Patent Publication Number: US-2013244514-A1

Title: Fin for surf craft

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fins and methods for making them as may be applied to surf craft such as surfboards, windsurfers, paddleboards, wave and surf skis, kite-boards, wake boards and the like. 
     2. Description of the Art 
     Surf craft (including surfboards) often have one or more fins located on an underside of the surf craft that, for example, may be used for stability, controlling direction and facilitating turning of the surf craft. In addition surfboards may have multiple fins with different functions, for example an uppermost side fin with a curved or airfoil profile may function so as to provide to provide lift when the surfboard is travelling across the face of a wave and the uppermost side fin is located within the face of the wave. It also follows that extra acceleration and drive to the surfboard results. 
     Fin/s of turning surf craft may experience substantial side ways and other forces to the face of the fin/s. How the fin/s respond to these sideways and other forces in turns and other manoeuvres may strongly affect the performance of the surf craft for a particular set of surf conditions. The construction of a fin may in particular affect its response to sideways and other forces in use. 
     Current surfing trends, particularly in competitive surfing, involve multiple, high speed, sharp turns of a surfboard whilst a wave is being ridden. Such manoeuvring of a surfboard places very significant forces on the fins of the surfboard. Under such forces, the fins tend to experience bending (e.g. between the base and the tip of the fin) and twisting (e.g. between the leading and trailing edges of the fin). The fin&#39;s ability to return sharply to its normal state following the removal of the experienced force (e.g. via a turn) affects the performance of the fin and, consequently, the surfboard. 
     Commonly available fins for surfboards may be a composite structure of layers of bi-directional fibreglass fabric imbedded in a suitable resin and then moulded and/or shaped to the form of a fin. The word “bi-directional” in the following is taken to include the direction of the fibreglass strands within the closely woven fabric. The fibreglass strands being often made up of multiple fibres or filaments of fibreglass. Bi-directional fibreglass or other reinforcing fabric often has a basket weave pattern where the strands are closely interwoven orthogonally to form the fabric. 
     The reinforcing fibreglass fabric together with the impregnating resin or other suitable material typically determines the physical properties of the fin in terms of, by way of example, the stiffness characteristics, bending resistance, twisting resistance and/or flexibility of the fin to sideways and other forces in a turn or other manoeuvres. However for typical fins, varying the stiffness characteristics, flexibility or other such properties of the fin in an easily manufacturable and controllable fashion is difficult due to the many layers of reinforcing fabric with impregnating resin matrix contributing to the stiffness or flexibility across the fin. There is also the additional limitation of what is commercially available in reinforcing fabrics and the strand materials forming them. 
     None of these prior art fin devices and methods of construction for fins provides an entirely satisfactory solution to the provision of fins for surf craft where the desired stiffness characteristics and other physical properties may be varied in a controllable fashion, nor to the ease of providing a convenient and reliable way of manufacturing fins having different degrees of stiffness or other desirable physical properties. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide an alternative method for constructing a fin in which the stiffness characteristics and other physical properties of the fin may be better controlled and/or varied as well as to the provision of fins with different, controlled stiffness characteristics which overcomes or ameliorates the disadvantages of the prior art, or at least provides a useful choice. 
     In one form, the invention provides a fin for surf craft comprising: a fin body and at least one layer of structural strands, located within the fin body; wherein the structural strands are in one or more non-woven arrangements; and the structural strands have a physical property greater than a corresponding physical property of other material forming the fin body; and wherein the physical property is selected from at least one of a toughness, a tensile strength, an elastic moduli and a Youngs modulus. Preferably at least a portion of the structural strands extend substantially from a base portion to a tip portion of the fin. Preferably at least a portion of the structural strands extend substantially from a base portion to a leading edge portion of the fin. Preferably at least a portion of the structural strands extend substantially from a leading edge portion to a trailing edge portion of the fin. Preferably at least one layer of structural strands in one or more arrangements is located within the fin body such that the at least one layer of structural strands is substantially parallel to opposing faces of the fin. 
     Preferably the structural strands of at least one layer are substantially parallel to each other. Preferably in a first arrangement, the parallel structural strands are generally parallel to a sweep angle of the fin. In an alternate first arrangement, the parallel structural strands are at a first angle to a sweep angle of the fin, the first angle being in the range of up to 20 degrees, more preferably the parallel structural strands are at a first angle of approximately 10 degrees to a sweep angle of the fin. Preferably a second arrangement the parallel structural strands are at a second angle to the vertical of the fin, the second angle being in the range of 20 to 40 degrees more preferably the parallel structural strands are at a second angle of approximately 30 degrees to the vertical of the fin. Preferably a third arrangement, the parallel structural strands are generally vertical. 
     Preferably in a primary arrangement, the parallel structural strands are generally perpendicular to a sweep angle of the fin. Preferably in a secondary arrangement, the parallel structural strands are at a first angle to a sweep angle of the fin, the first angle being in the range of 20 to 40 degrees, more preferably the parallel structural strands are at a first angle of approximately 30 degrees to a sweep angle of the fin. Preferably in a tertiary arrangement, the parallel structural strands are generally vertical. 
     Preferably at least one layer of structural strands comprises of a plurality of structural strands extending from at least one substantially common point in a substantially radial formation. Preferably at least one substantially common point is adjacent the base portion of the fin. Preferably at least substantially common point is adjacent at least one of a leading edge portion and a trailing edge portion of the fin. 
     Preferably at least one structural strand comprises of a plurality of filaments. Preferably at least one structural strand is made of at least one of carbon fibre, Kevlar, aramide, natural fibres and synthetic fibres. Preferably at least one structural strand has a tensile strength that is at least 1.5 times greater than the tensile strength of the other material forming the fin body. Preferably at least one structural strand has a Youngs modulus that is at least 1.5 times greater than a Youngs modulus of the other material forming the fin body. Preferably at least one structural strand has a toughness that is greater than a toughness of the other material forming the fin body. Preferably at least a portion of the structural strands comprises unidirectional filaments in a ribbon configuration. Preferably at least a portion of the structural strands have a width in the range of 0.5 to 3 mm. Preferably at least a portion of the structural strands has a width in the range of 1 to 2 mm. Preferably at least a portion of the structural strands comprises of at least about 3,000 filaments per structural strand. 
     Preferably a spacing between at least a portion of the structural strands is less towards the base portion compared with the tip portion of the fin. Preferably a spacing between at least a portion of the structural strands is in the range of 1 to 30 times a width of one structural strand, more preferably a spacing between at least a portion of the structural strands is in the range of 4 to 13 times a width of one structural strand. Preferably a spacing between at least a portion of the structural strands is in the range of 4 to 15 mm, more preferably a spacing between at least a portion of the structural strands is in the range of 9 to 13 mm. 
     In one form, the invention provides a fin for surf craft comprising: a fin body; and at least one layer of structural strands, located within the fin body; wherein the structural strands are in one or more woven arrangements that are at least one of an open weave and a scrim; wherein the structural strands have a physical property greater than a corresponding physical property of other material forming the fin body; and wherein the physical property is selected from at least one of a toughness, a tensile strength, an elastic moduli and a Youngs modulus. Preferably, further including a core structure located within the fin body. Preferably at least one layer of structural strands in one or more arrangements is embedded within a body of the fin such that the layer of structural strands is substantially parallel to a face of the core structure. Preferably at least one layer of structural strands are located intermediate the core structure and at least one of the opposing faces of the fin. Preferably the core is at least one of a foam core structure and a solid, non-foam core structure. Preferably at least a portion of the core structure is made of at least one of PVC foam, polyurethane foam, resin impregnated fibreglass, hardened resin, polyester mat, microspheres, plastic, bamboo and wood. 
     Preferably further including at least one layer of unidirectional carbon fibre fabric towards a base portion of the fin body, more preferably at least one layer of carbon fibre fabric is located about a periphery of the fin body. 
     Preferably a sweep angle of the fin is in the range of 20 to 60 degrees. 
     In yet another form, the invention provides a method of controlling a fin physical property for a surf craft, the method comprising: selecting one or more structural strands having a structural strand physical property greater than a corresponding physical property of other materials in a body of the fin; selecting a number of structural strands to provide the fin physical property; providing a layer of the structural strands in one or more arrangements; and embedding the layer of structural strands in the body of the fin; whereby varying at least one of the structural strands selection, the number of structural strands or the arrangement of the structural strands varies the fin physical property; and wherein the fin physical property is selected from at least one of: a stiffness characteristic, a bending resistance, a twisting resistance, a resistance to a deflection, a flexibility and a high elastic recoil; and wherein the structural strand physical property is selected from at least one of: a toughness, a tensile strength, an elastic moduli and a Youngs modulus. Preferably the step of providing a layer of structural strands includes the use of a template to locate one or more structural strands of one or more arrangements. Preferably the step of using a locating template further includes providing at least one of pins, adherents and securing systems to locate one or more structural strands. Preferably the step of using a locating template further includes the steps of: providing one or more reliefs machined into the template, and laying individual structural strands into respective reliefs to form a three dimensional structural strand layer. Preferably the step of providing a layer of structural strands includes the use of a numerically or a computer controlled machine to locate one or more structural strands of one or more arrangements. 
     Preferably the step of providing a layer of structural strands further includes a step of: configuring the arrangement of structural strands in a layer to vary the fin physical property. Preferably further including providing one or more structural strands largely parallel to a sweep angle of the fin such that the fin is provided with an increased resistance to a twisting of the fin. Preferably further including providing one or more structural strands at a first angle of up to 20 degrees to a sweep angle of the fin to provide the fin with an increased resistance to a twisting of the fin. Preferably further including providing one or more structural strands at a second angle in the range of 20 to 40 degrees to the vertical axis of the fin such that the fin is provided with an increased resistance to a deflection from the vertical axis. 
     A fin for surf craft produced according to the methods described above 
     In an alternate form, the invention provides a fin for surf craft substantially as described herein and a method of controlling a stiffness characteristic or other desired physical property of a fin for a surf craft substantially as described herein. 
     Further forms of the invention are as set out in the appended claims and as apparent from the description. 
    
    
     
       DISCLOSURE OF THE INVENTION 
       Brief Description of the Drawings 
       The description is made with reference to the accompanying drawings; of which: 
         FIG. 1  is a perspective, representative view of a surfboard 
         FIG. 2  is a side elevation view of a fin from the surfboard of  FIG. 1 . 
         FIG. 3  is a bottom view of a fin of  FIG. 2 . 
         FIG. 4  is a “peel-away” or partially exploded perspective view of a side fin in an embodiment of the present invention. 
         FIG. 5  is a side elevation view of the fin embodiment of  FIG. 4 . 
         FIG. 6  is a plan view of a template board. 
         FIG. 7  is a schematic showing a layup of three arrangements of a structural strands layer embodiment on the template board. 
         FIG. 8  is an enlarged view of the circled region in  FIG. 7 . 
         FIGS. 9 to 13  are respective side, plan, end and bottom elevation views of a FEA analysis of homogeneous fin under an applied force. 
         FIGS. 14 to 18  are the same elevation views of the fin of  FIGS. 9 to 13  with no force applied. 
         FIGS. 19 to 42  are to further embodiments of the invention in side elevation views only, unless otherwise indicated. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective, representative view of a surfboard  110  to illustrate the main features associated with surfboards in general. The surfboard  110  has a board  112  with a deck  114  that the surfer stands on. The board  112  has a nose  116 , a tail  118  and two rails  120  defining the generally longitudinal edges of the board  112 . To an underside  122  of the board  112  one or more fins  124 ,  126  are typically attached, usually towards the tail  118  but for high performance surfboards and other surf craft the fins may be located at a variety of locations along the underside of the board. The surfboard  110  illustrated as an example has three fins  124 ,  126  in a “thruster” configuration however surfboards may also have one, two (“twin fin”), four or more fins in a variety of configurations. An outside face  310  and inside face  312  of the side fin  124  are described in detail below with respect to  FIG. 3 . 
     The overall general axes of orientation to a surfboard  110  may be a vertical axis  128 , a transverse or “sideways” axis  130  and a longitudinal or “stringer” axis  132 . 
       FIG. 2  is a side elevation view of a fin  124 ,  126  to additionally illustrate the main features associated with a fin. The fin may have a base  210  with attachment features, tabs or attachment means  212  which enable the fin  124 ,  126  to be suitably attached or secured to the underside  122  of the board  112 . It will be readily appreciated that there may be a variety of attachment or securing means for a fin to an underside of a board. The fin may have a leading edge  214  that is towards the nose  116  of the board  112  and a trailing edge  216  which is towards the tail  118 . A tip  218  of the fin may be also used to define a sweep angle  220  of the fin, as shown with the dashed lines, with respect to the vertical axis  128 . For reference in the following detailed description a rotation or twist  222  about the vertical axis  128  of the fin  124 ,  126  may occur in use. Alternatively a rotation or twist component/s may occur about an axis corresponding to the dashed line in  FIG. 2  corresponding to the sweep angle from the base  210  to the tip  218  of the fin. A vertical height or depth  224  dimension from the base  210  to the tip  218  of the fin may also be defined as shown in  FIG. 2 . The base  210  may have a base length  226  dimension as shown in  FIG. 2 . 
       FIG. 3  is a bottom view of the fin in  FIG. 2 . The fin shown is an example of a side fin  124  where an outside face  310  may be more curved than an inside face  312  of the fin  124 . The outside face  310  of the fin  124  corresponds to the face closest to the rail  120  of the board  112  whilst the inner face  312  is the opposing face to the outer face  310 . The different respective curvatures of the faces  310 ,  312  are configured to form an airfoil which induces a sideways hydrodynamic force upon the side fin  124  and thereby providing lift, as the fin travels through a wave and in particular across the face of a wave. 
     The side fin/s  124  and/or centre fin/s  126  may also experience a variety of other hydrodynamic forces upon them during turns and complex manoeuvres which may cause them to deflect and/or twist from their at rest positions. 
       FIG. 4  is a “peel-away” or partially exploded perspective view of a side fin  410  in an embodiment of the present invention. For reference  FIG. 5  is a side elevation view of the fin  410  of  FIG. 4 , viewing the outside face  310 . In  FIG. 4  a core  412  may optionally be included to reduce the weight of the fin, provide positive buoyancy in water and/or as further described below. The core  412  may be a solid (non-foam) or a foam core, where a foam core includes air pockets within that may be partially or fully filled with impregnating resin. Solid cores may be made of resin impregnated fibreglass, hardened resin, plastic, bamboo or wood. Alternatively, foam cores may be PVC foam, polyurethane (PU) foam or an advanced foam core materials such as Lantor Coremat as described at www.lantor.nl. The Lantor Coremat being a nonwoven polyester mat containing microspheres. Layers of fibreglass fabric  414  may optionally be present, the two layers illustrated in  FIG. 4  being only illustrative. Many more layers of fibreglass fabric  414  may be present on either side of the core  412  depending on the particular fin type and shape being designed/manufactured. An optional outer layer of black polyester veil  416  for each face  310 ,  312  of the fin may be included to promote resin flow, as well as improve the external finish and appearance of the fin  410 . A further, optional outer layer of uni-directional carbon fibre fabric  418  may be included near the base  210  of the fin  410 ; possibly extending to the attachment means  212  to improve stiffness and strength at those parts of the fin body. 
     A description of the commonly available materials used to manufacture fins as illustrated in  FIG. 4  is provided in the following by way of example only. The fibreglass fabric  414  may be of 6 oz, close, plain weave or of other readily available fibreglass reinforcing fabrics. The core may be a PVC foam of a 1.3 to 2.5 lb/cubic foot density PVC foam, black silk or polyester veil and urn-directional carbon fibre fabric of 300 gsm (grams/metre squared) weight. It will be readily appreciated that these commonly available materials may be varied in terms of whether they are included in a fin body and what may be selected for their use as would be exercised by a person skilled in the art of surf craft, surfboards in particular, design and manufacture. 
     The embodiment of the invention in  FIG. 4  shows a structural strand layer  420  that may include high tensile carbon fibre strands  422 ,  424  and high tensile strength and toughness Kevlar  426  strands. Structural strand examples are described in detail further below. The layer of structural strands  420  features structural strands which may have a tensile strength substantially greater than the other materials typically used in a fin body. The use of a discrete structural strand layer  420  provides an economic and ready technique to vary and control a stiffness characteristic physical property of the fin in a variety of directions or about a variety of axes of rotation; without varying the other common components that may be used in a fin body. 
     The term “stiffness characteristic” as a physical property in the following detailed description and claims is taken to include:
         The resistance of the fin to deflection or twist forces in a variety of directions. Or in other words a twisting resistance and/or a bending resistance.   The flexibility of the fin in a variety of directions or about a variety of axes.   High elastic recoil or restoration of the fin after a force or a twist is applied to it is released. For example the snapping back of the fin after it has been deflected due to forces applied during a turn or complex manoeuvres. In other words energy or work put into the fin in a turn is returned, with little or no loss, to the surfer or rider of a surf craft at the completion of a turn.   Stiffness, resilience and/or flexibility physical properties imparted to the fin by the combination of various materials of various tensile strengths, elastic moduli and other properties into the fin construction.       

     In addition toughness as a physical property in the following detailed description and claims is taken to include a comparatively moderate tensile strength material with improved ductility, for example Kevlar/aramide fibres may have a higher toughness compared with carbon fibres. Fibres with superior toughness have a high degree or resistance to repeated twisting and/or bending. 
     The carbon fibre strands  422  may be largely parallel to the sweep  220  of the fin  410  or offset from the sweep angle by up to 20 degrees, preferably approximately 10 degrees in the example shown in  FIG. 4 . The structural strands  422  may be introduced as a first arrangement to control how much the fin resists twisting, in particular about the vertical axis. Or in other terms how much energy is retained or stored in the fin from its twisting in use. The use of more structural strands in the first arrangement will increase the resistance to twisting by the fin. The carbon fibre strands  424  that are offset to the vertical by 20 to 40 degrees, preferably approximately 30 degrees, may be introduced as a second arrangement to modify how much the fin resists deflection from the vertical axis. Preferably the direction of the offset to the vertical for strands  424  is towards the fin leading edge  214 . The use of more structural strands in the second arrangement will increase the resistance to deflection from the vertical axis for the fin. As a general comment, the stiffness characteristic that may be imparted by a structural strand, to a fin, being largely in the longitudinal/length direction of the strand and proportional to the number of structural strands and the structural strand physical properties. The largely vertical Kevlar strands  426  may be additionally introduced as a third arrangement to improve the stiffness characteristic as well as the toughness and strength of the fin so that it may resist breakage. 
     A description of the structural strands used in the structural strand layer  420  is provided in the following by way of example only. “3 k” (3,000 filaments per strand) unidirectional carbon fibre strands in a largely ribbon form, “toe” form, may be used. “3 k” unidirectional Kevlar or Aramide equivalents strands in a substantially ribbon form may be used. Typically the ultimate tensile strengths of carbon and Kevlar/Aramide fibres may be at least 1.5 times or 2 times (2×) or more than commonly used fibreglasses such as E-glass and more than the other commonly used materials in a fin body. Similarly the elastic moduli such as Youngs modulus for carbon fibre and Kevlar/Aramide equivalents may be at least 1.5 times (1.5×), 2 times, 5 times or more than commonly used fibreglasses such as E-glass and more than the other commonly used materials in a fin body. The width of ribbon strands may be in the approximate range of 0.5 to 3 mm or more preferably in the range of 1 to 2 mm. The ribbon strands may have a thickness. The thickness of a ribbon strand may be greater than 0.1 mm. Natural fibres and synthetic fibres (in addition to those mentioned already) may also be suitable with appropriate resin, plastic and/or binder systems. It will be readily appreciated that these structural strand materials may be readily varied in terms of what may be selected for their use as would be exercised by a person skilled in the art of surf craft, surfboards in particular, design and manufacture. Furthermore the person skilled in the art would be guided in their choice of structural strands by their superior physical properties in comparison to the other materials used in the construction of the fin; for example carbon fibre over fibreglass and via a property such as tensile strength. 
     In one embodiment a layer of the structural strands may be fabricated by use of an aluminium template  610  as shown in  FIG. 6  in plan view. The template may have marked upon it the outline  612  of the fin  410 , marker lines  614  for the location for the carbon fibre strands  422  in the sweep direction or angle  220 , marker lines  616  for the carbon fibre strands  424  that may be 30 degrees from the vertical axis of the fin outline  612  and marker lines  618  for the Kevlar strands  426 . 
       FIG. 7  shows the layup of three arrangements of structural strands  422 ,  424 ,  426  on the template board  610  to form a layer of structural strands  420 . A first parallel arrangement  710  of carbon fibre strands  424  that are offset approximately 30 degrees to the vertical axis of the fin outline  612  may be hand laid first. The spacing between the parallel carbon fibre strands  424  may be chosen to be in the range of 9 to 13 mm for a fin of approximate depth  224  of 100 mm. A second parallel arrangement  712  of Kevlar strands  426  may then be laid down. The spacing between the parallel Kevlar strands  426  may be chosen to be in the range of 4 to 8 mm for a fin of depth  224  of 100 mm. The Kevlar arrangement may then be followed by a third parallel arrangement  714  of carbon fibre strands  422  that may be in the sweep angle  220  direction of the fin outline  612 . The spacing between the parallel carbon fibre strands  422  may be chosen to be in the range of 9 to 13 mm for a fin of depth  224  of 100 mm. It will be readily appreciated that the numerical values for strand spacing and orientation are to obtaining a particular stiffness characteristic and are only illustrative. For example a fin of approximate depth of 111 mm may have a spacing between the parallel carbon fibre strands  424  in the range of 9 to 15 mm or more preferably in the range of 10 to 12 mm. Other examples are given below with respect to  FIGS. 19 to 42 , where the relative, proportional and/or angular relationships between the structural strands are shown. 
     To aid in the laying up of the strands for each arrangement, pins or other locating, fixing, securing or otherwise aid devices (not shown) may be used at the periphery of the template  610  to locate and/or secure the strands in a desired arrangement. More complex arrangements or configurations may also be laid up and these are described below in detail with respect to  FIGS. 19 to 42 . For these more complex arrangements further locating/securing systems such as pins, adherents and the like may be used to facilitate the forming of more complex arrangements of the structural strands. 
     In the course of laying up the arrangements  710 ,  712 ,  714  of structural strands the individual strands may be impregnated with a suitable resin or binder in order that overlapping structural strands may be adhered together.  FIG. 8  is an enlarged view of the circled region in  FIG. 7 .  FIG. 8  shows the resin or binder  810  adhering overlapping  812  structural strands  422 ,  424 ,  426  together. The template board  610  may be pre-coated with a release agent to prevent the adhering of the resin or binder  810  to the template  610 . 
     In the above example of forming a structural strand layer  420  the layer is not woven, that is the structural strands are not interlaced. In addition the layer is in the form of a scrim with clear apertures  814 . From the above examples of ribbon strand widths and strand spacing the relative clear aperture may be from approximately 1 to 30 times (1×-30×) one ribbon strand width or more preferably from 4 to 13 times (4×-13×) one ribbon strand width. In further embodiments of the structural strand layer, described in detail below with respect to  FIGS. 19 to 42 , the structural strands from various arrangements may have some or all of their strands interlaced in some fashion to form a woven arrangement or scrim for the structural strand layer. 
     The technique for forming the structural strand layer may also be adapted to a computer or numerically controlled apparatus to manufacture the structural strand layer. A numerically controlled (NC) machine (and/or computer controlled) may be particularly suited for the arrangements/configurations described below with respect to  FIGS. 19 to 42 . For example an embroidery machine may be adapted to lay out the structural strand layer. 
     The scrim structural strand layer may then be die or otherwise cut into the desired outline which for the example above is the full outline  612  of the fin. The structural strand layer may then be appropriately inserted into a mould of a fin with the other fin components, for example described above with respect to  FIG. 4 . In order to form the fin body a suitable resin system or plastic together with possible additives such as fillers and/or colour agents may then be injected into the mould to impregnate all the reinforcing fabrics and the structural strand layer to form the fin body. Resin Transfer Moulding (RTM) is one common example of a mass production technique for forming the fin. Compression moulding may also be used, by way of example. 
     Alternatively the scrim structural strand layer may be directly removed from the template board  610  without cutting to the fin outline  612 . The scrim structural strand layer may then be appropriately incorporated into a traditional fin panel of fibreglass sheet and resin, formed by machine and/or hand. A desired fin may then be machine cut (for example NC machine) from the fin panel incorporating the structural strand layer. The machine cut fin may then be hand finished and polished. 
     Without wishing to be bound by theory, Finite Element Analysis (FEA) may be readily done for a typical homogeneous fin (not incorporating a structural strand layer).  FIGS. 9 to 13  show the results of an FEA model of a homogeneous fin being subjected to a force applied to the normal of face  312  of a side fin  124 ,  410 . The applied force simulates the sideways force that a side fin may experience when: travelling across the face of a wave, in a manoeuvre and/or when a fin is at a high angle of attack to the bulk fluid flow stream under the surfboard.  FIG. 9  is a side view,  FIG. 10  is a plan view,  FIG. 11  is an end view,  FIG. 12  is a bottom view and  FIG. 13  is a front view. Contour lines  910  to  918  have been placed on each of the views to show the amount of horizontal displacement of the fin, from its rest position, by an applied force. Contour line  910  is approximately 20 mm at the tip  218 , contour line  912  is approximately 13 mm, contour line  914  is approximately 10 mm, contour line  916  is approximately 3 mm and contour line  918  at the secured base  210  is 0 mm. For comparison purposes  FIGS. 14 to 18  are views of the same fin with no force applied.  FIG. 14  is a side view,  FIG. 15  is a plan view,  FIG. 16  is an end view,  FIG. 17  is a bottom view and  FIG. 18  is a front view. 
     It is apparent from  FIGS. 9 to 18  that a side fin travelling along the face of a wave, or otherwise as per above, may bend sideways in the direction of the transverse axis  130  as well as twisting/rotating about the vertical axis  128 . Altering the stiffness characteristic of such a fin by incorporating a structural strand layer may readily affect the response of the fin to applied forces in a number of directions. 
     The technique described above for producing a structural strand layer allows for arrangements or configuration of the structural strands within the structural strand layer which may be very difficult or impossible to attain with commercially available stock reinforcing fabrics. In the following figures of  FIGS. 19 to 42  further embodiments of the invention are illustrated in side elevation views only.  FIGS. 19  to  42  primarily illustrate the layup of the structural strands; the other common components of a fin have been omitted for clarity. In addition in  FIGS. 19 to 42  the Kevlar strands have been omitted for clarity as well as indicating that they may be considered optional. 
     In a number of the  FIGS. 19 to 42  a core  412  may be shown, but as for the embodiments disclosed above: the core  412  is an optional component. However in some instances in the below the core may also serve as a useful locational reference where the embodiment may have two structural strand layers or arrangements of a structural stand layer continue over two layers about the core. An example of a structural strand for the embodiments of  FIGS. 19 to 42  may be carbon fibre strands. 
       FIGS. 19 and 20  are the opposing side elevation views of a fin  1910  featuring a structural strand layer with two arrangements. The first arrangement  1912  has radial structural strands  1912  with a common origin  1914  at the intersection of the base  210  and leading edge  214  of the fin  1910 . Or in other words, the structural strands may extend from one common point to form a radial pattern or formation. The second arrangement  1916  has arc strands  1912  with a common arc centre being also the origin  1914 . In this structural strand layer the first and second arrangements may be laid up either in a non-woven or woven (interlaced) manner to form a scrim. However in comparison to commercially available reinforcing fabrics there are no substantially unidirectional structural strands or structural strands that are orthogonal to each other over their full length of use within the structural strand layer. 
       FIGS. 21 and 22  are again opposing side elevation views of a fin  2110 . The first arrangement  2112  also has radial structural strands  2112  but with a virtual origin (not shown) below the base  210 . The second arrangement  2114  is also radial structural strands but with a different virtual origin (not shown) which is below the base  210  but forward of the leading edge  214 . 
       FIGS. 23A and 23B  are again opposing side elevation views of a fin  2310 . However this structural strand layer features two arrangements of partially continuous radial strands. The first arrangement  2312  of radial strands originates from a virtual origin (not shown) to the rear of the trailing edge  216 . The radial strands  2312  radiate to the base  210  and leading edge  214 . At the leading edge  214  a portion of the radial strands  2312  are re-directed (or “reflected”) from the leading edge  214  to form a second arrangement of continuing radial strands  2314  that continue to the base  210 . This structural strand layer embodiment may have the effect of providing additional structural strands and consequently stiffness to the base  210  of the fin  2310  in comparison to the portion of the fin  2310  towards the tip  218 . 
       FIGS. 24 to 26  illustrate two related fin embodiments  2410 ,  2510  where both structural strand layers originate from the leading edge  214 . The first arrangement of structural strands  2412 ,  2512  radiates to the tip portion  218  of the fins  2410 ,  2510 . The second arrangement  2414 ,  2514  radiates to the base  210  and lower portion of the trailing edge  216 . However the second embodiment  2510  employs the use of a core  412  to separate a first arrangement  2512  from a second arrangement  2514 . 
       FIGS. 27 and 28  are a related embodiment to  FIG. 5 , however the structural strand layer for fin  2710  has only one arrangement  2712  of structural strands and the strand arrangement is slightly radiused with a substantial portion of the structural strands being in the general direction of the sweep  220  of the fin  2710 . The fin  2710  also features a portion of uni-directional carbon fibre fabric  418  as described for  FIGS. 4 and 5 . 
       FIGS. 29 and 30  are to a fin  2910  embodiment where the structural strand layer may have two arrangements of structural strands with the individual strands being continuous through both arrangements. The first arrangement  2912  of largely parallel structural strands projects in a generally vertical direction from the base  210  and then executes a fold over  2916  or strand re-direction as produced on the template  610  or the like. The re-direction  2916  of the structural strands may be such that the structural strands again continue in a parallel fashion for the second arrangement  2914  directly to the mid section of the trailing edge  216 . However the second arrangement  2914  features substantially closer adjacent structural strands than for the first arrangement  2912 . Such a reinforcing layup may not be achievable with commercial reinforcing fabrics. 
     The stiffness characteristic of the fin  2910  in the region of the second arrangement  2914  may be higher than that of the region of the first arrangement  2912  due to the combined effect of the reduced spacing between adjacent structural strands together with the overlap between the second  2914  and first  2912  arrangements. Accordingly the fin  2910  may have stiffness characteristic of being very stiff towards the base and in particular for a portion to the mid section of the trailing edge  216  but with a particularly flexible or whip-like tip  218 .  FIG. 30  shows the presence of a mirror structural stand layer  2912 ″,  2914 ″ to  FIG. 29 , which may further promote the stiffness characteristic described. 
       FIGS. 31 and 32  illustrate a fin  3110  embodiment with a structural strand layer with a first arrangement  3112  and a second arrangement  3114  to also vary the stiffness characteristic in different portions or regions of the fin  3110 . The first arrangement  3112  of parallel structural strands may feature a first narrow spacing  3116  and second larger spacing  3118  between adjacent structural strands. The first arrangement  3112  projects from a tip portion  218  towards the base  210  along the general sweep angle  220  direction. At a re-direction band  3120  the structural strands may be redirected approximately orthogonally as shown. The redirection  3120  may be such that in the second arrangement  3114  spacing between adjacent structural strands is uniform. This structural strand layer for fin  3110  may achieve a greater stiffness characteristic for the base portion of the fine  3110  compared with the rest of the fin body. This fin  3110  embodiment may have an advantage to that described with respect to  FIGS. 4 and 5  in that the urn-directional reinforcing fabric  418  may not be necessary. 
       FIGS. 33 and 34  are to a fin  3310  embodiment similar to that of  FIGS. 29 and 30 ; where the structural strand layer may have two arrangements of structural strands with the individual strands being continuous through both arrangements. However the first arrangement  3312  from the leading edge  214  portion of the base extends generally towards the tip  218 . At a re-direction or fold-over band  3316  the first arrangement  3312  is twisted through 180 degrees to form the second arrangement  3314  which continues to the tip  218  as shown. 
       FIGS. 35 and 34  are to a fin embodiment  3510  with four arrangements of structural strands. The first  3512  and second  3514  arrangements may be a zigzagged arrangement from one edge of the fin to another edge to approximately the mid portion of the fin  3510  as shown in  FIG. 35 . The first  3512  and second  3514  arrangements may be overlayed or interlaced. In  FIG. 36  the third  3612  and fourth  3614  arrangements are also shown in a zigzagged fashion, but extending from the mid-portion of the fin  3510  to the tip  218 . 
       FIG. 37  is a fin embodiment  3710  where the first arrangement  3712  zigzags up the leading edge  214  with one side of the first arrangement interlaced/woven into the second arrangement  3714  which zigzags up the trailing edge  216 , from base  210  to tip  218 . 
       FIG. 38  is to a fin embodiment  3810  that is an alternate embodiment to that of  FIG. 37 . In  FIG. 38  the structural strand layer  3812  features lighter gauge structural strands  3814 ,  3816  but in a higher density/pitch in the weaving/interlacing. This fin  3810  embodiment of the structural strand layer may have an increased stiffness to the leading edge  214  but allows the rest of the fin  3810  to twist and flex. 
       FIGS. 39 and 40  are to a fin embodiment  4010  where a three dimensional structural strand layer  3912  may be formed by the use of a template block  3910  with a relief machined  3914  into it. The structural strand layer  3912  may have the individual structural strands  3916  laid up into the relief  3914 . Once all the strands  3916  have been placed a layer of resin may then be applied to form the three dimensional structural strand layer  3912  as a shell. The three dimensional structural stand layer  3912  may then be incorporated into a fin body as described previously; however because of the relief of this structural strand layer  3912 , it may be positioned close to the surface of the fin face  310 . One or more layers of fibreglass fabric may be located between the fin face  310  surface and the three dimensional structural strand layer  3912 . 
       FIGS. 41 and 42  are to another fin embodiment  4110  incorporating a number of elements from the prior embodiments described above. In this embodiment  4110  the primary arrangement  4112  of structural strands generally originates from the base  210  of the fin and may then be directed to the fin leading edge  214 . The primary arrangement may then be folded over or re-directed at the fin leading edge  214  to then continue as the secondary arrangement  4114  of structural strands proceeding generally to the fin trailing edge  216  as shown. It will be readily appreciated that the folding over or redirecting from the first to the secondary arrangement may be achieved using the lay-up template  610  described above with respect to  FIGS. 6 and 7 . For example the fold over or redirection may be slightly offsetted to the fin leading edge as allowed for by use of the lay-up template  610 . Alternatively the lay-up may be with two different carbon strands for each arrangement, the intersection of the strands for the primary and secondary arrangement being along all or part of the leading edge of the fin. 
     The spacings between the structural strands of the primary and secondary arrangements  4112 ,  4114  vary from the base  210  to the tip  218  so as to provide an increased stiffness characteristic towards the base  210  of the fin. A reduced spacing of the structural strands towards the base consequently increases the stiffness characteristic as well as providing a gradient of the stiffness characteristic across the depth of the fin. 
     The carbon fibre strands of the secondary arrangement  4114  may be largely perpendicular to the sweep angle of the fin as shown in  FIGS. 41 and 42 . Whilst the carbon fibre strands of the primary arrangement  4112  may be offset to the secondary arrangement  4114  by an angle in the range of 20 to 40 degrees or preferably approximately 30 degrees. 
     The primary and secondary arrangements  4112 ,  4114  of structural strands may be analogous to the embodiments of  FIGS. 23A ,  31  and  35 , for example. The closer spacing of the structural strands towards the fin base  210  may be analogous to  FIGS. 23A and 31  for example. 
     In  FIGS. 41 and 42  a core  412  is shown which for this embodiment may be of Lantor Coremat as previously described or any other suitable material. In  FIGS. 41 and 42  the core  412  is shown on one side of the two arrangements  4112 ,  4114 , however as described in detail below the structural strand arrangements or scrims may be on both sides of the core  412  as may be used for the centre fin of a thruster configuration,  FIG. 1 , whilst the single sided structural strand arrangement of  FIGS. 41 and 42  may be for a side fin of a thruster configuration. In an alternate embodiment for a centre fin a scrim/structural strand arrangement may be sandwiched between two cores such that the centre fin has the appearance of  FIG. 42  from both sides. 
     Optionally, another arrangement of largely horizontal, parallel fibreglass strands  4116  may be further included in the fin construction. Alternatively the tertiary arrangement  4116  may use structural strands of Kevlar or aramide equivalents instead of fibreglass in order to improve the toughness performance of the fin as well as its stiffness characteristic. The fin embodiment  4110  may be constructed using RTM injection with vinyl ester as described above. 
     The embodiments of  FIGS. 19 to 42  are also examples of how the spacing and gauge of the structural strands may differ between different structural strand layers and between different strand arrangements within a structural strand layer. 
     It will be readily appreciated that elements from the described embodiments may be used to formulate other embodiments of the invention and still be within the scope of the invention. 
     In addition, between side fin/s  124  and centre fin/s  126  of surfboards the number and type of structural strand layers may differ. A greater stiffness characteristic for the centre fin  124  compared with the side fins  126  may be obtained by the use of a structural strand layer imparting a greater stiffness characteristic and/or multiple structural strand layers. For example: to the multiple structural strand layers for a centre fin, two structural strand layers may be used, one on each side of the core  412 . In addition the choice of a core material and the dimensions of the core may also be varied in order to further change the stiffness characteristic or toughness of a fin. It will be readily appreciated that greater stiffness for a fin may be also achieved by changing the fin geometry/shape but this would also impact upon the hydrodynamic drag and other hydrodynamic properties. 
     The above described method and product of using a discrete structural strand layer allows the stiffness characteristics in terms of the amount of stiffness and distribution of the stiffness to be readily varied across the face of the fin and thru the fin body. For example to produce a component of twist about the horizontal/longitudinal axis of a fin. In addition the deflection and twist characteristics of stiffness may be varied from one face to the other face of a fin by either the layup of strands within an arrangement of a discrete structural strand layer and/or the position of the structural strand layer within construction of the fin. Fins with customised, multi-axis deflection and twist characteristics may be readily produced and tested. The technique disclosed here may be suitable for both small experimental and custom-built production runs common in surf craft fin research and development work and custom-built professional competition supply as well as readily adaptable to mass production of a fin product range with particular stiffness or flexibility characteristics. 
     For surfboards a fin product range incorporating a structural strand layer may be, for example, to:
         A surf board rider&#39;s proficiency, strength and style of surfing. For example experienced surfers may prefer a stiffer fin range to improve surfboard performance. Professional surfers may require a custom-built fin with a stiffness characteristic tailored to their particular requirements.   A surfboard rider&#39;s weight: heavier surfers may require stiffer fins to maintain hold through turns. The term “hold” is often used to describe the level of slippage movement of the tail of the surfboard during turns, particularly aggressive turns.       

     An example fin product range for surfboards may have the approximate dimensions and angles of:
         “Large”, a depth/height  224  dimension of 119 mm, a base length  226  dimension of 118 mm and a sweep angle  220  of 34 degrees.   “Medium”, a depth/height  224  dimension of 113 mm, a base length  226  dimension of 111 mm and a sweep angle  220  of 34 degrees.   “Small”, a depth/height  224  dimension of 110 mm, a base length  226  dimension of 105 mm or 109 mm and a sweep angle  220  of 34 degrees.   “Custom-Built/Competition”, a depth/height  224  dimension of 119 mm, a base length  226  dimension of 114 mm and a sweep angle  220  of 36 degrees.   Sweep angles for surfboard fins according to the invention may be in the range of 20 to 60 degrees or more preferably in the range 26 to 56 degrees or in another preferred embodiment approximately 33 degrees.       

     A broad, simple example of a stiffness characteristic specification for a fin product range may be the amount of horizontal displacement of the fin tip  218  to an applied force as described above with respect to  FIGS. 9 to 18 . By way of example fins with various structural strand layers may provide a range in horizontal displacements from 5 to 25 mm or 10 to 20 mm of the tip  218  for applied forces typical in variety of surfboard uses. 
     Without wishing to be bound by theory we believe that the ability to readily vary the stiffness characteristic across a fin may enable further improvements in the performance of a surfboard in the areas of:
         Stall characteristics   The hold of the fin/s during a turn and complex manoeuvres.   The sensation of “drive”/acceleration into and out of a turn. Stiffer fins tend produce a greater sensation of drive.   The responsiveness of a surfboard may be affected by the stiffness of the fin/s. Stiffer fins may result in a more responsive surfboard. A more forgiving surfboard may result from more flexible fin/s.   When transitioning from one turn to another a stiff fin with a high degree of elastic recoil may provide increased speed and acceleration from one turn to another as the surfboard transitions from one side fin to the opposing side fin.   Flex: To make a fin that performs more efficiently the inventors had to ensure it could flex in multiple directions. This invention&#39;s technology is the latest development in fin flexion which draws on the material lay-up of the fin, the cambered foil, and the overall fin template. The result is a multi-directional flex pattern. This unique flex pattern allows the fin to ‘load-up’ and flex under pressure, and then de-coil once the pressure is released. Ultimately the fin stores energy during the transition between turns and then gives it back to the surfer in the form of superior speed and acceleration. The feeling can be compared to a slingshot, or whipping effect as the surfer enters and then exits through the turning arc.   Foil: A highly efficient foil in combination with the invention can be the defining element that makes for exceptional fin performance. The highly cambered foil in the base of the fin provides drive and hold, the low cambered foil in the tip provides stability and allows the fin to release with control, even when the fin is pushed to the limits. This cambered foil also increases the fin&#39;s stall angle which helps to produce down-the-line speed and maintain projection through the entire turning arc.   Template: The fin with the invention may feature an efficient, low aspect ratio elliptical template. The long base increases drive, moderate volume in the tip enhances the flex and coil characteristics, and the smooth transitional trailing edge reduces water separation, which is traditionally linked to cavitation. Translated, this means increased speed and drive through minimal water disturbance.   Construction: Visually it&#39;s easy to see how technology and performance overlap. Structurally, the fin may draw on a combination of engineered Bi-axial Carbon (via two arrangements of uni-directional Carbon) and Uni-directional Kevlar to achieve the invention&#39;s flex pattern. The Uni-directional carbon fibre fabric ( 418 ) base further increases stiffness in the base of the fin, and helps to distribute pressure away from the plugs (of the surfboard) by reducing the twisting forces on the fin tabs securing the fin to the board. The Resin Transfer Moulding (RTM) process delivers consistency across manufacturing and guarantees the integrity of the flex and foils. Epoxy resin may be used to provide strength and material stability, while a lightweight moulded core further reduces the overall weight of the fin.       

     It will be readily appreciated that the above described method for readily altering the stiffness or flexibility properties of a fin of a surfboard may be readily applied to other surf craft such as windsurfers, paddleboards, wave and surf skis, kite-boarding, wake boards, and the like. 
     Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiments, it is recognized that departures can be made within the scope of the invention, which are not to be limited to the details described herein but are to be accorded the full scope of the appended claims so as to embrace any and all equivalent assemblies, devices and apparatus. 
     In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise, comprised and comprises” where they appear. 
     It will further be understood that any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates.