Patent Publication Number: US-2019168855-A1

Title: A propulsion device

Description:
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&#39;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”. 
       FIG. 1  illustrates the typical path of a rowing blade, with the positive x axis in the direction of the boat&#39;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  FIG. 1 , it has been found that typical rowing blades generate very little lift in the region after 0.25 s compared to the region before 0.25 s. Furthermore, in the region between 0.35 s and about 0.5 s, 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&#39;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&#39;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&#39;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&#39;s span, and the second point may be midway along the second lifting element&#39;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. Wth 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&#39;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&#39;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, ora 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: 
         FIG. 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; 
         FIG. 2  is a perspective view of a rowing oar having a blade in accordance with the present invention; 
         FIG. 3  is a cross-section along the length of the blade illustrated at  FIG. 2 ; 
         FIG. 4  is a planar view of a blade according to another embodiment of the present invention; and 
         FIG. 5  is a perspective view of a blade according to a further embodiment of the present invention. 
     
    
    
     Referring to  FIGS. 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  200   a  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  310   a - 310   d  are connected. The first  310   a , second  310   b , third  310   c , and fourth  310   d  aerofoils are lined up in sequence with the first aerofoil  310   a  being positioned closest to the shaft  200  and the fourth aerofoil  310   d  being positioned the furthest away from the shaft  200 . 
     The aerofoils  310   a - 310   d  each have a top surface  360   a - 360   d  and an underside surface  360   a ′- 360   d ′ (see  FIG. 3 ). In use, the top surface  360   a - 360   d  experiences higher water flow velocity and lower static pressure than the underside surface. The pressure difference between these two regions of an aerofoil  310   a - 310   d  generates lift. 
     The aerofoils  310   a - 310   d  also each have a leading edge  330   a - 330   d  and a trailing edge  340   a - 340   d . The trailing edges  340   a - 340   d  face the shaft  200  and the leading edges  330   a - 330   d  face away from the shaft  200 . The leading edge  330   a - 330   d  and the trailing edge  340   a - 340   d  of each aerofoil  310   a - 310   d  extend from a top side  350   a - 350   d  of the aerofoil  310   a - 310   d  to a bottom side  350   a ′- 350   d ′ of the aerofoil  310   a - 310   d . The top side  350   a - 350   d  of each aerofoil  310   a - 310   d  is connected to the upper peripheral frame  380 . The bottom sides  350   a ′- 350   d ′ are not connected—that is they are connection free. Leaving the bottom sides  350   a ′- 350   d ′ 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  350   a ′- 350   d ′ were connected together. 
     It will be appreciated that the bottom sides  350   a ′- 350   d ′ define a distal edge of the blade and that the central region of each aerofoil between their respective top side  350   a - 350   d  and bottom side  350   a ′- 350   d ′ are also connection free. 
     The span of the first  310   a , second  310   b , third  310   c , and fourth  310   d  aerofoils is set by the length of their respective leading edge  330   a - 330   d  which is longer than their respective trailing edge  340   a - 340   d . 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. 
       FIG. 3  is a cross sectional view of the blade  300  that illustrates the mid-span chord lines of each aerofoil  310   a - 310   d . However, to more clearly illustrate the arrangement of the aerofoils  310   a - 310   d  relative to each other and the longitudinal axis  210 , the upper peripheral frame  380  has been omitted from  FIG. 3 . As shown, the mid-span chord lines  320   a - 320   d  extend linearly between the respective leading edge  330   a - 330   d  and the respective trailing edge  340   a - 340   d  of each aerofoil  310   a - 310   d . In addition, the mid-span chord line  320   b - 320   d  of the second  310   b , third  310   c , and fourth  310   d  aerofoils are angled relative to the mid-span chord line  320   a  of the first aerofoil  310   a . To better illustrate the angle  550   b - 550   d  between the second  320   b , third  320   c , and fourth  320   d  mid-span chord lines relative to the first mid-span chord line  320   a ,  FIG. 3  illustrates a linear projection  320   a ′ of the first mid-span chord line  320   a  of the first aerofoil  310   a . In this embodiment the angle between the second  320   b , third  320   c , and fourth  320   d  mid-span chord lines relative to the first mid-span chord line  320   a  is 3 degrees, 10 degrees, and 23 degrees, respectively. 
     Accordingly, it will be appreciated that in this embodiment the second  310   b , third  310   c , and fourth  310   d  aerofoils are angularly offset relative to the first aerofoil  310   a  and to each other. Further, the aerofoils which are further away from the first aerofoil  310   a  have a larger angular offset than the aerofoils which are closer to the first aerofoil  310   a . It will also be appreciated that the first  310   a , second  310   b , third  310   c , and fourth  310   d  aerofoils are angularly offset relative to the longitudinal axis  210 . That is, the first  320   a , second  320   b , third  320   c , and fourth  320   d  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  320   a - 320   d  to be parallel with the longitudinal axis  210 , depending on the angle between the first mid-span chord line  320   a  and the longitudinal axis  210 . 
     Angularly offsetting the aerofoils  310   b - 310   d  relative to the first aerofoil  310   a  streamlines the water flow on the first  310   a , second  310   b , and third  310   c  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  FIG. 2 , the leading edges  330   a - 330   d  are arranged parallel to each other. In addition, the first aerofoil  310   a  is spaced apart from and partially overlaps the second aerofoil  310   b  to define a slot  370   a  therebetween which has constant width across the span of the aerofoils  310   a ,  310   b . The overlap is such that the leading edge  330   a  of the first aerofoil  310   a  is positioned along the longitudinal axis  210  between the leading edge  330   b  and the trailing edge  340   b  of the second aerofoil  310   b . Similarly, the second aerofoil  310   b  is spaced apart from and partially overlaps the third aerofoil  310   c  to define a slot  360   b  therebetween, and the third aerofoil  310   b  is spaced apart from and partially overlaps the fourth aerofoil  310   d  to define a slot  370   c  therebetween. The slots  370   a - 370   c  help to further streamline the flow of water incident on the first, second, and third aerofoils  310   a - 310   b  and also to reduce interference in the water flow in those regions. 
     Referring to  FIG. 2  again, it is seen that the spans of the aerofoils  310   a - 310   d  increase the further away they are from the shaft and the trailing edges  340   a - 340   d . Increasing the span in this way has been found to further streamline the water flow between the aerofoils  310   a - 310   d  and, in addition, reduce the formation of vortices at the bottom side  350   a ′- 350   c ′ of the first  310   a , second  310   b , and third  310   c  aerofoils. 
     As an example,  FIG. 3  illustrates the direction of incoming water flow  400 ′ at a certain time in a rowing stroke. It will be appreciated that the first  320   a , second  320   b , third  320   c , and fourth  320   d  mid-span chord lines each form a different angle relative to the direction of the incoming water flow  400  and therefore each aerofoil  310   a - 310   d  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  310   a - 310   d ) 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  310   a - 310   d ), 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  360   a - 360   d  and/or the underside surface  360   a ′- 360   d ′ of each lifting element (e.g. aerofoils  310   a - 310   d ) may be smooth with a surface roughness of between 0.8-12.5 μm. Alternatively, the top surface  360   a - 360   d  and/or the underside surface  360   a ′- 360   d ′ of each lifting element (e.g. aerofoils  310   a - 310   d ) may be treated or coated to have a rough surface with a surface roughness of greater than 12.5 μm. Additionally or alternatively, the top surface  360   a - 360   d  and/or the underside surface  360   a ′- 360   d ′ of the lifting elements (e.g. aerofoils  310   a - 310   d ) may be treated to have a so-called shark skin surface texture. 
     Additionally or alternatively, the top surface  360   a - 360   d  and/or the underside surface  360   a ′- 360   d ′ of the lifting elements (e.g. aerofoils  310   a - 310   d ) 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 0.2 mm to 3 mm in width and/or may be spaced apart to define a 0.2 mm to 3 cm 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  310   b  to the first lifting element such as first aerofoil  310   a ). This advantageously increases the amount of lift generated by the neighbouring lifting element. 
     The lifting elements (e.g. aerofoils  310   a - 310   d ) 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  310   b ) 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  310   a ) 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  310   a - 310   b ) 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&#39;s particular rowing style. 
     Optionally, the blade  300  may have a lower peripheral frame  380 ′ connected to the bottom sides  350   a ′- 350   d ′ of the aerofoils  310   a - 310   d  (i.e. lifting elements), as illustrated in  FIG. 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  350   a ′- 350   d ′ of the aerofoils  310   a - 310   d.    
     Optionally, the lifting elements (e.g. aerofoils  310   a - 310   d ) may each comprise a flange  510   a - 510   d  at their top side  350   a - 350   d  and/or their bottom side  350   a ′- 350   d ′. The flanges  510   a - 510   d  may connect a lifting element to one or more of its neighbouring lifting elements. Preferably, the flange  510   a - 510   d  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  FIG. 2 , a flange  510   a  may extend from the bottom side  350   a ′ of the first aerofoil  310   a  and connect to the bottom side  350   b  of the second aerofoil  310   b . The second  310   b  and third  310   c  aerofoils may have similar flanges to connect the second aerofoil  310   b  to the third aerofoil  310   c , and to connect the third aerofoil  310   c  to the forth aerofoil  310   d  aerofoil. Of course, it will be appreciated that the fourth aerofoil  310   d  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  510   a - 510   d  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  FIG. 5 . 
     Apart from the flanges  510   a - 510   c  and the absence of the upper peripheral frame  380 , the aerofoils  500   a - 500   d  in  FIG. 5  are the same as the corresponding aerofoils  310   a - 310   d  in  FIG. 2 . That is, for example, the aerofoils  500   a - 500   d  of  FIG. 5  are each angularly offset in the same way as the aerofoils  310   a - 310   d  of  FIG. 2 . Further, for example, the aerofoils  500   a - 500   d  of  FIG. 5  are each separated in the same way as the aerofoils  310   a - 310   d  of  FIG. 2  to define slots  570   a - 570   c  between neighbouring aerofoils (e.g. between the first  500   a  and the second  500   b  aerofoils). 
     Turning to the arrangement of the flanges  510   a - 510   c  in  FIG. 5 , the first aerofoil  500   a  is connected to the second aerofoil  500   b  via a flange  510   a  extending from the bottom side  550   a  of the first aerofoil  500   a  to the bottom side  550   b  of the second aerofoil  500   b . Similarly, the second aerofoil  500   b  is connected to the third aerofoil  500   c  via a flange  510   b  extending from the bottom side  550   b  of the second aerofoil  500   b  to the bottom side  550   c  of the third aerofoil  500   c . The third aerofoil  500   c  is connected to the fourth aerofoil  500   d  via a flange  510   c  extending from the bottom side  550   c  of the third aerofoil  500   c  to the bottom side  550   d  of the fourth aerofoil  500   d.    
     Although not shown in  FIG. 5 , the first  500   a , second  500   b , and third  500   c  aerofoil may also be respectively connected at their top sides to the second  500   b , third  500   c , and fourth  500   d  aerofoil via a respective flange. For example, the first aerofoil  500   a  may be connected to the second aerofoil  500   b  via a flange extending from the top side (not shown in  FIG. 5 ) of the first aerofoil  500   a  to the top side (not shown in  FIG. 5 ) of the second aerofoil  500   b . Similarly, the second aerofoil  500   b  may be connected to the third aerofoil  500   c  via a flange extending from the top side of the second aerofoil  500   b  to the top side  550   c  (not shown in  FIG. 5 ) of the third aerofoil  500   c . The third aerofoil  500   c  may also be connected to the fourth aerofoil  500   d  via a flange extending from the top side  550   c  of the third aerofoil  500   c  to the top side  550   d  (not shown in  FIG. 5 ) of the fourth aerofoil  500   d.    
     An advantage of using flanges is that it provides rigidity to the aerofoils  500   a - 500   d  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.