WO2016033651A1

WO2016033651A1 - Watercraft fin, fin assembly or fin attachment

Info

Publication number: WO2016033651A1
Application number: PCT/AU2015/050517
Authority: WO
Inventors: Brian Young
Original assignee: Blue Movement Pty Ltd
Priority date: 2014-09-03
Filing date: 2015-09-03
Publication date: 2016-03-10
Also published as: AU2015311617A1; AU2015311617B2

Abstract

Fin assembly, fin or fin attachment for watercraft such as, for example, surfboards, stand up paddle boards, windsurfers, kayaks, boats or the like. The assembly, fin or attachment typically being such to provide an elongate planar extension in the direction of the trailing edge of the main fin body.

Description

WATERCRAFT FIN, FIN ASSEMBLY OR FIN ATTACHMENT Field of the invention

[001] The present invention relates to fins, fin assemblies, of fin attachments for watercraft such as, for example, surfboards, stand up paddleboards, windsurfers, kayaks, boats or the like.

Background

[002] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[003] Conventional surfboards are typically formed of a foam core reinforced with fiberglass and/or epoxy resin. Generally fins are attached to the underside of surfboards to aid with control and manoeuvring while wave riding.

[004] Surfboard fins are available in a wide array of shapes, sizes and have different levels of flexibility. Typically a surfer will select fins in line with their surfing style or for a particular board type (e.g. fish, longboard, thruster).

[005] It is also not unusual to see a wide range of fin set ups/arrangements on surfboards. For example, some boards have a single fin set up, while others have a three fin (thruster) or four fin arrangement (quad).

[006] In seeking to improve performance, some fin manufacturers have attempted to include a foil or lateral component to their fins to increase lift of the board while wave riding. However these attempts have so far failed to provide a substantial performance increase so as to warrant take up by the mainstream surfing community. Summary of the Invention

[007] In one broad form the present invention provides a fin assembly for a watercraft, including: a main fin body; and an extension member, releasably engageable with the main fin body.

[008] In various forms, the extension member is substantially planar, flexible and/or elongate. In one form, the main fin body includes a leading edge and a trailing edge, the extension member being releaseably attachable to the main fin body to extend from the main fin body in the direction of the trailing edge.

[009] In one form, the main fin body includes a slot extending inwardly from the trailing edge for receiving the extension member. In a further form, the main fin body includes a substantially central wall portion.

[0010] In another form, the extension member is substantially oval shaped. In a further form the extension member has a rounded end, and a tapered end leading to a tip region. In one form, at the tapered end, the lateral edges of the extension member are concave.

[0011] In a further form, when the main fin body and extension member are in the engaged position, the lateral edges of the extension member at the tip region of the tapered end are substantially flush with the lateral surfaces of the main fin body.

[0012] In one form, the extension member includes a slot therein extending inwardly from the tip region so as to provide two fingers. In one form, when the main fin body and the extension member are engaged, the fingers of the extension are adapted to sandwich the central wall portion.

[0013] In a further form, the slot of the main fin body is formed in a thickened portion of the main fin body. In one form, the thickened portion of the main fin body is a bulbous portion.

[0014] In one form, an inner wall of the slot in the main fin body includes a protrusion therein for engagement with a corresponding recess in a surface of the extension member. [0015] In one form, the extension member is substantially planar with a first surface substantially opposite a second surface. In a further form, the lateral edges of the extension member are at least partly rounded between the first and second surfaces. In one form, the first surface curves to meet the second surface at an edge.

[0016] In a further form, the extension member is biased to resist flection in one direction. In another form, the extension member includes a stiffening member. In one form, the extension member is formed of a plurality of layers. In one form, the stiffening member is a stiffening layer. In another form, the extension member is formed of a base layer and stiffening layer.

[0017] In another form, the stiffening layer acts to resist flexion of the base layer in one direction. In one form, the stiffening layer and base layer are at least partially separable.

[0018] In one form, the watercraft is a surfboard.

[0019] In a further form the present invention provides a main fin body for a fin assembly as described in any of the above forms. In another form the preset invention provides an extension member for a fin assembly as described in any one of the above forms.

[0020] In a further broad form, the present invention provides a fin for a watercraft, the fin including: a main fin body including a leading edge and a trailing edge; and an extension portion extending from and in the direction of the trailing edge of the main body. In one form, the watercraft is a surfboard.

[0021] In various forms, the extension portion is substantially planar, substantially flexible and/or elongate. In one form, the extension portion is substantially oval shaped.

[0022] In one form, the extension portion is planar and substantially perpendicular to the plane of the main fin body.

[0023] In one form, the extension portion has a leading end and a trailing end, the extension portion decreasing in thickness from the leading end to the trailing end. [0024] In a further broad form the present invention provides a fin attachment for a fin of a watercraft, the fin including a leading edge and a trailing edge, wherein the fin attachment is configured to attach to the fin so as to extend from and in the direction of the trailing edge.

[0025] In one form, the fin attachment is substantially planar. In another form the attachment is elongate. In one form, the attachment is substantially flexible.

[0026] In one form, the fin attachment is substantially oval shaped. In another form, the attachment having a first end and a second end, the first end including a slot for receiving the trailing edge of the fin.

Brief description of the drawings

[0027] This invention may be better understood with reference to the illustrations of embodiments of the invention in which:

Figure 1 is a perspective view of a fin assembly according to one example;

Figure 2 is a perspective view of the fin assembly of figure 1 with extension portion detached;

Figure 3 is a side view of the fin assembly of figure 1;

Figure 4 is a side view of the fin assembly of figure 1 with the extension member detached; Figure 5 is rear view of the main fin body of the fin assembly of figure 1;

Figure 6 is a front view of the main fin body of the fin assembly of figure 1 with extension member engaged;

Figure 7 is a cross sectional side view of the fin assembly of figure 1;

Figure 8 is a top view of the fin assembly of figure 1;

Figure 9 is a top view of the extension member of the fin assembly of figure 1;

Figure 10 is a side view of a further example of a fin assembly;

Figure 11 is a side view of the fin assembly of figure 10, with the extension member detached;

Figure 12 is a rear view of the fin assembly of figure 10;

Figure 13 is a rear view of the main fin body of the fin assembly of figure 10; Figure 14 is a top view of the fin assembly of figure 10

Figure 15 is a top view of the extension member of figure 10;

Figure 16 is rear view of a variation of the fin assembly of figure 10;

Figure 17 is rear view of a variation of the fin assembly of figure 10;

Figure 18 is a perspective view of a main fin body and extension/attachment according to one example;

Figure 19 is a side view of the fin extension/attachment in figure 18 showing flexion during parallel water flow;

Figure 20 is a side view of the fin extension/attachment in figure 18 showing flexion with water flow toward the underside of a watercraft;

Figure 21 is a side view of the fin extension/attachment in figure 18 showing flexion with water flow away from the underside of a water craft;

Figure 22 is one example of a fin with integrated extension according to one example; and Figure 23 is one example of a fin with integrated extension according to one example.

Detailed description of the drawings

[0028] Embodiments of the present invention provide fin assemblies, fins or fin attachments for watercraft. Generally, the assemblies, fins or attachments are such to provide an elongate extension in the direction of the trailing edge of the main fin body. Typically, the extension is substantially planar and substantially flexible yet resilient. It will be appreciated that watercraft may include, for example, surfboards, stand up paddle boards, windsurfers, kayaks, boats or the like.

[0029] Figures 1 to 9 show one example of the invention when embodied as a fin assembly for a surfboard. The fin assembly (100) includes a main fin body (102) and an extension member (103) that is releasably engageable with the main fin body (102).

[0030] The main fin body (102) resembles that of a conventional surfboard fin with a base portion (106) and base elements (107) for engagement with a fin box at the underside of a surfboard. It will be appreciated that that the base portion/elements of the main fin body may vary according to the fin connection method (i.e. to correspond with a particular board type or particular fin box e.g. 'Fin Control Systems (FCS)' type or 'Future' type connection mechanisms). It will also be appreciated that in some forms the base portion of the main fin body (102) may be permanently fixed to the underside of a surfboard (e.g. using fiberglass or otherwise).

[0031] The main fin body includes (102) a leading edge (104) and a trailing edge (105) and opposite lateral surfaces (108, 109). It would be appreciated by a skilled person that the leading edge would typically be considered as the edge facing forward during forward motion of the surfboard and the trailing edge as the edge facing rearward. As would also be appreciated by a skilled person, the general shape of the main fin body may vary depending on the purpose/nature of the fin/fin assembly (e.g. for longboard use, use as a side fin or a center fin etc.). For example, the overall shape, curvature of the leading or trailing edge, and/or the contours of the lateral surfaces of the main fin body may vary.

[0032] The extension member (103) is releaseably attachable to the main fin body (102) to extend from the main fin body (102) in the direction of the trailing edge (108) (i.e. in rearward direction from main fin body). The attachment is such that the extension member (103) would extend substantially parallel to the underside of a surfboard to which the main fin body (102) would be attached. It will be appreciated that in alternate forms the attachment may be such that the extension member extends at angle with respect to the underside of the watercraft (i.e. non parallel).

[0033] The plane of the extension member (103) is substantially perpendicular to the plane of the main fin body (102). However, it will also be appreciated that this may not always be the case, and in some forms, the plane of the extension member may be angled otherwise (i.e. non perpendicular) with respect to the main fin body.

[0034] In the example of figures 1 to 9, the main fin body (102) includes a slot (110) extending inwardly from the trailing edge (105) for receiving the extension member (103). The extension member (103) is typically formed of a material that is flexible yet resilient. As shown most clearly in figures 2 and 9, the extension member (103) is substantially planar and elongate with a generally oval shape. However, whilst one end (115) of the extension member is rounded like a typical oval, the opposite end (116) tapers to a tip region (117). The lateral edges (118, 119) of extension member are for the most part convex, however, at the tapered end (116), the lateral edges include concave regions (120).

[0035] The extension member (103) includes a slot (111) therein extending inwardly from the tip region (117) for engagement with the slot (110) of the main fin body (102). The slot (111) forms two fingers (112, 113) and during engagement with the main fin body (102), the fingers (112, 113) in the extension member sandwich a central wall portion (114) in the main fin body slot (110). The central wall portion (114) extends partway the length of the main fin body slot (110), and the extension member slot (111) is typically sized to fit the central wall portion (114). It will be appreciated that engagement with the central wall portion (114) typically contributes to reducing lateral movement of the extension member (103) when engaged to the main fin body (102). Also during engagement, the inner walls (121, 122) of the slot (110) in the main fin body (102) sandwich the extension member (103), contactingly engaging opposite planar surfaces (130, 131) of the extension member.

[0036] As shown in figures 1 and 8, the nature of the tapered end (116) of the extension member (103) is such that when engaged to the main fin body (102), the lateral edges (118, 119) of the extension member (103) at the tip region (117) of the tapered end (116) are substantially flush with the lateral surfaces (108, 109) of the main fin body (102). The lateral edges of the tips (140, 141) of the fingers matching outer surface profile of the main fin body (102).

[0037] The slot (110) of the main fin body (102) is formed in a thickened/bulbous portion (160) relative to the rest of the main fin body (102). Having the slot formed in a thickened/bulbous portion, makes the inner walls (121, 122) of the slot (110) larger/wider and thus provides a greater contact area with the opposite surfaces (130, 131) of the extension member (103). The bulbous portion (160) thereby contributing to the security of the engagement between the main fin body and extension member.

[0038] To further contribute to secure engagement between the main fin body and the extension member, one of the inner walls (122) of the slot (110) in the main fin body includes a protrusion (125) therein for engagement with a corresponding recess (126) in a surface (130) of the extension member. The protrusion (125) is located on the slot internal wall (121) after the termination of the central wall portion (114). Similarly the recess (126) is correspondingly positioned at the end of slot (111) in the extension member (103).

[0039] In use, the extension member/attachment (103) is typically slidingly engaged into the slot (110) by hand with a reasonable force, and an audible 'click' sound indicates the extension member has snapped locked into place. The extension member (103) may be also typically removed by hand by applying suitable pulling force to the extension member (103) and, if necessary, flexing the main fin body (102) slightly.

[0040] It will also be appreciated that in other embodiments the positioning of the protrusion and corresponding recess may differ and may be on other corresponding surfaces of main fin body and extension member. For example, in other forms, it may be that the protrusion extends from the extension portion to engage with a recess found in an inner wall in the slot of the of the main fin body.

[0041] The above described connection mechanism is advantageous in that it allows the extension member/attachment to be interchanged without any tools. The extension member/attachment may therefore be easily removed/interchanged in the water or on land, as weather and water conditions change.

[0042] Some variations of this embodiment may also include a grub screw or other mechanical fastening element to further secure the extension member with the main fin body.

[0043] Generally, the extension member extends from the main fin body at a height from the base (106) part way to the tip or top (117) of the main fin body. It will be appreciated that the height at which the slot in the main fin body is positioned and the therefore the height at which the extension member extends from the main body may be variable. For example, in some embodiments the extension member (103) may extend from the top or tip of the main fin body, or nearer to the base portion of the main fin body. [0044] As most clearly shown in figures 7 and 9 the extension member (103) is substantially planar with a first surface (130) opposite a second surface (131) and the lateral edges (118, 119) of the extension member are at least partly rounded between the surfaces. In one example form, the first surface curves to meet the second surface at hard or pronounced edge line (or vice versa). In some forms, one or more of the surfaces of the extension member (103) may also include concave regions or shallow depressions therein.

[0045] It will be appreciated that in alternate forms the connection/engagement between extension member (103) and the main fin body (102) may take other forms. Also, in some embodiments, the main body and extension member may be provided as a unitary construction or the extension member permanently fastened to the main fin body. Furthermore, a fin attachment with characteristics of the extension member may be provided for attachment to a conventional fin body that does not include a receiving slot.

[0046] Another particular embodiment of the fin assembly is shown in figures 10 to 15, wherein an alternate extension member is illustrated. This embodiment shows main fin body (701) for releasable attachment with an extension member (702) that is more completely oval shaped with both leading and trailing ends (715, 716) being substantially rounded. The extension member (702) is typically flexible yet resilient and the main fin body (701) is configured to receive and engage the extension member (702) such it is retained securely in the main fin body (701) until release or interchange is required.

[0047] The main fin body (701) includes an elongate slot (705) extending from the trailing edge (708) toward the leading edge of the fin (709). The slot (705) is configured for receiving the extension member (702), however, in this embodiment, there is no central wall portion within the slot (705).

[0048] As best shown in figure 15, the attachment/extension member (702) also includes a slot (706) therein to slidingly engage with the slot (705) of the main fin body (701). When engaged, the fingers (712, 713) sandwich the main fin body (701), contactingly engaging the outer lateral surfaces (717, 718) of the main fin body. Similarly, the internal walls (707,711) of the slot (705) of the main fin body (701) sandwiches the extension member (702). This configuration typically contributes to stabilising the extension member, for example from lateral movement (i.e. in the plane of the extension member) or tilting.

[0049] It will also be appreciated that the width of the slots (705, 706) in the main fin body (701) and extension member (702) are typically configured such that a snug fit is provided between extension member (702) and main fin body (701) (i.e. the width of the slot (705) in the main fin body (701) substantially matches the thickness the extension member (702) and vice versa).

[0050] In one form (see figures 12 and 13), the thickness of the main fin body (701) increases to provide a bulbous or thickened region around the slot. The slot (705) extends within this bulbous region (710) which, as described in respect of the earlier described embodiment, provides larger slot side walls and thus a larger contact area between the inner walls (707, 711) of the slot (705) of the main fin body (701) and any inserted extension member (702). Thus the bulbous portion can contribute to further stabilisation (e.g. from tilting) of the extension member (702). The bulbous portion typically has a curved outer surface in seeking to encourage laminar/smooth water flow over the fin. However, it will be appreciated that in alternate embodiments, the region around the slot may vary. For example, figures 16 and 17 show other possible variations around the slot region including no substantial change in thickness (figure 17) and an increase in thickness towards the slot with concave outer surface portions (810) (figure 16).

[0051] To assist with secure engagement, one of the inner side walls (707) of the slot (705) includes a protrusion (703). The protrusion (703) is substantially hemispherical/pimple/dome shaped and is for engagement with a corresponding recess/depression (704) in the extension member (702).

[0052] Whilst the extension member/attachments of the previously described embodiments have been of a single body or layer, it will be appreciated that this may not always be the case. For example, the extension member/attachment may be formed of a plurality of layers. Furthermore, the extension member may also be such that that is based to allow flection more readily in one direction. A stiffening member or portion may be included to achieve this, and/or to simply stiffen portions of the typically flexible extension member.

[0053] A further particular embodiment is shown in figures 18 to 23, which illustrate one example wherein the fin assembly (1) includes of multi layered extension member (3) engaged to a main fin body (2). The extension member (3) includes a leading end (3 a) and a trailing end (3b), with the leading end (3a) being attachable to the main fin body (2). Typically, the leading end (3a) includes a slot extending towards the trailing end (3b) for receiving the main fin body (2). However, it will be appreciated that the extension member may be connected in a variety of ways, such as, for example, those described in relation to other embodiments.

[0054] The extension member/attachment (3) has a curved leading edge (4) in seeking to reduce drag as the main fin body and extension member/attachment travel through the water. In this example, the extension member is substantially oval shaped has an oval perimeter. The fin attachment (3) is attached to the fin such that it extends from the leading edge of the fin (2a) beyond the trailing edge (2b).

[0055] The extension member (3) of the fin assembly (1) includes a stiffening member or layer (7). The stiffening member (7) acts such that the extension member is biased to be resistant to flection in one direction (e.g. toward the surfboard). The extension member (3) is thus formed of a base layer (6) and a stiffening member/layer (7). The stiffening layer (7) extends from the leading end of the base layer (3a) part way toward the trailing end (3b). It will be appreciated that whilst the base layer may be biased to be resistant to flexion in one direction it may not be completely prevented flexion in that direction.

[0056] Typically, the stiffening layer extends over a portion of an area of the base layer and it will be appreciated that the stiffening layer may also be flexible. The stiffening layer generally sits between the base layer and the underside of the watercraft so as to resist flection of the base layer towards the watercraft surfboard. [0057] Without wishing to be bound by theory figures 19 to 21 illustrate proposed flexion of the extension member (3) during different water flow conditions (such as when travelling through different parts of a wave). As shown in figure 19 when the direction of water flow is substantially parallel to the extension member/ attachment (3), the attachment (3) remains in a non-flexed/neutral state. Figure 20 illustrates how the attachment (3) flexes toward the surfboard when water flow is upward toward the underside of the surfboard. This may be the case when a surfer is travelling down the face of a wave. The stiffening layer (7) acts to resist flection of the base layer (1) which deflects water flow to provide additional forward propulsion to the surfboard (or other watercraft).

[0058] During turbulent flow, or when water is flowing in a direction to flex the attachment away from the underside of the surfboard (see figure 21), the base layer flexes accordingly. However, the base layer is more readily flexed in this direction (away from the surfboard) as there is no stiffening member/layer as on that side. The base layer therefore more gradually returns to the neutral position. As shown in figure 21, the base layer may be separated from the stiffening layer in this type of water flow. The base layer and stiffening layer may or may not be are at least partially separable.

[0059] Where the surfboard is not in trim or is involved in turbulent flow the attachment/extension will return to the neutral position or readily deflect downward due to the leaved one way flex configuration of the layers and thus accommodate the existing conditions without increase in drag (see figure 21).

[0060] Whilst flexion of the extension member (3) is illustrated in relation to the fin assembly (1) of figures 18 and 19, it will be appreciated similar flexion capability/functionality may be provided by other forms of the fin assembly and extension members/attachments/portions, such as those described in figures 1 to 17.

[0061] It will be appreciated that the invention may be provided as: a fin assembly including a main fin body configured to be releasably attachable with an extension member; a stand alone fin attachment for attachment to a conventional fin body; main fin body with extension portion formed as a single unitary construction; or main fin body with permanently fixed extension portion. Typically, the extension member/attachment/portion is elongate and formed of a material or combination of materials which provides it is flexible yet resilient. In some forms, the elongate axis of the extension member/attachment/portion extends substantially in line or parallel to the plane of the main fin body (or the line between the leading and trailing edge of the main fin body).

[0062] For some forms of the invention one aim is to deflect the sub surface laminar flow of a wave to increase the speed of a surfcraft. When riding a wave for example, the upward water pressure of the wave may be generally constant and hence thrust generally constant as long as the surfboard is kept in trim on the wave by the rider. Up and down oscillations of the extension member/attachment/portion are not required to produce a constant thrust.

[0063] In a position on a wave where flow is parallel to the bottom of the surfboard, the extension/attachment is in a neutral position creating minimum thrust. As a wave increases in steepness and flow is more from beneath the attachment, bearing upwards (i.e. towards the surfboard), the extension member/attachment/portion are able to flex and provide a deflection of water flow to the rear which creates a thrust in a forward direction (e.g. figure 20).

[0064] If the extension member/attachment/portion is too rigid and not flexible, it may create a destabilizing deflection upon the surfboard as well as a drag due to unsatisfactory angle of incidence. The surfboard would therefore tend to tip forward out of trim as well as lose speed and maneuverability. Hence, when the extension member/attachment/portion is flexible the surfboard is able to maintain its trim with the extension member/attachment/portion assuming a natural deflection that accommodates both the balanced descent of the surfboard down the face of the wave as well as the upward relative movement of water from the wave while producing thrust in the forward direction. Thus, the forms of the invention have particular advantages over existing rigid winged fins.

[0065] It will be appreciated that extension member/attachment/extension portion typically has a relatively narrow frontal/leading aspect rather than a wide frontal aspect. A wider frontal aspect may lead to greater drag and therefore, a decrease in performance. The extension member/attachment/portion typically has an elongate shape in seeking to maximize deflection and compensate for the narrow frontal aspect. A shorter/stubbier shape may result in lesser deflection of the attachment/extension and therefore may result less thrust.

[0066] It will also be appreciated that the extension member/attachment/portion may include other features not shown in the embodiments of the figures. For example, the extension member/attachment/portion may have a thickness that decreases from the leading end to the trailing end. Accordingly, as the thickness decreases towards the trailing end, the amount of flex may increase. It will be appreciated that in extreme cases, where the decrease in thickness is substantial, the attachment/extension may not considered be substantially planar.

[0067] Furthermore, the shape of the extension member/attachment/portion may vary. For example, rather that having a substantially oval shape, the extension member/attachment/portion may have a squared off leading and/or trailing end. For example, the extension member/attachment/portion may be substantially rectangular or rectangular with slightly rounded lateral sides. Similarly, the lateral edges may not be rounded between opposing planar surfaces of the extension member. For example, a square or other edge may be provided between opposing planar surfaces (e.g. between top and bottom surfaces). Furthermore, either of the substantially planar surfaces of the extension member/attachment/portion may also include one or more concaved regions and/or channels therein.

[0068] In other forms, the extension member/attachment/portion may have a concave underside and/or may curve downward (i.e. away from the watercraft) from leading end to trailing end. The extension member/attachment/portions may also have hydrofoil characteristics such as appropriately cambered surfaces. It will be appreciated the extension member/attachment/portion, when planar, may be perpendicular or angled otherwise with respect to the plane of the main fin body. It will also be appreciated the extension member/attachment/portion may be such that it extends parallel or angled (i.e. non parallel) with respect to the underside of the watercraft. [0069] The elongated planar shape of some parts may reflect the generic shape of a surfboard and may exhibit similar characteristics in behavior during manoeuvers, albeit beneath the surface of the water.

[0070] Further advantages of the extension member/attachment/portion may be realised when used in aerial surfing or kite surfing. In particular, the horizontal foil/attachment cushions the landing of a surfboard (or kite board or other water craft) from an aerial position as it breaks the surface tension of the water prior to the underside of the surfboard hitting the water surface. It will also be appreciated that in some forms, a main fin body may be used with a range of different shaped extension members (or vice versa). For example, a user may elect to change their extension member/attachment to suit changing surf or weather conditions.

[0071] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[0072] Although a preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

[0073] It will be appreciated that various forms of the invention may be used individually or in combination.

Claims

The claims defining the invention are as follows:

1. A fin assembly for a watercraft, including: a main fin body; and an extension member, releasably engageable with the main fin body.

2. A fin assembly as claimed in claim 1, wherein the extension member is substantially planar.

3. A fin assembly as claimed in claim 1 or 2, wherein the extension member is flexible.

4. A fin assembly as claimed in any one of the preceding claims, wherein the extension member is elongate.

5. A fin assembly as claimed in any one of claims 1 to 4, wherein the main fin body includes a leading edge and a trailing edge, the extension member being releaseably attachable to the main fin body to extend from the main fin body in the direction of the trailing edge.

6. A fin assembly as claimed in claim 5, wherein the main fin body includes a slot extending inwardly from the trailing edge for receiving the extension member.

7. A fin assembly as claimed in claim 6, wherein the slot of the main fin body includes a substantially central wall portion.

8. A fin assembly as claimed in any one of the preceding claims, wherein the extension member is substantially oval shaped.

9. A fin assembly as claimed in any one of the preceding claims, wherein the extension member has a rounded end, and a tapered end leading to a tip region.

10. A fin assembly as claimed claim 9, wherein at the tapered end, the lateral edges of the extension member are concave.

11. A fin assembly as claimed in claim 9 or 10, wherein in the engaged position, the lateral edges of the extension member at the tip region of the tapered end are substantially flush with the lateral surfaces of the main fin body.

12. A fin assembly as claimed in any one of claims 9 to 11, wherein the extension member includes a slot therein extending inwardly from the tip region so as to provide two fingers.

13. A fin assembly as claimed in claim 12, wherein when enagaged, the fingers of the extension are adapted to sandwich the central wall portion.

14. A fin assembly as claimed in claim 6, wherein the slot of the main fin body is formed in a thickened portion of the main fin body.

15. A fin assembly as claimed in claim 14, wherein the thickened portion of the main fin body is a bulbous portion.

16. A fin assembly as claimed in claim 6, wherein an inner wall of the slot in the main fin body includes a protrusion therein for engagement with a corresponding recess in a surface of the extension member.

17. A fin assembly as claimed in any one of the preceding claims, wherein the extension member is substantially planar with a first surface substantially opposite a second surface.

18. A fin assembly as claimed in claim 17, wherein the lateral edges of the extension member are at least partly rounded between the first and second surfaces.

19. A fin assembly as claimed in claim 17 or 18, wherein the first surface curves to meet the second surface at an edge.

20. A fin assembly as claimed in any one of the preceding claims, wherein the extension member is biased to resist flection in one direction.

21. A fin assembly as claimed in claimed in any one of the preceding claims, wherein the extension member includes a stiffening member.

22. A fin assembly as claimed in any of the preceding claims, wherein the extension member is formed of a plurality of layers.

23. A fin assembly as claimed in claim 22, wherein the stiffening member is a stiffening layer.

24. A fin assembly as claimed in claim 22 or 23, wherein the extension member is formed of a base layer and stiffening layer.

25. A fin assembly as claimed in claim 24, wherein the stiffening layer acts to resist flexion of the base layer in one direction.

26. A fin assembly as claimed 24 or 25, wherein the stiffening layer and base layer are at least partially separable.

27. A fin assembly as claimed in any one of the preceding claims, wherein the watercraft is a surfboard.

28. A main fin body for a fin assembly as claimed in any one of the preceding claims

29. An extension member for a fin assembly as claimed in any one of claims 1 to 27.

30. A fin for a watercraft, the fin including: a main fin body including a leading edge and a trailing edge; and an extension portion extending from and in the direction of the trailing edge of the main body.

31. A fin as claimed in claim 30, wherein the extension portion is substantially planar.

32. A fin as claimed in claim 31, wherein the extension portion is substantially flexible.

33. A fin as claimed in any one of claims 30 to 32, wherein the extension portion is elongate.

34. A fin as claimed in any one of claim 30 to 33, wherein the extension portion is substantially oval shaped.

35. A fin as claimed in claim 31, wherein the planar extension portion is substantially perpendicular to the plane of the main fin body.

36. A fin as claimed in any one of claims 30 to 35, wherein the watercraft is a surfboard.

37. A fin as claimed in any one of claims 30 to 36 , wherein the extension portion has a leading end and a trailing end, the extension portion decreasing in thickness from the leading end to the trailing end.

38. A fin attachment for a fin of a watercraft, the fin including a leading edge and a trailing edge, wherein the fin attachment is configured to attach to the fin so as to extend from and in the direction of the trailing edge.

39. A fin attachment as claim in claim 38, wherein the fin attachment is substantially planar.

40. A fin attachment as claimed in claim 38 or 39, wherein the attachment is elongate.

41. A fin attachment as claimed in any one of claim 38 to 40, the attachment having a first end and a second end, the first end including a slot for receiving the trailing edge of the fin.

42. A fin attachment as claimed in any of one claims 38 to 41, wherein the attachment is substantially flexible.

43. A fin attachment as claimed in claim any one of claims 38 to 42, wherein the fin attachment is substantially oval shaped.