EP3497012A1

EP3497012A1 - A propulsion device

Info

Publication number: EP3497012A1
Application number: EP17752443.6A
Authority: EP
Inventors: Bob Sharp; Alexandra Roy; Ronald Price
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-08-08
Filing date: 2017-08-07
Publication date: 2019-06-19
Also published as: US20190168855A1; CA3033214A1; WO2018029451A1

Abstract

A blade (300) for a rowing oar (100), comprises: a first lifting element (310a) having a first chord line (320a) at a first point along its span, the first chord line extending from a trailing edge (340a) to a leading edge (330a) of the first lifting element; a second lifting element (310b) having a second chord line (320b) at a second point along its span, the second chord line extending from a trailing edge (340b) to a leading edge (330b) of the second lifting element; and a slot (370a) through which water may flow when in use, the slot (370a) having a constant width across more than half of the slot's length. The constant width extends between the trailing edge of the second lifting element (340b) and the nearest point on the first lifting element. The first and the second chord lines (340a, 340b) are non-parallel so as to form a first angle (550b) therebetween.

Description

A Propulsion Device

The invention relates to a propulsion device. In particular, but not exclusively, the invention relates to a rowing oar and a blade for a rowing oar.

Rowing oars typically comprise a shaft with a handle at one end of the shaft and a blade at the other end of the shaft. Rowing oars and rowing strokes are different to paddles and paddling strokes. During a rowing stroke, a rower faces the stern of a boat and uses a rowing oar as a lever to move the boat through water. The rowing oar's blade in the water acts as the pivot point, staying approximately stationary in the water, as the boat moves forward.

On close examination of the course of the blade in a rowing stroke it is apparent that, as the boat moves forwards, the blade moves outboard (i.e. away from the boat) as the oar sweeps through an arc up until the oar is perpendicular (i.e. at 90 degrees) to the side of the boat. Subsequently, the blade moves inboard (i.e. towards the boat) as the oar is swept further along the arc past the perpendicular point and towards the back of the boat.

During the stroke, the rowing oar may also move slightly towards the back of the boat as a result a phenomenon called "negative slip".

Figure 1 illustrates the typical path of a rowing blade, with the positive x axis in the direction of the boat's travel, and the positive y axis in a direction perpendicular to the direction of travel(i.e. out board of the boat).

If the oar experienced zero slip, the blade would act as a stationary pivot point and enable the rower to lever the boat forward. The degree of slip experienced by a blade is dependent on the combined effect of the lift and the drag that is generated by the blade in the water, together with the forces imparted on the blade by the boat and the efforts of the rower. Negative slip reduces the forward motion of the boat provided by each stroke. Accordingly, it is preferable to have zero slip in order to provide maximum forward motion in a given stroke. Increasing the lift generated by the blade in the forward direction as it moves through the water reduces slip. In addition, the efficiency of a rowing stroke can be increased by (i) reducing the drag created in the region of stroke where the motion of the blade changes from an outboard direction to an inboard direction, and (ii) increasing the lift to reduce the slip.

The amount of lift and drag generated by a rowing blade depends on the angle of the blade relative to the inflow of the water. For typical blades at shallow angles relative to the inflow of water, a region of higher pressure is created on the aft surface of the blade and a region of lower pressure is created on the forward surface of the blades. This pressure difference generates lift. Any object such as the rowing blade that generates lift by such a pressure difference across its surfaces may be considered a lifting element.

The angle of inflow has been found to vary considerably during the rowing stroke and therefore, at certain points of the rowing stroke, the angle of inflow becomes so large that the flow of water around the blade detaches from the blade of the oar. This effect generates a large amount of drag and almost no lift.

For example, with regard to the typical path of a rowing blade in Figure 1 , it has been found that typical rowing blades generate very little lift in the region after 0.25s compared to the region before 0.25s. Furthermore, in the region between 0.35s and about 0.5s, it is found that the flow has detached from the blade of a typical blade and, as a result, the blade primarily generates drag. Accordingly, it is seen that conventional rowing blades have a varying lift profile in a rowing stroke.

The present invention seeks to address the above issues and provide an improved rowing blade for a rowing oar.

When viewed from a first aspect the invention provides a blade for a rowing oar, the blade comprising:

a first lifting element having a first chord line at a first point along its span, the first chord line extending from a trailing edge to a leading edge of the first lifting element; a second lifting element having a second chord line at a second point along its span, the second chord line extending from a trailing edge to a leading edge of the second lifting element; and

a slot through which water may flow when in use, the slot having a constant width across more than half of the slot's length, the constant width extending between the trailing edge of the second lifting element and the nearest point on the first lifting element,

wherein the first and the second chord lines are non-parallel so as to form a first angle therebetween.

In use, a top surface of each lifting element experiences higher water flow velocity and lower static pressure than an underside surface of each lifting element. It will be appreciated that the pressure difference between these two regions of a lifting element generates lift.

It will be seen by those skilled in the art that in accordance with the invention the blade comprises at least two lifting elements for generating lift. The two lifting elements are positioned such that the chord lines at the first point and the second point (i.e. the first and second chord lines) are non-parallel and form a first angle therebetween. Thus, it will be seen that the second lifting element is angularly offset from the first lifting element and has a different angle of attack to the first lifting element. The angle of attack of the first lifting element is defined by the angle of the first chord line at the first point (e.g. the mid-span chord line of the first lifting element) relative to the direction of oncoming water. Similarly, the angle of attack of the second lifting element is defined by the angle of the second chord line at the second point (e.g. the mid-span chord line of the second lifting element) relative to the direction of oncoming water.

The different angles of attack result in each lifting element generating a different lift profile across the rowing stroke. The interaction between the elements generates a lift greater than the sum of the lift generated by each individual element and the total drag is reduced compared to the sum of the drag from each individual element.

Accordingly, it will be appreciated that the net lift profile of the blade may be tuned by modifying the individual lift profile of each lifting element which, for example, may be done by adjusting the angular offset between the lifting elements. In this way, the angular offset between the lifting elements may be set to provide an improved (e.g. more uniform) lift profile across the rowing stroke than a conventional single piece rowing blade.

Another benefit of angularly offsetting the second lifting element relative to the first lifting element is that it directs (e.g. streamlines) the water flow from the second lifting element to the first lifting element at a more optimal angle (e.g. optimal angle of attack). It will be appreciated that directing the water flow incident on the first lifting element in this way results in more lift. In addition, directing the water flow incident on the first lifting element reduces drag effects that may arise, for example, from the formation of vortices next to the first lifting element.

To reduce the turbulence in the water flow incident on the first lifting element and therefore increase the amount of lift it generates, the blade comprises a slot through which water may flow when in use, wherein the slot has a constant width across more than half of the slot's length the constant width extending between the trailing edge of the second lifting element and the nearest point on the first lifting element. It will be appreciated that the slot provides an outlet that channels the water flow directed from the second lifting element to the first lifting element. It has been found that a slot with a constant width across more than half of the slot's length i.e. for the majority of the length of the slot advantageously reduces the flow separation across the first element which reduces the drag and increases the lift. The first and/or second lifting element may be selected from the group comprising an aerofoil, hydrofoil, or a flat sheet.

In the case of a flat sheet lifting element, the chord line comprises the centre line extending between two opposing peripheral faces of the flat sheet. Where the peripheral edges are perpendicular to the main plane, the trailing edge and leading edge of the flat sheet may be defined as either edge where the respective peripheral face join the main surface,

The first point may be midway along the first lifting element's span, and the second point may be midway along the second lifting element's span. That is, the first chord line may be the mid-span chord line of the first lifting element. Similarly, the second chord line may be the mid-span chord line of the second lifting element.

Alternatively, the first and/or second chord line may be an end-span chord line of the first/second lifting element, respectively.

In some embodiments, the slot may be defined by a gap between the first lifting element and the second lifting element.

In some embodiments, the slot may be defined by at least one gap between the first lifting element and the second lifting element.

The first angle formed between the first and the second chord lines may be between -10 and -1 degree or between 1 and 60 degrees. The trailing edge of each lifting element may extend from a top side to a bottom side of the lifting element. The first chord line may be a top side chord line (i.e. end- span chord line) or a bottom side chord line (i.e. end-span chord line) of the first lifting element. Similarly, the first chord line may be a top side chord line or a bottom side chord line of the second lifting element.

The first lifting element may be positioned so as at least partially to overlap the second lifting element when viewed normally to the first chord line. Overlapping the lifting elements further reduces turbulence in the water flow incident on the first lifting element and thereby generates more lift, as well as reducing the separation of the flow from the second element.

The first lifting element may overlap between 1 % and 50% of the second chord line.

Optionally, the blade may comprise a proximal edge and a distal edge opposite the proximal edge, wherein the first and the second lifting elements extend from the proximal edge to the distal edge and are in connection with each other at the proximal edge and/or distal edge.

The first and the second lifting elements may only be in connection with each other at one of the proximal edge and the distal edge. Connecting the lifting elements only at the proximal edge and/or distal edge keeps the central region of the lifting elements between the top side and the bottom side free of any connections to each other. The central regions of the lifting elements are where most of the lift is generated. Therefore keeping the central regions free of connections ensures that they are kept free of any features that could perturb the water flow and thus lift generation in these regions. With this arrangement, the inventors have recognised that the proximal and distal edges of the blade generate less lift in comparison to the central region and thus provide a good place to connect the lifting elements together without excessively impacting the amount of lift generated by the lifting elements.

Connecting the lifting elements only at one of the proximal edge or distal edge keeps the other edge free. Keeping one of the proximal edge or distal edge free of connections between the lifting elements (e.g. free of a peripheral frame connecting the lifting elements together) allows that edge to be more easily removed from the water during a rowing stroke.

The top side of each lifting element may define the proximal edge of the blade and/or the bottom side of each lifting element may define the distal edge of the blade.

The second lifting element may be connected to the first lifting element at the proximal edge and/or the distal edge via a flange extending from the second lifting element to the first lifting element. Of course, the flange may extend from the first lifting element to the second lifting element. Also, the flange may be a portion of the first and/or second lifting element.

Additionally or alternatively, the second lifting element may be connected to the first lifting element at the proximal edge via an upper peripheral frame, and/or the second lifting element may connected to the first lifting element at the distal edge via a lower peripheral frame.

Edge vortices are undesirable as they generate drag acting against the movement of the blade in water. In some embodiments, the upper and/or the lower peripheral frame may comprise a hydrodynamic fence. The fence may restrict the flow of water over the top/bottom side of the lifting elements and in this way inhibit the formation of edge vortices at the top/bottom sides of the lifting elements. In some embodiments, the span of the second lifting element may be larger than the span of the first lifting element, or vice versa. Increasing the span modifies the individual lift profile of the second lifting element and therefore the net lift profile of the blade in the rowing stroke. The blade may comprise further lifting elements and preferably may comprise between two and fifteen lifting elements in total.

In some embodiments, the blade may comprise:

a third lifting element having a third chord line at a third point along its span, the third chord line extending from a trailing edge to a leading edge of the third lifting element; and

a second slot through which water may flow when in use, the second slot having a constant width across more than half of the second slot's length, the constant width extending between the trailing edge of the third lifting element and the nearest point on the second lifting element, and

wherein the second and the third chord lines are non-parallel so as to form a second angle therebetween.

The addition of the third lifting element generates additional lift due to the slot effect. Further, angularly offsetting the third lifting element relative to the second lifting element such that the second and the third chord lines are non-parallel and form a second angle therebetween, advantageously directs (e.g. streamlines) the water flow from the third lifting element to the second lifting element at a more optimal angle (e.g. optimal angle of attack). In this way, the third lifting element enhances the amount of lift generated by the second lifting element and reduces drag. In addition, by improving the water flow over the second lifting element in this way, the second lifting element better directs (e.g. streamlines) the water flow incident on the first aerofoil element. Accordingly, the third lifting element also enhances the amount of lift generated by the first lifting element and reduces drag on the first lifting element. The second slot further reduces turbulence in (i.e. streamlines) the water flow from the third lifting element that falls incident on the second lifting element. The third point may be midway along the third lifting element's span. That is, the third chord line may be the mid-span chord line of the third lifting element.

Alternatively, the third chord line may be an end-span chord line of the third lifting element, respectively. The second slot may be defined by a gap between the second lifting element and the third lifting element.

The second angle may be greater than the first angle. The inventors have found that this enhances the streamlining effect that the third lifting element has on the second lifting element and on the first lifting element. Preferably, the second angle is greater than the first angle by an amount in the range 1 to 20 degrees.

In some embodiments, the second lifting element is positioned so as at least partially to overlap the third lifting element when viewed normally to the second chord line. Overlapping the second lifting element with the third lifting element further reduces turbulence in (e.g. streamlines) the water flow incident on the second lifting element.

The third lifting element may extend from the proximal edge to the distal edge and may be in connection with the second lifting element at the proximal edge and/or distal edge.

The third lifting element may only be in connection with the second lifting element at the proximal edge and/or distal edge. This ensures that the central region of the third lifting element is free of any connections to other lifting elements.

The third lifting element may be connected to the second lifting element at the proximal edge and/or the distal edge via a flange extending from the third lifting element to the second lifting element. Of course the flange may extend from the second lifting element to the third lifting element. The flange may be a portion of the third and/or second lifting element.

The third lifting element may be connected to the second lifting element at the proximal edge via the upper peripheral frame, and/or the third lifting element is connected to the second lifting element at the distal edge via the lower peripheral frame.

The span of the third lifting element may be larger than the span of the second lifting element. Increasing the span modifies the individual lift profile of the third lifting element and therefore the net lift profile of the blade across the rowing stroke.

The blade could have four, five or more lifting elements. One or more of the lifting elements may be connected to the upper/lower peripheral frame via a rotatable joint such as a ball joint to allow the angular offset between the lifting elements to be adjusted.

One or more of the lifting elements may be twisted along their span. Twisting a lifting element modifies the individual lift profile of the lifting element.

In some examples, the blade may be monolithically formed.

The lifting elements, flanges, and/or upper/lower peripheral frame(s) may be formed of wood, metal such as aluminium, plastic, or a composite material such as carbon fibre.

The invention extends to an oar comprising a blade according to the present invention and a shaft extending away therefrom. For example, the blade may comprise any of the above-mentioned blades.

The shaft may be formed as a separate piece attached to the blade. Optionally, the shaft may be monolithically or integrally formed with the blade. The second lifting element may be further away from the shaft than the first lifting element. The third lifting element may be further away from the shaft than the second lifting element. Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 plots the position of the blade relative to the plane of the water adjacent to the boat at various times during a rowing stroke;

Figure 2 is a perspective view of a rowing oar having a blade in accordance with the present invention;

Figure 3 is a cross-section along the length of the blade illustrated at Figure 2;

Figure 4 is a planar view of a blade according to another embodiment of the present invention; and

Figure 5 is a perspective view of a blade according to a further embodiment of the present invention.

Referring to Figures 2 and 3, an oar 100 comprises a blade 300 and a shaft 200 having a longitudinal axis 210 extending linearly across its two opposing ends. The blade 300 is connected to one end 200a of the shaft 200 and comprises an upper peripheral frame 380 defining a proximal edge of the blade 300 upon which a set of four lifting elements in the form of aerofoils 310a-310d are connected. The first 310a, second 310b, third 310c, and fourth 31 Od aerofoils are lined up in sequence with the first aerofoil 310a being positioned closest to the shaft 200 and the fourth aerofoil 31 Od being positioned the furthest away from the shaft 200.

The aerofoils 310a-310d each have a top surface 360a-360d and an underside surface 360a'-360d' (see Figure 3). In use, the top surface 360a-360d experiences higher water flow velocity and lower static pressure than the underside surface. The pressure difference between these two regions of an aerofoil 310a-310d generates lift. The aerofoils 310a-310d also each have a leading edge 330a-330d and a trailing edge 340a-340d. The trailing edges 340a-340d face the shaft 200 and the leading edges 330a-330d face away from the shaft 200. The leading edge 330a-330d and the trailing edge 340a-340d of each aerofoil 310a-310d extend from a top side 350a-350d of the aerofoil 310a-310d to a bottom side 350a'-350d' of the aerofoil 310a-310d. The top side 350a-350d of each aerofoil 310a-310d is connected to the upper peripheral frame 380. The bottom sides 350a'-350d' are not connected - that is they are connection free. Leaving the bottom sides 350a'-350d' connection free allows the blade 300 to be more easily inserted (bottom side first) into and removed out of the water during the rowing stroke than if the bottom sides 350a'-350d' were connected together.

It will be appreciated that the bottom sides 350a'-350d' define a distal edge of the blade and that the central region of each aerofoil between their respective top side 350a-350d and bottom side 350a'-350d' are also connection free.

The span of the first 310a, second 310b, third 310c, and fourth 31 Od aerofoils is set by the length of their respective leading edge 330a-330d which is longer than their respective trailing edge 340a-340d. Of course, if an aerofoil had a trailing edge that was longer than its leading edge then the length of the trailing edge would define the span of that aerofoil.

Figure 3 is a cross sectional view of the blade 300 that illustrates the mid-span chord lines of each aerofoil 310a-310d. However, to more clearly illustrate the arrangement of the aerofoils 310a-310d relative to each other and the longitudinal axis 210, the upper peripheral frame 380 has been omitted from Figure 3. As shown, the mid-span chord lines 320a-320d extend linearly between the respective leading edge 330a-330d and the respective trailing edge 340a-340d of each aerofoil 310a-310d. In addition, the mid-span chord line 320b-320d of the second 310b, third 310c, and fourth 31 Od aerofoils are angled relative to the mid-span chord line 320a of the first aerofoil 310a. To better illustrate the angle 550b-550d between the second 320b, third 320c, and fourth 320d mid-span chord lines relative to the first mid-span chord line 320a, Figure 3 illustrates a linear projection 320a' of the first mid-span chord line 320a of the first aerofoil 310a. In this embodiment the angle between the second 320b, third 320c, and fourth 320d mid-span chord lines relative to the first mid-span chord line 320a is 3 degrees, 10 degrees, and 23 degrees, respectively. Accordingly, it will be appreciated that in this embodiment the second 310b, third

310c, and fourth 31 Od aerofoils are angularly offset relative to the first aerofoil 310a and to each other. Further, the aerofoils which are further away from the first aerofoil 310a have a larger angular offset than the aerofoils which are closer to the first aerofoil 310a. It will also be appreciated that the first 310a, second 310b, third 310c, and fourth 31 Od aerofoils are angularly offset relative to the longitudinal axis 210. That is, the first 320a, second 320b, third 320c, and fourth 320d mid-span chord lines form an angle with respect to the longitudinal axis 210. However, it will be appreciated that in other embodiments it is possible for one of the mid-span chord lines 320a-320d to be parallel with the longitudinal axis 210, depending on the angle between the first mid-span chord line 320a and the longitudinal axis 210.

Angularly offsetting the aerofoils 310b-310d relative to the first aerofoil 310a streamlines the water flow on the first 310a, second 310b, and third 310c aerofoils and as result enhances the amount of lift generated by each of these aerofoils and the amount of drag each of these aerofoils experience.

As best seen in Figure 2, the leading edges 330a-330d are arranged parallel to each other. In addition, the first aerofoil 310a is spaced apart from and partially overlaps the second aerofoil 310b to define a slot 370a therebetween which has constant width across the span of the aerofoils 310a, 310b. The overlap is such that the leading edge 330a of the first aerofoil 310a is positioned along the longitudinal axis 210 between the leading edge 330b and the trailing edge 340b of the second aerofoil 310b. Similarly, the second aerofoil 310b is spaced apart from and partially overlaps the third aerofoil 310c to define a slot 360b therebetween, and the third aerofoil 310b is spaced apart from and partially overlaps the fourth aerofoil 31 Od to define a slot 370c therebetween. The slots 370a-370c help to further streamline the flow of water incident on the first, second, and third aerofoils 310a-310b and also to reduce interference in the water flow in those regions. Referring to Figure 2 again, it is seen that the spans of the aerofoils 310a-310d increase the further away they are from the shaft and the trailing edges 340a-340d. Increasing the span in this way has been found to further streamline the water flow between the aerofoils 310a-310d and, in addition, reduce the formation of vortices at the bottom side 350a'-350c' of the first 310a, second 310b, and third 310c aerofoils.

As an example, Figure 3 illustrates the direction of incoming water flow 400' at a certain time in a rowing stroke. It will be appreciated that the first 320a, second 320b, third 320c, and fourth 320d mid-span chord lines each form a different angle relative to the direction of the incoming water flow 400 and therefore each aerofoil 310a-310d has a different angle of attack.

Setting the aerofoils to have a different angle of attack improves the lift profile and reduces the drag and improves the lift-across the rowing stroke. Further the different angles of attack allow lift to be generated in regions of the rowing stroke where usually a conventional single surface rowing blade may stall (i.e. provide no lift). It will be appreciated that the number of lifting elements (e.g. aerofoil 310a-310d) is not limited to four lifting elements and in other embodiments the blade 300 may comprise between two and fifteen lifting elements. Preferably, the blade 300 may comprise three, four, five, or six lifting elements. Optionally, the blade 300 and the shaft 200 may be monolithically formed to provide a single continuous piece. In particular, the shaft and the blade may be made out of one single moulding.

Preferably, the upper peripheral frame 380, lifting elements (e.g. aerofoils 310a- 31 Od), and/or shaft 200 may be made out of one of the group of materials comprising; a composite material such as a carbon fibre composite; wood; metal such as aluminium; and plastic.

Optionally, the surface texture of the top surface 360a-360d and/or the underside surface 360a'-360d' of each lifting element (e.g. aerofoils 310a-310d) may be smooth with a surface roughness of between 0.8- 12.5 μηι. Alternatively, the top surface 360a-360d and/or the underside surface 360a'-360d' of each lifting element (e.g. aerofoils 310a-310d) may be treated or coated to have a rough surface with a surface roughness of greater than 12.5 μηι. Additionally or alternatively, the top surface 360a-360d and/or the underside surface 360a'-360d' of the lifting elements (e.g. aerofoils 310a-310d) may be treated to have a so-called shark skin surface texture.

Additionally or alternatively, the top surface 360a-360d and/or the underside surface 360a'-360d' of the lifting elements (e.g. aerofoils 310a-310d) may have a plurality of ribs. Optionally, the length of the ribs on each lifting element may be parallel to the mid-span chord line of the lifting element and/or extend between the leading edge and the trailing edge of the lifting element. Further optionally, the ribs may be 0.2mm to 3mm in width and/or may be spaced apart to define a 0.2mm to 3cm gap therebetween. By channelling water in the gaps between the ribs, the ribs further streamline and improve the uniformity of water flow incident on a

neighbouring lifting element (e.g. the ribs may further streamline the water directed from the second lifting element such as second aerofoil 310b to the first lifting element such as first aerofoil 310a). This advantageously increases the amount of lift generated by the neighbouring lifting element.

The lifting elements (e.g. aerofoils 310a-310d) may be twisted or otherwise contoured to account for differences in hydrostatic water pressure across the lifting element (e.g. differences in hydrostatic water pressure in the central region of the lifting element) during a rowing stroke and/or to increase/decrease the water flow rate in the slots between the lifting elements. For example, the second lifting element (e.g. second aerofoil 310b) may be twisted to curve its trailing edge towards its leading edge to reduce the flow of water in the slot between the first (e.g. first aerofoil 310a) and the second lifting elements.

The cross section of the shaft 200 may be round, oval, or shaped to provide lift (such as an aerofoil) to make it easier to move the shaft in air during a rowing stroke. Additionally or alternatively the lifting elements (e.g. aerofoils 310a-310b) may be connected to the upper peripheral frame 380 via a swivel or rotatable joint. The swivel/rotatable joint enables the relative angular offset between each lifting element to be adjusted. In this way, the angular offset of the lifting elements may be tuned to provide a different net lift profile for different water conditions or to better match a rower's particular rowing style.

Optionally, the blade 300 may have a lower peripheral frame 380' connected to the bottom sides 350a'-350d' of the aerofoils 310a-310d (i.e. lifting elements), as illustrated in Figure 4. The lower peripheral frame 380' makes the blade 300 more rigid. Optionally, the lower peripheral frame 380' may be shaped to form a hydrodynamic fence to reduce vortex effects at the bottom side 350a'-350d' of the aerofoils 310a-310d. Optionally, the lifting elements (e.g. aerofoils 310a-310d) may each comprise a flange 510a-510d at their top side 350a-350d and/or their bottom side 350a'-350d'. The flanges 510a-510d may connect a lifting element to one or more of its neighbouring lifting elements. Preferably, the flange 510a-510d of a lifting element may connect the lifting element to its neighbouring lifting element that is further away from the shaft 200. For example, in the embodiment of Figure 2, a flange

510a may extend from the bottom side 350a' of the first aerofoil 310a and connect to the bottom side 350b of the second aerofoil 310b. The second 310b and third 310c aerofoils may have similar flanges to connect the second aerofoil 310b to the third aerofoil 310c, and to connect the third aerofoil 310c to the forth aerofoil 31 Od aerofoil. Of course, it will be appreciated that the fourth aerofoil 31 Od may not have a flange since it does not have a neighbouring aerofoil that is further away from the shaft 200.

In some embodiments, the blade 300 may not have an upper 380 or lower 380' peripheral frame to connect the lifting elements together. In such arrangements, flanges 510a-510d extending from the top side or the bottom side of a lifting element may be used to connect the lifting element to its neighbouring lifting element(s). An example of this arrangement is provided in Figure 5. Apart from the flanges 510a-510c and the absence of the upper peripheral frame 380, the aerofoils 500a-500d in Figure 5 are the same as the corresponding aerofoils 310a-310d in Figure 2. That is, for example, the aerofoils 500a-500d of Figure 5 are each angularly offset in the same way as the aerofoils 310a-310d of Figure 2. Further, for example, the aerofoils 500a-500d of Figure 5 are each separated in the same way as the aerofoils 310a-310d of Figure 2 to define slots 570a-570c between neighbouring aerofoils (e.g. between the first 500a and the second 500b aerofoils). Turning to the arrangement of the flanges 510a-510c in Figure 5, the first aerofoil

500a is connected to the second aerofoil 500b via a flange 510a extending from the bottom side 550a of the first aerofoil 500a to the bottom side 550b of the second aerofoil 500b. Similarly, the second aerofoil 500b is connected to the third aerofoil 500c via a flange 510b extending from the bottom side 550b of the second aerofoil 500b to the bottom side 550c of the third aerofoil 500c. The third aerofoil 500c is connected to the fourth aerofoil 500d via a flange 510c extending from the bottom side 550c of the third aerofoil 500c to the bottom side 550d of the fourth aerofoil 500d. Although not shown in Figure 5, the first 500a, second 500b, and third 500c aerofoil may also be respectively connected at their top sides to the second 500b, third 500c, and fourth 500d aerofoil via a respective flange. For example, the first aerofoil 500a may be connected to the second aerofoil 500b via a flange extending from the top side (not shown in Figure 5) of the first aerofoil 500a to the top side (not shown in Figure 5) of the second aerofoil 500b. Similarly, the second aerofoil 500b may be connected to the third aerofoil 500c via a flange extending from the top side of the second aerofoil 500b to the top side 550c (not shown in Figure 5) of the third aerofoil 500c. The third aerofoil 500c may also be connected to the fourth aerofoil 500d via a flange extending from the top side 550c of the third aerofoil 500c to the top side 550d (not shown in Figure 5) of the fourth aerofoil 500d.

An advantage of using flanges is that it provides rigidity to the aerofoils 500a-500d of blade 500. It will be appreciated by those skilled in the art that the invention has been illustrated by describing several specific embodiments thereof, but is not limited to these embodiments. Many variations and modifications are possible, within the scope of the accompanying claims.

Claims

Claims:

A blade for a rowing oar, the blade comprising:

a first lifting element having a first chord line at a first point along its span, the first chord line extending from a trailing edge to a leading edge of the first lifting element;

a second lifting element having a second chord line at a second point along its span, the second chord line extending from a trailing edge to a leading edge of the second lifting element; and

a slot through which water may flow when in use, the slot having a constant width across more than half of the slot's length, the constant width extending between the trailing edge of the second lifting element and the nearest point on the first lifting element, and

The blade according to claim 1 wherein the first and/or the second lifting element is selected from the group comprising an aerofoil, hydrofoil, or a flat sheet.

The blade according to claim 1 or 2 wherein the first point is midway along the first lifting element's span, and the second point is midway along the second lifting element's span. 4. The blade according to claim 1 , 2, or 3 wherein the slot is defined by a gap between the first lifting element and the second lifting element.

The blade according to any preceding claim wherein the first angle is between 10 and -1 degrees or between 1 and 60 degrees.

The blade according to any preceding claim wherein the first lifting element is positioned so as at least partially to overlap the second lifting element when viewed normally to the first chord line. The blade according to any preceding claim further comprising a proximal edge and a distal edge opposite the proximal edge, wherein the first and the second lifting elements extend from the proximal edge to the distal edge and are in connection with each other at the proximal edge and/or distal edge.

The blade according to claim 7 wherein the first and the second lifting elements are only in connection with each other at one of the proximal edge and the distal edge.

The blade according to claim 7 or 8 wherein the second lifting element is connected to the first lifting element at the proximal edge and/or the distal edge via a flange extending from the second lifting element to the first lifting element.

10. The blade according to any of claims 7-9 wherein the second lifting element i connected to the first lifting element at the proximal edge via an upper peripheral frame.

1 1. The blade according to any of claims 7-10 wherein the second lifting element i connected to the first lifting element at the distal edge via a lower peripheral frame.

12. The blade according to claim 10 or 1 1 wherein the upper and/or the lower

peripheral frame comprises a hydrodynamic fence.

13. The blade according to any preceding claim wherein the span of the second lifting element is larger than the span of the first lifting element.

14. The blade according to any preceding claim further comprising:

a second slot through which water may flow when in use, the second slot having a constant width across more than half of the second slot's length, the constant width extending between the trailing edge of the third lifting element and the nearest point on the second lifting element, and wherein the second and the third chord lines are non-parallel so as to form a second angle therebetween.

15. The blade according to claim 14 wherein the third point is midway along the third lifting element's span.

16. The blade according to claim 14 or 15 wherein the second slot is defined by a gap between the second lifting element and the third lifting element. 17. The blade according to claim 14, 15 or 16 wherein the second angle is greater than the first angle.

18. The blade according to claim 17 wherein the second angle is greater than the first angle by an amount in the range 3 to 20 degrees.

19. The blade according to any of claims 14-18 wherein the second lifting element is positioned so as at least partially to overlap the third lifting element when viewed normally to the second chord line. 20. The blade according to any of claims 14-19 when dependent on claim 7 wherein the third lifting element extends from the proximal edge to the distal edge and is in connection with the second lifting element at the proximal edge and/or distal edge. 21. The blade according to claim 20 wherein the third lifting element is only in

connection with the second lifting element at the proximal edge and/or distal edge.

22. The blade according to claim 20 or 21 wherein the third lifting element is

connected to the second lifting element at the proximal edge and/or the distal edge via a flange extending from the third lifting element to the second lifting element.

23. The blade according to any of claims 20-22 when dependent on claim 10 wherein the third lifting element is connected to the second lifting element at the proximal edge via the upper peripheral frame.

24. The blade according to any of claims 20-23 when dependent on claim 1 1

wherein the third lifting element is connected to the second lifting element at the distal edge via the lower peripheral frame.

25. The blade according to any of claim 14-24 wherein the span of the third lifting element is larger than the span of the second lifting element.

26. An oar comprising the blade according to any of claims 1 to 25 and a shaft extending away therefrom.

27. The oar according to claim 26 wherein the shaft is formed as a separate piece attached to the blade.

28. The oar according to claim 27 wherein the shaft is monolithically or integrally formed with the blade.

29. The oar according to any of claims 26 to 28 wherein the second lifting element is further away from the shaft than the first lifting element.

30. The oar according to any of claims 26 to 29 when dependent on any of claims 14 to 25 wherein the third lifting element is further away from the shaft than the second lifting element.