WO2005066018A1

WO2005066018A1 - Surfboard fin

Info

Publication number: WO2005066018A1
Application number: PCT/AU2005/000007
Authority: WO
Inventors: Andrew Dovell
Original assignee: Surfing Hardware International Holdings Pty Limited
Priority date: 2004-01-09
Filing date: 2005-01-07
Publication date: 2005-07-21

Abstract

A cambered airfoil-shaped surfcraft side fin having a camber of from about 2 % to about 6 %, wherein said camber is substantially constant throughout length of the side fin from adjacent a base to adjacent an end tip of said side fin.

Description

SURFBOARD FIN

Field of Invention

The present invention relates to a fin for a surfcraft, such as a surfboard. More particularly, the invention relates to a side fin for such a surfcraft.

Background of Invention

Surfcraft typically include surfboards (including shortboards, longboards and kneeboards and bodyboards) and surfskis which support a rider and enable a rider to catch waves whilst lying, sitting, kneeling or standing on the surfcraft.

Surfboards, surfskis and other surfcraft are customarily provided with one or more fins extending from a rearward section of the lower surface of the surfcraft. Each fin helps to maintain the orientation of the surfcraft in the direction of travel and facilitates turning of the surfcraft by the rider. The fins of a surfboard may also provide lateral stability to the board when the rider stands on the board to ride a wave.

Modem surfboards generally have two or three fins, three-fin surfboards being by far the most common. The fins of two-fin surfboards are located near the rear of the underside of the surfboard on opposite sides of the surfboard centre-line. Each fin is located intermediate the centre-line and a side of the surfboard. Three-fin surfboards have a centre fin positioned along the centre line of the board close to the rear end of the board, and a pair of opposed side fins located nearer to the side rails of the surfboard and being positioned forward of the rear fin. The side fins may be orientated parallel to the rear fin but, for better performance, they are generally toed in by up to 3 or 4 degrees relative to the centre line of the board. The side fins of two-fin surfboards may be similarly toed in.

In this specification, the term "side fin" means a fin adapted to be located intermediate the centre-line and a side rail of a surfcraft. Although variations in fin shape have occurred over the years, surfboard fins are generally shaped substantially in the profile of a dolphin fin, having a relatively long base adjacent to the lower surface of the board, a rounded tip and highly curved, arcuate leading and trailing edges between the base and the tip.

The centre fin of a surfboard is symmetrical about its centre plane. However, side fins are generally not symmetrical, usually having a flat inner surface (being the surface nearer to the center-line of the surfboard) and a curved (convex) outer surface. The cross-sectional profile (along planes which are parallel to the bottom surface of the surfboard) of such a side fin is not consistent between the base and the tip of the fin. Traditionally, surfboard fins have been made by shaping them from a fibreglass slab which were then typically securely affixed to the body of a surfboard and 'glassed over' so as to be integrally formed with (and, therefore, not removable from) the body of the surfboard. Alternatively, fins have also been made from plastic by injection moulding techniques. Substantial improvements have been made in the past decade in relation to systems for removably attaching fins to a surfboard. However, little advance has been made in the design of fins themselves and the surfboard fins used today are predominantly moulded plastic analogues of the old fibreglass glassed-in fins, with altered bases to fit the new removable fin attachment systems. Also, little attention has been paid to the design of the side fins which, typically, have adopted the same shape as the centre fin save that, instead of being symmetrical, the inside surface of the fin is generally flat along the nose to tail line of the fin.

Relatively little data exists in relation to the performance of surfboard fins, including surfboard side fins. As a result, surfboard fins have generally been designed by a "look and feel" approach. In the design of surfboard fins, the starting point has often been the shape and profile of fins (typically dorsal fins) of dolphins, sharks and other similar sea creatures, on the assumption that "nature" has determined optimum shape/profile characteristics which can be adapted to surfboard fins. Apart from lacking any scientific basis, this approach is unjustified for surfboard side fins because the purposes, functions and requirements of surfboard side fins are quite different from those of fins (typically dorsal fins) of sea creatures. For example, a surfboard fin (particularly a side fin) must be designed to enable a surfboard to turn, whilst maintaining forward speed, without stalling and, in doing so, to absorb very substantial lateral forces.

Early surfboard fins were not designed with the above performance criteria in mind. Over recent years, attempts have been made to improve the performance characteristics of surfboard fins but generally these attempts have been made in an unstructured and unscientific way. Consequently, standard surfboard fins still have relatively low performance characteristics.

When riding a surfboard (or other surfcraft) on a wave, the rider generally controls its movement by applying force (eg by shifting weight) through the contact with the rider's feet and the top deck of the board. By shifting the rider's weight sideways, the surfboard is able to turn towards the direction of the weight shift. This changes the direction of travel of the board and thus changes the angle of attack of the fins (relative to the direction of water flowing under the surfcraft). Gentle turns only require relatively low levels of force to be applied through the fins. However, sharp or radical turns generally require substantial force to be applied through the fins. Turning manoeuvres performed by surfboard riders, particularly those of a high standard, require high board speed and predictable performance of the board (including the fins). Tests conducted by the applicant, which are consistent with aerodynamic theory, show that high aspect ratio fins generally have lower drag than low aspect ratio fins. However, high aspect ratio fins generally have lower stall angles than low aspect ratio fins, meaning that, during a turn, high aspect ratio fins will cause a surfboard to stall sooner than would be the case with low aspect ratio fins. Accordingly, low aspect ratio fins tend to have gentle stall characteristics enabling a surfer to recover from a stall (or partial stall) situation much more smoothly and easily.

The tests also show that fins having a relative high sweep angle will generally have more drag at moderate angles of attack but less drag at zero angle of attack than fins having lower degrees of sweep. The drag experienced by a fin is in the same direction as the water flow. Swept fins also generally have higher lift capacity than unswept fins. The lift and drag characteristics of a fin, and therefore the performance of the fin, are dependant upon a number of the above variables including aspect ratio and sweep. The lift and drag characteristics are also affected by the angle of attack which, obviously, varies depending on whether the surfboard is travelling straight or undergoing a turn. As noted above, typical surfboard side fins have a flat inside surface and a convex outside surface. Tests conducted by the applicant have shown that such side fins have some (but relatively poor) airfoil characteristics. When an airfoil moves through a fluid (particularly at moderate angles of attack) lift is created. This general concept applies whether the fluid is a gas (eg air) or a liquid (eg , water). The lift provided by a such a surfboard side fin moving through water is substantially perpendicular to the direction of water flow. Through testing conducted by the applicant, the applicant has determined that a side fin which provides desirable lift will result in sharper and more controlled turns. As indicated above, generating more lift usually results in generating more drag. However, the amount of drag generated varies based on various parameters such as foil section, camber, aspect ratio and sweep. Accordingly, the design of high performance side fins is a difficult exercise - requiring consideration of numerous variables and which often involves trade- offs to achieve desirable performance results. For instance, increasing the lift capacity of the fin (eg by increasing the sweep angle) will generally result in increased drag at moderate angles of attack. Similarly, increasing the aspect ratio has the benefit of increasing the stall angle but this also often results in increased drag. Accordingly, the present invention is directed towards providing a side fin for a surfcraft, such as a surfboard, which has improved performance characteristics over standard side fins. In the present specification, reference will be had to the following terms which are illustrated with reference to Figures 1A to 3B relating to a conventional fin for a surfboard (or other surfcraft). "Profile area" (PA) - the area of the fin when viewed from the side. "Span" (s) - the vertical height from the base to the tip of the fin.

"Aspect ratio" (AR) - 2 x span² divided by profile area (AR = 2 x S²/PA)

"Nose to tail line" - a straight line between the front edge and the rear edge of the fin at its base. "Chord" - a straight line (eg the distance) between the leading edge and the trailing edge of the fin at a given height.

"Root chord" - the chord adjacent the base of the fin.

"Tip chord" - the chord adjacent the tip of the fin.

"Camber line" - the centre line of the fin cross section. "Camber curve" - a plot of the offset between the camber line and the "nose to tail" line (Y axis) against the distance from the frontage of the fin along the chord (X axis).

"Camber" (when expressed as a percentage) - the ratio of the maximum camber offset (ie maximum distance between the camber line and the chord) to the chord length. Camber is, therefore, a measure of the curvature of an airfoil.

"Thickness" - the maximum distance between the 2 opposed external surfaces of the fin at a given height.

"Foil" - a plot of the offset from the camber line to the fin face (Y axis) against the distance from the frontage of the fin along the cord (X axis). "Angle of attack" - the angle between the direction of fin travel and the nose-to- tail line.

"Lift" - the maximum sideways force which the fin can resist for a given angle of attack.

"Drag" - the resistance to fin travel through the water. "Construction line" - a line following a locus at a separate portion (eg 25%) between the front edge and rear edge of the fin profile (which is generally used to measure sweep angle).

"Sweep" (also known as "rake") - the angle between the vertical and the construction line of the fin profile. "Thickness to chord ratio" (t/c) - the ratio of the maximum thickness along a given chord to the chord length, typically expressed as a percentage.

"Cant angle" (also known as "dihedral angle") - the angle by which the central plane of the fin body departs from being perpendicular to the lower surface of the surfboard.

Summary of the Invention

According to a first aspect of this invention there is provided a cambered airfoil- shaped surfcraft side fin having a camber of from about 2% to about 6%, wherein said camber is substantially constant throughout length of the side fin from adjacent a base to adjacent an end tip of said side fin.

The camber of an airfoil which is consistent from near the base to near the tip of the airfoil is said to be a geosymmetric camber. In the present invention, the camber of the side fin is preferably in the range of 3% to 5% and, more preferably, is about 4%. A particularly preferred camber which may be incorporated in the side fin of the present invention is that which is embodied in the Selig Donovan 7043 foil.

It is further preferred that the surfcraft fin has a sweep of from about 25° to about 50°. More preferably, the sweep will be between about 30° and 45°. A particularly preferred sweep is an angle of about 35°. In fins which have a profile area which represents an elliptical segment, it is generally easy to determine the angle of the sweep (typically being the angle between the symmetric-centre line of the ellipse and the base line of the fin). However, where a fin is designed "freehand", it may not have such an elliptical- segment shape. An example of a freehand fin is one which has a shape based on the shape of a "dolphin fin". In such cases, nevertheless, approximate sweep angles can be determined by reference to a notional construction line. In "freehand" fins according to the invention, the sweep is generally as described above, namely from about 25° to about 50°; preferably from about 30° to 45°, and more preferably about 30°. The various design parameters of the surfcraft side fin of this invention may vary depending in the nature and size of the surfcraft. The desired parameters may also vary depending on the conditions in which the side fin is to be used.

When the surfcraft is a surfboard, the side fin of this invention preferably has a profile area of between about 7000mm² and 11 ,000mm². A profile area of between 8000mm² and 10,000mm² is more preferred. In one embodiment of the invention, a particularly preferred profile area is about 9000mm². The preferred profile area will however depend on the conditions in which the fin is intended to be used. A side fin having a larger profile area may be desirable in larger surf conditions and a smaller profile area may be desirable for fins used in light surf conditions.

Preferably, a surfboard side fin according to this invention will have a span of between about 90mm and 140mm, A preferred range is between about 110mm and 125mm. In one embodiment of the invention, a particularly preferred span is about 120mm. Again, the actual preferred dimensions are likely to depend upon the conditions in which the side fin is to be used. A longer span may be desirable in conditions of larger surf.

The surfboard side fin preferably has an aspect ratio of between about 2.5 and 4.0. A preferred aspect ratio is between about 3.0 and 3.5. In one embodiment of the invention, a particularly preferred aspect ratio is about 3.2.

It is generally preferred that the thickness:chord (t/c) ratio is substantially constant throughout the length of the fin. A preferred t/c ratio range is from about 6% to about 12%. A more preferred range is between about 8% and 10%. In one preferred embodiment of the present invention, a particularly preferred t/c ratio is about 9%.

In a further preferred embodiment of the invention, the surfcraft side fin has a substantially rounded leading edge, meaning that the portions of the inside and outside surfaces of the fin which are immediately adjacent the leading edge are curved. According to a second aspect of this invention, there is provided a surfcraft, such as a surfboard, comprising one or more surfcraft side fins according to the first aspect of this invention.

The surfcraft preferably comprises two of said side fins, one being located between a centre line of the surfcraft and a first side rail of the surfcraft and the other of said fins being located between the centre line and a second side rail of said surfcraft, wherein the fins are orientated to provide lift when substantially only one of said side fins is travelling through water. As will be readily appreciated, it is important that the side fins are orientated correctly meaning that the appropriate side fins are located adjacent the appropriate rails of the surfcraft.

Preferably, the surfcraft will also include a centre fin located on said centre line and positioned rearward of the two side fins of this invention.

It is further preferred that the two side fins are toed inwards, meaning that, at any point along the span, the leading edge of the fin is nearer said centre line than the trailing edge. It is preferred that each of said two side' fins are toed inwards by about 4° relative to the centre line.

It is further preferred that the cant angle of each of said side fins is from about 7° to about 12°. In one embodiment of the invention, the preferred cant angle is about 10°. Where the side fins are attached to the surfcraft by means of one or more tabs extending from the base of the fins, the cant angle may be effected by having the fin tabs (which removably engage the surfboard by slotting into vertical cavities within the surfboard) angularly offset from the central plane of the fin. Alternatively, the cant angle may be effected by having the tab-receiving cavities in the surfboard angled inwardly (and the tabs being in substantially the same plane as the side fin).

The surfcraft side fin of this invention may be sold as part of a kit. A typical kit may include a centre fin (which would generally have a symmetrical cross section) and two side fins of this invention. One of the side fins would be adapted to be attached near the rear left side rail of the surfcraft to generate lift in a leftwards direction and the other side fin (typically a mirror image of the first side fin) would be adapted to be attached near the rear right side rail of the surfcraft to generate lift in a leftwards direction. Description of the drawings

Figures 1A and 1B, are, respectively, a side view and a cross sectional view of the profile of a conventional "dolphin fin" surfboard side fin;

Figure 2 is a sketch illustrating change in the angle of attack of a fin as a surfboard turns;

Figures 3A and 3B are sketches of typical graphs of lift against angle of attack and of drag against lift, respectively;

Figures 4A to 4C are side, cross sectional and front views, respectively, of the base profile of fin according to a preferred embodiment of the present invention; Figure 5A to 5C are side, cross sectional and front views, respectively, of the base profile of a fin according to another preferred embodiment of the present invention;

Figures 6A to 6C, 7A to 7C and 8A to 8C are side, cross sectional and front views, respectively, of the base profile of fins (not being fins of the present invention) used in tests to determine the effect of sweep angle on fin performance;

Figure 9 is a graph showing the lift curves of lift coefficient v. angle of attack for the results of the tests on the fins of Figures 6, 7 and 8;

Figure 10 is a graph showing the lift/drag polar curves of drag coefficient v. lift coefficient for the results of the tests on the fins of Figures 6, 7 and 8;

Figures 11A to 11 C, 12A to 12C, 13A to 13C, 14A to 14C, 15A to 15C and 16A to 16C are side, cross sectional and front views of the base profile of fins according to embodiments of the present invention used in tests to determine the effect of camber on fin performance (said fins being substantially identical in all respects except for the camber);

Figures 17 to 22 are graphs showing the lift curves of lift coefficient v. angle of attack for the results of the tests on the fins of Figures 11 to 16;

Figures 23 and 24 are graphs showing the lift/drag polar curves of drag coefficient v. lift coefficient for the results of the tests on several of the fins of Figures 11 to 16; Figures 25A to 25G are schematic representations of water flow over a surfboard side fin at varying angles of attack, namely 0°, 5°, 10°, 15°, 20°, 25° and 30°, respectively;

Figure 26 is a set of drawings representing different views of a side fin according to a preferred embodiment of this invention;

The following description, relative to Figures 1A, 1B, 2, 3A and 3B, illustrates terminology for fin design parameters described in the present specification.

With reference to Figures 1A, 1B, 2, 3A and 3B, the fin 10 has an exposed body, which is adapted to project below the bottom surface of the surfboard, with opposed lateral surfaces 12, 14. Extending from the base of the exposed portion are one or more fixing projections, such as tabs 16, adapted to co-operate with cavities in fixing elements within the surfboard for removably fixing the fins to the board. The illustrated fin has a pair of tabs 16 adapted for fitting to the fixing plugs of the fixing system described in US patent nos. 5,328,397, 5,464,359 and 5,671 ,081.

Figure 1A is a side view and Figure 1B is a plan cross-sectional view of a conventional prior art surfboard fin 10. The item numbers designating particular parameters of the fin include span 18, chord 20 (at an arbitrary point), root chord 22, sweep (rake angle) 24, camber line 30, thickness and foil 34. As shown in Figure 1A, the typical prior art surfboard fin has a profile area very similar in shape to a "dolphin fin", having pronounced arcuate leading and trailing edges.

Typically, prior art fins of this type have a span of about 110 to 120mm, a base length (root chord) of about 100mm and a profile area of about 10,000mm². With reference to Figure 1 B, the inside lateral surface 12 of the prior art fin is flat - extending along the nose-to-tail line, whilst the outer lateral surface 14 is curved (convexed). The camber line 30, being the centre line of the fin section, therefore follows a locus halfway between the inside and outer lateral surfaces, while the foil 34 (the offset between the camber line and surface) is equal to the camber offset at any point. The fin thickness 32 is substantially constant over the length of the fin, usually about 6 to 9mm, such that the thickness:chord ratio of the fin increases significantly from the base to the tip.

In use, the fins provide lateral stability to the surfboard by resisting lateral forces on the fin. The maximum lateral resistance (eg lift) which can be provided by the fin will depend on a number of factors including the angle between the fin axis (as denoted by the nose-to-tail line) and its direction of travel through the water (ie the angle of attack). This is illustrated in Figure 2.

Figure 3A is a sketch of typical relationship between the lift (ie the maximum sideways force which the fin can resist) against the angle of attack. It can be seen that, up to a certain angle of attack, the lift performance of the fin increases but, with further increase in the angle of attack, then peaks and declines as the forward flow pattern around the fin stalls. The shape of the lift v. angle of attack curve, including the lift at 0°, gradient, the peak lift, the angle of attack at peak lift and the gradient of the drop-off in lift after the peak, is determined by design parameters of the fin.

Figure 3B is a sketch of a typical relationship between the drag (eg the resistance force to fin travel through the water) against the lift for the fin. It can be seen that the drag is minimised at zero lift and increases exponentially with increased lift. Again, the actual shape of the curve, including the rate of increase of the gradient, is determined by the design parameters of the fin.

In designing a surfboard fin with the above in mind, it is desirable to maximise the lift whilst minimising the drag. Various features of a fin contribute, to varying degrees to the lift and drag characteristics of the fin. Accordingly, it is desirable to design a fin having design features which result in good lift whilst having a relatively low degree of drag.

Figures 4A to 4C illustrate a preferred embodiment of the present invention, with dimensions as shown. The design parameters of this fin include:

' Profile area - 9,620mm² • Span - 120mm • Aspect ratio - 3.0 • Sweep - 25°

• Thickness/Chord at root - 9%

• Thickness/Chord at tip - 9%

• Cant angle - 15° • Foil section - Selig Donovan 7043

Figures 5A to 5C and Figure 26 illustrate a further preferred embodiment of the present invention, with dimensions as shown. The design parameters of this fin include:

^• Profile area - 9,000mm² • Span - 118.5mm Aspect ratio - 3.2 Sweep - Freehand Thickness/Chord at root - 9% Thickness/Chord at tip - 9% • Cant angle - 10° Foil section - Selig Donovan 7043 The camber of the fin section is distributed so as to result in a curved inside lateral face. In particular, the maximum camber is located rearwards of the maximum foil offset to form a slightly concave portion on the inside face towards the rear of the fin, crossing back over the nose to tail line.

Experiments and Experimental Results

Test 1 - Effect of sweep.

Different surfboard fins were tested under varying conditions in order to ascertain various performance criteria in relation to the surfboard fins. The fins were installed into a specially designed dynamometer in a cavitation tunnel. Each fin was tested at varying speeds through water from 8 to 20 knots and at varying angles of attack from negative 10 degrees to 26 degrees.

The data collected during these tests included data on lift, drag and vertical force. These data were used to create lift and drag curves for each fin. Three fins were tested having reference numbers SHI-006R, SHI-021S and SHI-022S. Representations of the fins and their design data are given in Figure 6, Figure 7 and Figure 8, respectively.

The results of the tests, at a particular water speed (or Reynolds number (Re)), are shown in Figure 9 and Figure 10. Figure 9 is a graph showing the lift curve resulting from plotting the lift co-efficient against the angle of attack. As can be seen from the lift curves, the lift co-efficient increases substantially linearly with increasing angle of attack up until a critical angle (eg stall angle) beyond which the lift co-efficient plateaus or decreases, owing to the fin being in (or approaching) a stall situation.

Figure 10 is a graph showing the drag co-efficient of the relevant fins against the lift co-efficient. As can be seen, as the lift co-efficient increases the drag coefficient also increases. Up until a particular region of the graph, the drag coefficient increases generally linearly with increases in the lift co-efficient. However, at a particular region in the graph, the drag co-efficient rapidly increases as the lift co-efficient increases. This region represents the situation where the fins have gone into a stall or partial stall situation.

Test 2 - Effect of Camber

Tests were conducted on different surfboard side fins in the abovementioned cavitation tunnel to quantify the lift and drag characteristics of side fins having varying cambers. In particular, side fins having substantially identical design data but having different cambers were tested. The reference number of each fin tested and its camber are listed below. SHI-007R 2% camber (see Figure 11). • SHI-008R 4% camber (see Figure 12).. SHI-012R 6% camber (see Figure 13). SHI-013R 8% camber (see Figure 14). SHI-010R Selig Donovan 7043 Foil (Figure 15). SHI-011R S3025 Foil (see Figure 16). * (*Sourced from the University of Indiana's Public Domain Foil Section Library) The results of the tests for the abovementioned fins are reflected in the "lift coefficient v. angle of attack" graphs referred to below.

• SHI-007R - see Figure 17.

• SHI-008R - see Figure 18. • SHI-012R - see Figure 19.

• SHI-013R - see Figure 20.

• SHI-01 OR - see Figure 21.

• SHI-011 R - see Figure 22.

Graphs showing comparative lift drag curves for various of the above tested fins are shown in Figure 23 and Figure 24.

The results of these tests showed that increasing the section camber from 2% to * 4% increases the maximum lift capacity of a fin by about 10% with a corresponding increase in drag at 0° angle of attack of approximately 40%. The maximum lift capacity of the 6% camber fin was about 10% higher than for the 4% camber fin. The trade off was a 50% increase in the drag at 0° angle of attack. The maximum lift capacity for the 8% camber fin is another 10% higher than for the 6% camber fin. However, the drag at 0° angle of attack is 60% greater than for the 6% fin. Given the strong drag trade-off in high cambered fins, the results indicate that cambers of 6% or more generally have unacceptably high drag characteristics.

Figures 25A to 25G are schematic representations of water flow over a surfboard side fin (according to an embodiment of this invention) at varying angles of attack, namely 0°, 5°, 10°, 15°, 20°, 25° and 30°, respectively. As can be seen, from 0° up to about 20°, increasing the angle of attack results in a corresponding increase in the lift applied to the fin. As shown in Figure 25F, at a 25° angle of attack, the flow of water over the fin is non-uniform and the fin is in a partial stall situation. The full stall situation is shown in Figure 25G where the angle of attack is 30° and there is a major disruption of the water flow over the fin.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the aspects of the invention, as shown in the specific embodiments contained herein, without departing from the spirit or scope of the invention as broadly described herein. The presently described embodiments are to be considered in all respects as illustrative of the invention and not restricting the scope of the invention.

Claims

The claims defining the invention are as follows:

1. A cambered airfoil-shaped surfcraft side fin having a camber of from about 2% to about 6%, wherein said camber is substantially constant throughout length of the side fin from adjacent a base to adjacent an end tip of said side fin.

2. A surfcraft side fin according to claim 1 , wherein said camber is in the range of from about 3% to about 5%.

3. A surfcraft side fin according to claim 2, wherein said camber is about 4%.

4. A surfcraft side fin according to any one of claims 1 to 3, wherein said side fin has a sweep angle of from about 25° to about 50°.

5. A surfcraft side fin according to claim 4, wherein the sweep angle is from about 30° to about 45°.

6. A surfcraft side fin according to claim 5, wherein the sweep angle is about 35°.

7. A surfcraft side fin according to any one of claims 1 to 6, wherein said surfcraft is a surfboard.

8. A surfcraft side fin according to claim 7, wherein said side fin has a profile area of between about 7000mm² and about 11 ,000mm².

9. A surfcraft side fin according to claim 8, wherein the profile area is between about 8000mm² and about 10,000mm².

10. A surfcraft side fin according to claim 9, wherein the profile area is about 9000mm².

11.A surfcraft side fin according to any one of the preceding claims, wherein said side fin has a span of between about 90mm and about 140mm.

12. A surfcraft side fin according to claim 11 , wherein the span is between about 110mm and about 125mm.

13. A surfcraft side fin according to claim 12, wherein the span is about 120mm.

14. A surfcraft side fin according to any one of the preceding claims, wherein said side fin has an aspect ratio of between about 2.5 and about 4.0.

15. A surfcraft side fin according to claim 14, wherein the aspect ratio is between about 3.0 and about 3.5.

16. A surfcraft side fin according to claim 15, wherein the aspect ratio is about 3.2.

17. A surfcraft side fin according to any one of the preceding claims, wherein said side fin has a thickness:chord (t/c) ratio which is substantially the same at all points from adjacent the base to adjacent the end tip of said side fin.

18. A surfcraft side fin according to claim 17, wherein said t/c ratio is from about 6% to about 12%.

19. A surfcraft side fin according to claim 18, wherein said t/c ratio is from about 8% to about 10%.

20. A surfcraft side fin according to claim 19, wherein said t/c range is about 9%.

21.A surfcraft comprising one or more surfcraft side fins according to any one of claims 1 to 20.

22. A surfcraft according to claim 21 , wherein said surfcraft comprises two of said side fins, one being located between a centre line of the surfcraft and a first side rail of the surfcraft and the other of said side fins being located between the centre line and a second side rail of said surfcraft, wherein the side fins are oriented to provide lift when substantially only one of said side fins is travelling through water.

23. A surfcraft according to claim 22, wherein said surfcraft further comprises a centre fin located on said centre line and positioned rearward of the two side fins.

24. A surfcraft according to claim 22 or claim 23, wherein each of the two side fins are toed inwards.

25. A surfcraft according to claim 24, wherein each of the two side fins are₍ toed inwards by about 4° relative to the centre line.

26. A surfcraft according to any one of claims 1 to 25, wherein each of said fins has a cant angle of from about 7° to about 12°.

27. A surfcraft according to claim 26, wherein said cant angle is about 10°.

28. A surfcraft according to any one of claims 1 to 27, wherein said side fin has a rounded leading edge.

29. A surfcraft side fin substantially as hereinbefore described with reference to any one or more of Figs 4A to 4C, 5A to 5C, 11 A to 11 C, 12A to 12C, 13A to 13C, 14A to 14C, 15A to 15C, 16A to 16C and 26.