface is concave.

29. A blade for an oar as claimed in claim 27, wherein the blade is adapted for fixing to one end of a shaft, such that the chord line is substantially angularly offset from the longitudinal axis of the shaft, and the longitudinal axis of the shaft intersects the chord line of the aerofoil section.

30. A blade for an oar as claimed in claim 29, wherein a tangent to the curved camber line is substantially angularly offset from the longitudinal axis of the shaft at the extremity of the curved camber line remote from the shaft.

31. A blade for an oar as claimed in claim 27, wherein the aspect ratio of the blade is greater than 0.7.

32. A blade for an oar as claimed in claim 27, wherein the aspect ratio of the blade is greater than 1.0.

33. A blade for an oar as claimed in claim 29, wherein the blade geometry comprises at least two aerofoil sections, the angular offset of the chord lines of the at least two aerofoil sections being different, resulting in the blade being twisted in shape.

34. A blade for an oar as claimed in claim 27, wherein the blade geometry comprises of at least two aerofoil sections, the thickness profiles of the at least two aerofoil sections being different, resulting in the blade being non-prismatic.

35. A blade for an oar as claimed in claim 29, wherein the shaft is substantially circular in cross section and the longitudinal axis of the shaft passes through the centre of each circular shaft cross section.

36. A blade for an oar as claimed in claim 29, wherein the shaft is non-circular in at least one cross section and the longitudinal axis of the shaft passes through the centroid of the at least one non-circular cross section.

37. A blade for an oar as claimed in claim 36, wherein the at least one non-circular cross section of the shaft is an elliptical or tear-drop shaped aerofoil section.

38. A blade for an oar as claimed in claim 29, wherein the chord line of the at least one aerofoil section is angularly offset from the longitudinal axis of the shaft by an angle greater than 10 degrees.

39. A blade for an oar as claimed in claim 29, wherein the chord line of the at least one aerofoil section is angularly offset from the longitudinal axis of the shaft by an angle greater than 15 degrees.

40. A blade for an oar as claimed in claim 29, wherein the shaft is faired into the lower surface of the aerofoil section of the blade, adjacent to the top edge of the blade when the blade is in the thrust position.

41. A blade for an oar as claimed in claim 29, wherein the longitudinal axis of the shaft intersects the chord line of the at least one aerofoil section at an intersection point which has a distance of greater than one quarter of the length of the chord line from the leading edge of the aerofoil section.

42. A blade for an oar as claimed in claim 29, wherein the longitudinal axis of the shaft intersects the chord line of the at least one aerofoil section at an intersection point which has a distance of greater than one third of the length of the chord line from the leading edge of the aerofoil section.

43. A blade for an oar as claimed in claim 27, wherein the blade material comprises of at least one of carbon fibre, boron fibre, fibre glass, plastic or timber.

44. An oar for rowing a boat, the oar comprising a shaft with a longitudinal axis, a blade and a handle, the blade fixed at one end of the shaft and the handle fixed at the opposite end of the shaft, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, the at least one horizontal cross sectional form being an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the chord line being substantially angularly offset from the longitudinal axis of the shaft, the aerofoil section defined by an upper and lower surface, the curvature of the upper surface being greater than the lower surface, a sleeve surrounding the shaft in the region of the shaft between the blade and the handle, the sleeve comprising a substantially vertical reaction surface when the blade is in the thrust position, characterised in that a straight line extending from the top surface of the handle to the centre of pressure of the blade in the thrust position lies substantially within the vertical extent of the vertical reaction surface of the sleeve.

45. An oar for rowing a boat, the oar comprising a shaft with a longitudinal axis, a blade and a handle, the blade fixed at one end of the shaft and the handle fixed at the opposite end of the shaft, in use the blade adapted to engage water in a substantially vertically aligned thrust position, the geometry of the blade in the thrust position comprising at least one horizontal cross sectional form, the at least one horizontal cross sectional form being an aerofoil section with a curved camber line and a substantially varying thickness profile along the longitudinal extent of its chord line, the chord line being substantially angularly offset from the longitudinal axis of the shaft, the aerofoil section defined by an upper and lower surface, the curvature of the upper surface being greater than the lower surface, characterised in that the longitudinal axis of the shaft is curved in an S-shape over a portion of its length adjacent to the handle when the shaft is unloaded.