BACKGROUND TO THE INVENTION It has long been recognized that fins are essential for stability of watercraft and particularly surf craft. Surfboards commonly employ a single fin on the underside of the board at the rear. The fin is centrally placed between the sides of the board, which are known as the rails.

As surfboard riders became more skilled they shortened the boards to increase maneuverability. These shorter boards were able to make much tighter turns on a wave then the earlier long boards.

However, the shorter boards had less stability and were therefore much harder to ride. This problem was addressed by employing multiple fins on the underside of the boards. Numerous configurations are known but the most popular are double fin and triple fin configurations. The double fin configuration has two identical side fins position equidistant from a longitudinal center axis of the board. The triple fin configuration (known as a thruster) is similar to the double fin configuration but with the addition of a slightly larger center fin.

The maneuverability of a surfboard is achieved by the combination of the effect of the rails and the fins acting against the water. However, the effective area of the fin in the water is greatly reduced in many riding maneuvers, particularly sharp turns and fast runs across the face such as riding a tube. In fact, for a double or triple fin surfboard the side fin away from the wave face may be completely out of the water and the fins in the water will be at a less than optimum angle.

A number of inventions have been proposed for adjustable fins that trim the craft, be it a surfboard, surf ski or other surf craft, for optimum performance.

Reference may be had to United States patent number 4733496, in the name of Michael Wallner, of Australia. The Wallner fin includes a pivoting rudder-like section that swings out relative to a fixed section when a turn is commenced. The fixed section incorporates a flange with a groove of varying depth that is deepest in the middle. An adjustable spring-loaded pin in the rudder-like section seats in the groove to bias the sections towards alignment.

The flange extends laterally and therefore produces drag that reduces the effectiveness of the Wallner innovation.

United States patent number 5567190, in the name of Oates, describes a fin box assembly that is adjustable to allow the angle of attack of the fin to be varied with respect to the longitudinal axis of the surfboard. The fins on the board can be trimmed to suit the prevailing conditions and the experience of the rider. The static adjustment proposed by Oates is useful for long boards but of limited advantage for shorter, more maneuverable boards.

Benham, United States patent number 5813890, recognized that the fin angle of watercraft, particularly windsurfer boards, needs to be continuously changing depending upon the action of the board.

When the windsurfer board is traveling in a straight line the fin is most suitably in a neutral position that is perpendicular to the underside of the board. If the board is turning, or keeling over under the force of the wind, the fin should be at an angle to the board so as to maintain maximum effective length in the water. Benham devised a pivoting fin that is biased towards the neutral position by an elastomeric spring. The fin is free to pivot in a ninety degree range under the influence of the water pressure as the watercraft turns or keels over. Benham also provides a mechanism for locking the fin in

a fixed angular position relative to the board, but does not provide any mechanism for limiting the range of pivot of the fin.

OBJECT OF THE INVENTION It is an object of the invention to provide an improved fin for surf craft.

It is a further objective to provide a self-adjusting fin with a selectable range of pivot.

Further objects will be evident from the following description.

DISCLOSURE OF THE INVENTION In one form, although it need not be the only or indeed the broadest form, the invention resides in an adjusting fin comprising: a fin box fixable to a watercraft; a spindle seated in the fin box for rotation about a longitudinal axis of the spindle ; a fin associated with the spindle so as to pivot relative to the watercraft with rotation of the spindle ; at least one cam associated with said spindle ; and at least one cam engaging stop adjustable to restrict the extent of rotation of the spindle, and thus limit the degree of pivot of the fin.

Suitably the adjusting fin further comprises biasing means that biases the fin towards a neutral position. The biasing means suitable acts upon the spindle, and most suitably upon the cam of the spindle.

Suitably the fin box comprises a base and a detachable cover with the spindle retained between the base and cover.

BRIEF DESCRIPTION OF THE DRAWINGS To assist in understanding the invention, preferred embodiments will be described with reference to the following figures in which: FIG 1 is a sketch of an assembled adjusting fin; FIG 2 is an end view of the adjusting fin of FIG 1 showing a range of movement; FIG 3 is a bottom view of the adjusting fin of FIG 1; FIG 4 is a sketch of a spindle ; FIG 5 is a sketch of the underside of the spindle of FIG 4, showing spring locating stubs; FIG 6 is a sketch of the fin box base; FIG 7 is a sketch of the fin box cover; FIG 8 is a sketch of the rear part of the underside of a triple fin surfboard employing two adjusting fins; and FIG 9 is a rear view of the surfboard of FIG 8.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG 1, there is shown a fin assembly 1 consisting of a fin box 2 and fin 3. The fin box 2 has a sculptured outer surface for glassing into the underside of a surfboard in conventional manner.

The fin 3 is held in a spindle 4 by screws 5 in a manner described in greater detail below. For simplicity the adjusting fin will be described by reference to a surfboard. It will be appreciated that the adjusting fin is not limited to this application but may equally as well be employed on a surf ski, windsurfer, or other watercraft.

The fin box 2 is formed in two parts as a base 6 and a cover 7.

The spindle 4 is captured in the fin box 2 between the cover 7 and the

base 6. The cover is attached to the base by screws 8. The fin 3 pivots with rotation of the spindle 4, which is limited in range of rotation by adjustable stops 9. The possible range of rotation is shown in FIG 2 with the fin shown in the neutral position (perpendicular to underside of board) and the extent of pivot shown in dotted outline.

Also in FIG 2 is shown the depth of glassing of the fin box into a board. When correctly glassed in the surface of the board should be at about the dotted line 10. The locating stubs 11 are sanded off after installation.

The elements of the adjusting fin are seen more clearly in the bottom view of FIG 3, and the sketches of each element in FIG's 4-7.

The screws 8 attach the cover 7 to the base 6, so capturing the spindle 4. As seen in FIG 4, the spindle has cams 12 at each end.

The range of rotation of the spindle is limited when the cams contact the adjustable stops 9. Adjusting the depth of stops changes the range of rotation and therefore limits the range of pivot of the fin. The usual arrangement for a triple or double fin board is for the range to be set in the manner shown in FIG 2, that is that the fin only pivots away from the neutral position on one side. This is depicted more clearly in FIG's 8 and 9.

Also seen most clearly in FIG 4 is the cavity 13 that receives the base of the fin 3. The fin is removably positioned in the cavity and retained by screws 5. Although it has been found that there may be benefit to have the fin detachable from the spindle, it will be appreciated that the fin and spindle could equally as well be formed as a single unit. Furthermore, if the fin and spindle are formed separately, means other than screws could be used to retain the fin 3 in the cavity 13. Such means includes pins, bolts, clips or glue.

The screws 5 allow for a certain amount of fine trimming of the position of the fin 3. The fin can be moved forward or back, tilted about its own center or twisted relative to the longitudinal axis of the

fin box. For an expert rider these fine adjustments can add perceptibly to the performance of the board.

The fin 3 may be biased towards a neutral position where the fin is maintained generally perpendicular to the underside of the board. It has been found that biasing is not essential as the fin 3 tends towards an optimum position as the board moves through the water. However, as mentioned above, there can be situations in which a fin is completely out of the water. In this case it is convenient for the fin to return to the neutral position. This is achieved by locating a biasing means, such as a spring, under the cam 12 on tabs 14. As will be clear by reference to FIG 6, the spring locates between the base 6 and the tabs 14 to bias the rotation of the spindle 4. Adjustable stop 9 acts against the opposite side of the cam 12 to stop the spindle 4 at the neutral position. The spring will be on the right or left side as required for the position of the fin.

Turning now to FIG 6, it can be seen that the base has as a simple structure with a pair of seats 15 at each end to support the spindle. It is conveniently made as a single moulded plastics element.

The cams 12 travel in the recess formed by seat pairs 15. A spring boss 16 is formed in the base of the recess to seat the spring. Thus the spring operates between the boss 16 in the base and the tab 14 on the cam to bias the spindle towards the neutral position.

The cover 7 is shown in more detail in FIG 7. It is suitable formed as a moulded plastics element, like the base 6. A hole 17 is formed in the cover to allow passage of the fin. Flanges 18, at each end of the hole 17, act to restrain the spindle between the cover 7 and the base 6.

The adjusting fin has a number of advantages over the prior art fins. Firstly, the fin is self-adjusting by the pressure of water acting on the fin as the craft passes through the water. Secondly, the adjustable stops allow the rider to easily limit the range of pivot and therefore trim

the craft for prevailing conditions and individual skill. Thirdly, the fin can be actively biased towards the neutral position.

The invention is particularly advantageous for a professional surfer who currently requires a number of boards to suit different conditions. It is possible with the invention to have a single board that can be reconfigured in moments. The surfer is not only able to adjust the fins, but is able to change the fin as desired. This can lead to a significant reduction in the number of boards required by a professional surfer.

The invention also offers significant advantage in surfboard manufacture. At present, a precision jig is required to manufacture a double or triple fin board with the side fins having a set pitch. With the invention the fin box can be positioned accurately with the adjustment of the fins occurring later. There is no need for a precision jig to set the angle of the fins during glassing of the board.

Throughout the specification the aim has been to describe the invention without limiting the invention to any particular combination of alternate features.