Abstract:
A hydrofoil assembly for a waterborne vessel, comprising: a body; a hydrofoil mounted to the body, the hydrofoil being adjustable to vary its lift characteristics; and a control mechanism operative to control the adjustment of the hydrofoil assembly relative to the support.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation and claims priority under 35 U.S.C. §120 to PCT Application No. PCT/AU2011/001069, filed on Aug. 19, 2011, which claims priority to Australian Application No. 2010903746, filed on Aug. 20, 2010. The contents of both of these priority applications are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to hydrofoils used with water-borne vessels. The invention has particular application, to sailing vessels and will herein be described in that context. 
       BACKGROUND 
       [0003]    Water vessels fitted with hydrofoils offer improved performance for vessels at speed. A hydrofoil is generally a wing or wing like structure mounted on struts beneath the vessels which act to lift the vessel from the water during forward motion. As the hull of the vessel is lifted from the water, the drag of the vessel in the water is reduced, thereby increasing potential speed. 
       SUMMARY 
       [0004]    In a first aspect, there is provided a hydrofoil assembly for a waterborne vessel, comprising; a support mountable to the vessel; a body movably mounted to the support and movable relative to the support between a first and second position; and a hydrofoil mounted to the body by a coupling, the hydrofoil being able to articulate about the coupling to provide a variable-incidence hydrofoil. 
         [0005]    In one form of the hydrofoil assembly, the body is movable between the first and second positions at least in part by sliding of the body relative to the support. 
         [0006]    In one form, the body is movable between the first and second positions at least in part by rotating of the body relative to the support. 
         [0007]    In one form, the coupling has a pivot axis about which the hydrofoil articulates relative to the body. In a further form, the pivot axis extends through or proximal the centre of pressure of the hydrofoil. 
         [0008]    In one form, the variable-incidence hydrofoil provides a variable lift hydrofoil. 
         [0009]    In another aspect, there is provided a hydrofoil assembly for a waterborne vessel comprising: a support mountable to the vessel, the support having a first end which in use faces towards the bow of the vessel and a second end which in use faces towards the stern of the vessel; a body movably mounted to the support and movable relative to the support between a locked and an unlocked condition; the body having a forward facing surface and an opposite rearward facing surface and a hydrofoil assembly attached to the body, wherein the body is arranged to be movable from the locked to the unlocked condition by an impact above a threshold loading applied to the forward facing surface of the body causing rearward displacement of a distal end of the body. 
         [0010]    In another aspect, there is provided a hydrofoil assembly for a waterborne vessel comprising: a support mountable to the vessel, the support having a first end which in use faces towards the bow of the vessel and a second end which in use faces towards the stern of the vessel; a body rotatably mounted to the support and movable relative to the support between a locked and an unlocked condition; and a hydrofoil assembly attached to the body, wherein the body is arranged to be movable from the locked to the unlocked condition by rotation of the body in the support towards the first end of the support. 
         [0011]    In another aspect, there is provided a hydrofoil assembly for a waterborne vessel comprising: a support mountable to the vessel, the support having a first end which in use faces towards the bow of the vessel and a second end which in use faces towards the stern of the vessel; a body rotatably mounted to the support and movable relative to the support between a locked and an unlocked condition; and a hydrofoil assembly attached to the body, wherein the body is arranged to be movable from the locked to the unlocked condition by displacement of a distal end of the body toward the second end of the support. 
         [0012]    In one form of the hydrofoil assembly, the body is movable relative to the support from an extended position to a retracted position when in the unlocked condition. 
         [0013]    In one form, the body is slidable relative to the support between the extended and retracted positions. 
         [0014]    In one aspect, there is provided a hydrofoil assembly for a waterborne vessel, comprising: a body; a hydrofoil mounted to the body, the hydrofoil being adjustable to vary its lift characteristics; and a control mechanism operative to control the adjustment of the hydrofoil assembly relative to the support. 
         [0015]    In one form, at least part of the control mechanism is mounted to the body. 
         [0016]    In a particular form, at least part of the control mechanism is mounted within the body. 
         [0017]    In one form of the hydrofoil assembly, the control mechanism comprises a sensor responsive to the altitude of the vessel relative to the water, a controller to provide a control output based on responses of the sensor, and an actuator to vary lift in the hydrofoil based on the control output of the controller. 
         [0018]    In one form, the sensor comprises a displaceable wand pivotally mounted to the body. In a further form, the wand is located at an intermediate region of the body. In another form, the sensor is located on other parts of the vessel. 
         [0019]    In a particular form, the hydrofoil assembly further comprises a support mountable to the vessel, and wherein the body is mounted to the support. In one form, at least part of the control mechanism is mounted to the support. In one form, where the sensor comprises a displaceable wand, that wand is mounted to the support. 
         [0020]    In one form of the hydrofoil assembly, the controller provides a control output that is non-linear to the sensor responses. In a further form, the controller providing the non-linear control output is a mechanical cam connected to the actuator. 
         [0021]    In one form of the hydrofoil assembly, the hydrofoil is mounted to the body by a coupling, the hydrofoil being able to articulate about the coupling to provide a variable-incidence hydrofoil, and wherein the actuator is a pushrod connected to the hydrofoil to articulate the hydrofoil about the coupling. 
         [0022]    In an alternative embodiment, the hydrofoil assembly has a fixed incidence hydrofoil, with a variable incidence flap. In this embodiment, the flap is actuated by a pushrod connected to the flap, whereby actuation of the pushrod varies the incidence of the flap, and thus altering the lift of the hydrofoil assembly. 
         [0023]    In another aspect, there is provided a hydrofoil assembly for a waterborne vessel, comprising: a body; a hydrofoil pivotally attached to the body, wherein during forward motion of the vessel, water flowing across the hydrofoil biases the hydrofoil to pivot to a neutral position in which no lift is generated; and an actuator arranged to apply a biasing force to the hydrofoil to change the incidence of the hydrofoil relative to body to create lift, wherein the actuator provides a biasing force in only one direction and in the absence of biasing force by the actuator, the hydrofoil is biased in use to return to the neutral position by water flowing across the hydrofoil. 
         [0024]    In another aspect, there is provided a hydrofoil assembly for a waterborne vessel, comprising: a body; a hydrofoil attached to the body, wherein in use, the hydrofoil assembly forms part of the centreboard of the vessel. 
         [0025]    In one form of the hydrofoil assembly, the assembly is mounted to a centreboard insert. 
         [0026]    In another form of the hydrofoil assembly, the assembly is a retrofit assembly to replace at least part of a centreboard assembly of the vessel. 
         [0027]    In another aspect, there is provided a hydrofoil assembly for a waterborne vessel, comprising: a body and a hydrofoil attached to the body, wherein in use the hydrofoil assembly forms at least part of a rudder of the vessel. 
         [0028]    In one form, the hydrofoil assembly is mounted to a rudder box of the vessel. 
         [0029]    In another form, the hydrofoil assembly is a retrofit assembly to replace at least part of the rudder of the vessel. 
         [0030]    In one form of the hydrofoil assembly the hydrofoil has a symmetrical foil section. Advantageously, this allows the water to naturally bias the foil section to a neutral orientation. However, it can be appreciated that in other forms, the hydrofoil may have an asymmetric foil section. In a further form, the body is a foil and the body and hydrofoil have identical foil sections. In other forms, the body and hydrofoil may be of different section shape or size. 
         [0031]    In another aspect, there is provided a support for a hydrofoil assembly, wherein the support comprises; an aperture for slidingly receiving a portion of a hydrofoil assembly; and a surface for supporting the hydrofoil assembly. 
         [0032]    In one form, the support further comprises a lock mechanism for releaseably locking the hydrofoil assembly to the support. 
         [0033]    In one form, the support further comprises at least one channel for receiving a sensor wand in the hydrofoil assembly. 
         [0034]    In another aspect, there is provided a waterborne vessel comprising: a hull; and at least one hydrofoil assembly according to any form described above. In one further form, the vessel is a sailing vessel. 
         [0035]    In one form, a plurality of the hydrofoil assemblies are located along the centreline of the vessel. Advantageously, hydrofoil assemblies along the centreline allow effective heeling of the vessel on either side. However, it is to be appreciated that other hydrofoil configurations may be used, such as tri-foil configurations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    It is convenient to hereinafter describe embodiments of hydrofoil assemblies with reference to the accompanying drawings. The particularity of the drawings and related description is to be understood as not superceding the preceding broad disclosure. 
           [0037]    In the drawings: 
           [0038]      FIG. 1  is a side view of a vessel with hydrofoil assemblies in the extended configuration; 
           [0039]      FIG. 2  is a side view of the vessel in  FIG. 1  with the hydrofoil assemblies in the retracted configuration; 
           [0040]      FIG. 3  is an isometric view of the main hydrofoil assembly; 
           [0041]      FIG. 4  is an alternative isometric view of the assembly in  FIG. 3 ; 
           [0042]      FIG. 5  is a close up view of area C in  FIG. 3 ; 
           [0043]      FIG. 6  is a close up view of area D in  FIG. 4 ; 
           [0044]      FIG. 7  is a side view of the main hydrofoil assembly in  FIG. 3 ; 
           [0045]      FIG. 8  is an end view of the main hydrofoil assembly in  FIG. 3 ; 
           [0046]      FIG. 9  is a close up view of an end of the hydrofoil in  FIG. 3 ; 
           [0047]      FIG. 10  is an isometric view of a support of the main hydrofoil assembly in  FIG. 3 ; 
           [0048]      FIG. 11  is a side view of the support in  FIG. 10 ; 
           [0049]      FIG. 12  is an alternative isometric view of the support in  FIG. 10 ; 
           [0050]      FIG. 13  is a side view of the main hydrofoil assembly in an extended and locked configuration; 
           [0051]      FIG. 14  is a rear isometric view of  FIG. 13 ; 
           [0052]      FIG. 15  is a side view of the main hydrofoil assembly in an extended and unlocked configuration; 
           [0053]      FIG. 16  is a rear isometric view of  FIG. 15 ; 
           [0054]      FIG. 17  is a side view of the main hydrofoil assembly with the vertical foil slightly retracted and the wand orientating to slide into a channel of the support; 
           [0055]      FIG. 18  is a rear isometric view of  FIG. 17 ; 
           [0056]      FIG. 19  is a side view of the main hydrofoil assembly with the vertical foil slightly retracted and the wand orientated to slide into the channel of the support; 
           [0057]      FIG. 20  is a rear isometric view of  FIG. 19 ; 
           [0058]      FIG. 21  is a side view of the main hydrofoil assembly in the retracted configuration; 
           [0059]      FIG. 22  is a rear isometric view of  FIG. 21 ; 
           [0060]      FIG. 23  is a side view of the main hydrofoil assembly in an extended configuration and locked with a shear pin. 
           [0061]      FIG. 24  is a close up isometric view of the vertical foil and the support  3  in the extended and locked configuration. 
           [0062]      FIG. 25  is a side view of the main hydrofoil assembly when the altitude of the vessel is low; 
           [0063]      FIG. 26  is a close up of area B in  FIG. 25 ; 
           [0064]      FIG. 27  is a close up of area C in  FIG. 25 ; 
           [0065]      FIG. 28  is a side view of the main hydrofoil assembly when the vessel is at optimum altitude; 
           [0066]      FIG. 29  is a close up of area B in  FIG. 28 ; 
           [0067]      FIG. 30  is a close up of area C in  FIG. 28 ; 
           [0068]      FIG. 31  is a side view of the main hydrofoil assembly when the altitude of the vessel is too high; 
           [0069]      FIG. 32  is a close up of area B in  FIG. 33 ; 
           [0070]      FIG. 33  is a close up of area C in  FIG. 33 ; 
           [0071]      FIG. 34  is an isometric view of a rudder hydrofoil assembly in the extended configuration; 
           [0072]      FIG. 35  is a rear isometric view of the assembly in  FIG. 34 ; 
           [0073]      FIG. 36  is an isometric view of the rudder hydrofoil assembly in  FIG. 34  in the retracted configuration; 
           [0074]      FIG. 37  is rear isometric view of the assembly in  FIG. 36 ; 
           [0075]      FIG. 38  is a side view of the assembly in  FIG. 36 ; 
           [0076]      FIG. 39  is a side view of the rudder hydrofoil assembly in the extended configuration; 
           [0077]      FIG. 40  is a side view of the assembly in  FIG. 39  in a partially retracted configuration; 
           [0078]      FIG. 41  is a side view of the assembly in  FIG. 39  in a fully retracted configuration; 
           [0079]      FIG. 42  is an alternative embodiment of a main hydrofoil assembly in the extended configuration; 
           [0080]      FIG. 43  is the assembly in  FIG. 42  in a retracted configuration; 
           [0081]      FIG. 44  is an alternative embodiment of a rudder hydrofoil assembly in the extended configuration; 
           [0082]      FIG. 45  is the assembly in  FIG. 44  in the retracted configuration; 
           [0083]      FIG. 46  is an alternative embodiment of the main hydrofoil assembly with the wand in an alternative location; 
           [0084]      FIG. 47  is an isometric view of the control mechanism in the main hydrofoil assembly, illustrating the cam and pushrod; and 
           [0085]      FIG. 48  is an alternative embodiment of a main hydrofoil assembly in the extended configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0086]      FIG. 1  illustrates a vessel  1  having a hull  3 . Along the centreline of the hull  3 , there are two retractable hydrofoil assemblies  9 ,  11  in the extended configuration. The first main hydrofoil assembly  9  is located midship along the hull. The second rudder hydrofoil assembly  11  is located towards the stern. 
         [0087]    The main hydrofoil assembly  9 , comprises of a centre case support  13  to which a body in the form of a vertical foil  15  is slidingly attached. At the lower end of the vertical foil  15 , there is provided a main hydrofoil  17  that is pivotally attached to the vertical foil  15 . 
         [0088]    The rudder hydrofoil assembly  11 , comprises of a rudder support  19  mounted to a rudder box  21  of the vessel. A body in the form of a rudder foil  23  is pivotally attached to the rudder support  19 , and a rudder hydrofoil  25  is pivotally attached to the rudder foil  23 . 
         [0089]    As illustrated in  FIG. 1 , the vertical foil  15  and rudder foil  19  is locked in an extended configuration below the hull of the vessel  1 . This prevents sliding movement of the vertical support foil  15  relative to the centre case support  13 , and pivotal movement of the rudder foil  23  relative to the rudder support  19 . In this extended configuration, there is a vertical clearance between the baseline of the hull  1 , and the hydrofoils  17  and  25 . 
         [0090]    In forward motion of the vessel  1 , lift is generated by the hydrofoils  17  and  25  due to their angle of attack to the water flow, which in turn lifts the hull  3  of the vessel  1  above the surface of the water. 
         [0091]      FIG. 2  illustrates the vessel  1 , with the hydrofoil assemblies  9 ,  11  in the retracted configuration. The vertical foil  15  of the main hydrofoil assembly  9  is slidingly retracted and locked into the hull  3  of the vessel  1 . The rudder foil  23 , is pivotally swung and locked in a trailing orientation. 
         [0092]    As clearly shown in  FIG. 2 , the components of the hydrofoil assemblies  9  and  11  in the retracted position are much closer to the baseline of the hull  3 , thereby reducing the draft of the vessel  1 . This is advantageous when launching and retrieving the vessel in shallow waters. 
         [0093]    The structure of the main hydrofoil assembly  9  will now be described with reference to  FIGS. 3 to 12 . The vertical foil  15  has a handle  31  at the upper end for the user to manipulate the main hydrofoil assembly  9  from the extended and retracted configurations. Rearward of the handle  31  is a support surface  32  to abut with the support  13  in the extended configuration. A sensor wand  33  is pivotally mounted at pivot  35 , at an intermediate region of the vertical foil  15 . In this particular embodiment, a sensor wand is located on both sides of the vertical foil  15 . The sensor wands form part of a control mechanism (which also includes pushrod  41 , cam  59  and pivot  35  as described in more detail below) to control the pivoting of the hydrofoil  17 . Advantageously, having a sensor wand on both sides of the vertical foil  15 , allows the sensor wands to operate effectively when the vessel  1  is heeled on either side Therefore, the response of the control mechanism is the same on both tacks. In an alternative form, one or more of the sensor wands are mounted on other parts of the vessel. In the form as shown in  FIG. 48 , a sensor wand  134  is mounted to the trailing end of the support  13 . The sensor wand is connected to other components of the control mechanism by any suitable connection, such as through linkage assembly  135  as shown schematically in  FIG. 43 . 
         [0094]    At the lower end of the vertical foil  15 , the main hydrofoil  17  articulates about a coupling  37 , so that the incidence (i.e. the angle of attack) of the main hydrofoil  17  can be varied to adjust the lift provided by the hydrofoil  17 . The foil section of the main hydrofoil is symmetrical and the ends of the main hydrofoil  17  are provided with wings  39 . 
         [0095]    As illustrated in  FIGS. 7 and 8 , a pushrod  41  of the control mechanism can provide downward force to the hydrofoil  17 , to increase or maintain the angle of attack of the hydrofoil. The incidence (angle of attack) of the hydrofoil, in conjunction with other factors such as speed of the hydrofoil relative to the water affects the lift generated. As the main hydrofoil  17  is coupled  37  to the vertical foil  15  with an axis near the centre of pressure of the hydrofoil, only minimal force is required by the pushrod to change the angle of attack of the hydrofoil during forward movement of the hydrofoil assembly  9  through water. In another embodiment, the axis may be slightly forward of the centre of pressure. 
         [0096]      FIGS. 10 to 12  illustrate a support  13  that is provided with an upper aperture  43 , and a lower aperture  45  to slidingly receive the vertical foil  15 . A retaining surface  47 , is provided to abut with the support surface  32  of the vertical foil  15  in the extended configuration. A pair of channels  49  accommodates and retains the wands  33  of the vertical foil  15  when in the retracted configuration. 
         [0097]    A flange  48  is provided at the lower portion of the support  13 , which in use is in abutment with the hull  3  to transfer upwardly directed forces to the hull  3 , as illustrated in  FIG. 23 . 
         [0098]    The sequence of changing the main hydrofoil assembly  9  from a retracted configuration to an extended configuration will now be described with reference to  FIGS. 13 to 22 . 
         [0099]      FIGS. 13 and 14  illustrate the main hydrofoil assembly  9  in the extended and locked configuration. The support surface  32  of the vertical foil  15  abuts the retaining surface  47  to prevent the vertical foil  15  from moving vertically through the support  13 . 
         [0100]      FIGS. 15 and 16  illustrate the main hydrofoil assembly  9  extending from the support  13 , but unlocked from the support  13 . This is achieved by moving the handle  31  forward towards the bow of the vessel so that the support surface  32  is free from the retaining surface  47 . The other end of the vertical foil  15  where the main hydrofoil  17  is attached correspondingly moves rearwards towards the stern of the vessel during this unlocking movement. 
         [0101]      FIGS. 17 and 18  illustrate the vertical foil  15  as it is retracted through the support  13 , by pulling up on the handle  31 . As the wand  33  is introduced into the channel  49 , the channel  49  guides the wand  33  downwards so the length of the wand is parallel to the length of the channel. This is best illustrated in  FIGS. 19 and 20 , where the wand  33  is orientated with the channel  49  to allow the vertical foil  15  to be further retracted upwards into the support  3 . 
         [0102]    In the fully retracted configuration, as illustrated in  FIGS. 21 and 22 , the vertical foil  15  is retracted with the main hydrofoil  17  in a proximal position to the hull  3 . The wands  33  are substantially parallel with the vertical foil  15 , with a portion of the wand  33  captured in the channel  49  of the support  3 , thus preventing any pivotal movement of the wand  33 . In the retracted configuration, the main hydrofoil  17  is allowed to freely rotate. As the hydrofoil  17  is symmetrical in foil section, the foil section would naturally bias to a neutral orientation (i.e. substantially zero angle of attack), when water flows past the freely pivoting hydrofoil  17 . Alternatively, the main hydrofoil  17  may be locked with a substantially zero angle of attack. In either case, having the main hydrofoil  17  in a neutral orientation reduces drag of the hydrofoil  17  when in a retracted configuration. 
         [0103]    To change the main hydrofoil assembly  9  from the retracted configuration to the extended configuration, the steps above are reversed. Thus the vertical foil  15  is lowered to the extended configuration, and the handle  31  moved to a rearward position so that the support surface  32  and the retaining surface  47  abut to prevent retraction of the vertical foil  15 , as illustrated in  FIG. 24 . To prevent the vertical foil  15  from moving out of the locked position, a pin  51  connected to the support  13  or hull  3 , is passed through the handle  31  as illustrated in  FIG. 23 . 
         [0104]    In one embodiment, the pin  51  is a shear pin designed to shear after a particular threshold of shear force is applied. If the hydrofoil assembly  9  is extended and locked, and the hydrofoil assembly  9  hits rocks or the bed of the body of water during forward motion, a rearward force at the lower end of the vertical foil  15  will be imparted. This force will be cantilevered at the support  13 , such that the top of the vertical foil  15  near the handle  31  would be forced forward. If the force is sufficient, the pin  51  would shear, allowing the handle  31  to move forward so that hydrofoil assembly  9  is unlocked. This allows the vertical foil  15  to then retract through the support  13 . 
         [0105]    This safety feature allows the hydrofoil assembly  9  to automatically retract in the event of hitting the bed in shallow waters, thus preventing serious damage or capsizing of the vessel. 
         [0106]    It is to be appreciated that other forms of locking the hydrofoil assembly  9  in the extended position may be used, such as a friction surface. The friction surface may be on the support surface  32  or the retaining surface  47 , and may comprise of a serrated surface. The friction surface holds the vertical foil  15  in the extended and locked configuration by friction and is designed to release the vertical foil  15  from the locked configuration after a force is applied. 
         [0107]    The operation of the control mechanism which determines variable lift of the main hydrofoil  9  will now be described with reference to  FIGS. 25 to 33 . 
         [0108]    When the altitude of the vessel  1  is in a low position, the flow of water pushes the wand  33  in a substantially horizontal position as illustrated in  FIGS. 25 and 26 . A cam  59  (see  FIG. 47 ) is connected to the wand  33  and engages the top of the pushrod  41 . When the wand is in the generally horizontal position, the cam  59  is in an orientation such that the pushrod is depressed as the cam  59  pushes downwards on pushrod  41 . The pushrod  41  in turn pushes down on a trailing portion of the main hydrofoil  17 , thereby pivoting the hydrofoil  17  to have a positive large angle of attack to create more lift, as illustrated in  FIG. 27 . This lift generated by the angle of attack of the foil in turn is designed to increase the altitude of the vessel  1 . 
         [0109]    As illustrated in  FIGS. 28 and 29 , when the altitude of the vessel  1  increases (i.e. the vessel lifts), the wand  33  continues to trail on the surface of the water and as such rotates from its horizontal position to an intermediate orientation between horizontal and vertical. This rotation of the wand causes rotation of the cam  59  which changes the amount the pushrod  41  is depressed by the cam. In particular the rotation of the cam (which is in a clockwise direction when arranged as shown in  FIG. 47 ) allows the push rod to rise slightly (as it is forced upwardly by the hydrofoil). This reduces the angle of attack in the main hydrofoil  9  and thereby reduces the amount of lift on the vessel until an equilibrium is reached where the amount of lift generated by the hydrofoil is the same as that required to maintain the vessel at that altitude, As will be appreciated less lift and less angle of attack is required when the vessel is in the equilibrium position as shown in  FIGS. 28 and 29  than when the altitude is in low position as described above and shown in  FIGS. 25 and 26 . The angle of attack of the main foil  17  to maintain the altitude of the vessel  1  is shown generally at  FIG. 30 . 
         [0110]    When the altitude of the vessel  1  is too high, the wand  33  drops to the surface of the water in a substantially downward orientation as illustrated in  FIGS. 31 and 32 . At this orientation, the cam  59  is further rotated in the clockwise direction which allows further rising of the pushrod  41  thereby allowing the main hydrofoil to reduce its angle of attack to the water flow, thus reducing lift which in turn causes the vessel  1  to lose altitude. 
         [0111]      FIG. 47  illustrates an embodiment of the control mechanism inside vertical foil  15 , comprising pushrod  41 , cam  59  and pivot  35 . The cam  59  is mechanically linked to the wands  33 , and pivoting of the wands  33  pivots the cam  59  in unison. On rotation of the cam, such that the cam surface  60  moves towards the pushrod, the cam surface  60  pushes and moves pushrod  41  downwards. However, movement of the cam surface  60  away from the pushrod  41  is non-reciprocating, and thus upward movement of the cam surface  60 , does not pull the pushrod  41  upwards. Instead, the flow of water over the main hydrofoil  17  biases the hydrofoil towards a neutral orientation, which causes the hydrofoil  17  to push upward on the pushrod  41 . 
         [0112]    The cam surface  60 , provides non-linear actuation of the pushrod  41  relative to the orientation of the wands  33 , thereby increasing or decreasing the change in displacement of the pushrod  41  at certain orientations of the wand. The amount and rate of change of displacement of the pushrod under rotation of cam is dependent of the profile of the contacting surface of the cam with the pushrod. As such the performance of the control mechanism can be “tuned” by varying this profile. In one embodiment, there is a decreasing change in the displacement of the pushrod as the orientation of the wand drops toward the surface of the water in a substantially downward orientation. 
         [0113]    The structure of the rudder hydrofoil assembly  11  will now be described with reference to  FIGS. 34 to 38 .  FIGS. 34 and 35  illustrate a rudder hydrofoil assembly  11  in the extended configuration. The rudder foil  23 , is orientated and locked substantially downwards from the rudder support  19 . The rudder hydrofoil  25 , is pivoted and locked so that the angle of the rudder hydrofoil  25  relative to the rudder foil  23  is fixed. In one embodiment, the angle may be fixed to provide a fixed angle of attack for lift. 
         [0114]      FIGS. 36 to 38  illustrate a rudder hydrofoil assembly  11  in the retracted configuration. The rudder foil  23  is substantially orientated horizontally, and the rudder hydrofoil  25  is allowed to freely pivot. The flow of water over the rudder hydrofoil  25  will naturally bias the rudder hydrofoil  25  to a neutral orientation with a substantially zero angle of attack. Alternatively, the rudder hydrofoil  25  may be locked horizontally with a substantially zero angle of attack. 
         [0115]      FIGS. 39 to 41  illustrate the sequence of the rudder hydrofoil assembly  11  retracted from the extended configuration. As illustrated in  FIGS. 40 and 41 , once the rudder hydrofoil assembly  11  is unlocked from the extended configuration, the rudder hydrofoil  25  is free to rotate to a neutral orientation. This ensure the rudder hydrofoil  25  does not create unnecessary drag, as would occur if the rudder hydrofoil  25  orientation was fixed relative to the rudder foil  23 . 
         [0116]    The main hydrofoil assembly  9  may form part of a centreboard assembly of a vessel  1 . In another form, the main hydrofoil assembly  9  is mounted to a centreboard insert. The main hydrofoil assembly  9  may form part of a retrofit kit to replace a centreboard assembly of a vessel. 
         [0117]    The rudder hydrofoil assembly  11  forms part of the rudder of the vessel, and may be mounted to the rudder box. The rudder hydrofoil assembly  11  may also form part of a retrofit kit to replace a rudder and rudder assembly of a vessel. 
         [0118]    To simplify manufacture of the hydrofoil assemblies  9  and  11 , it may be convenient to use common foil sections for the vertical foil, main hydrofoil, rudder foil and rudder hydrofoil. The foils may be manufactured by extrusion, and cut to length to suit respective foil and hydrofoil components of the hydrofoil assemblies. 
         [0119]    An advantage of an embodiment of the hydrofoil assembly is it allows the hydrofoil to be easily retracted when launching or recovering the vessel, or to allow navigating in shallow waters. 
         [0120]    Another advantage of an embodiment of the hydrofoil assembly is that the hydrofoil assembly can automatically retract if the hydrofoil hits the bed in shallow waters. This improves safety of the hydrofoil, and decreases the chance of damage to the vessel. 
         [0121]    Another advantage of an embodiment of the hydrofoil assembly is that the control mechanism is integral to the hydrofoil assembly. This simplifies construction, manufacture and maintenance. Furthermore, the integral control mechanism does not require components that intrude or interfere in other areas of the vessel. 
         [0122]    An advantage of an embodiment of the hydrofoil assembly is the natural biasing force of the hydrofoil to a neutral position. This reduces drag in the system, as well as the need for a reciprocating actuator in the control mechanism. 
         [0123]    Another advantage of an embodiment of the hydrofoil assembly is that it can be retrofitted to existing vessels without substantial modifications to the vessel. 
         [0124]    An alternative embodiment of the main hydrofoil assembly  109  will now be described with reference to  FIGS. 42 and 43 . The main hydrofoil assembly  109  comprises of a housing  161 , a bay  163 , a pivot arm  165 , a vertical foil  115  and a main hydrofoil  17 . The vertical foil  115  in this embodiment is pivotally attached to the housing  161  around pivot point  167 . 
         [0125]      FIG. 42  illustrates the main hydrofoil assembly  109  in the extended configuration, wherein the pivot arm  165  is at a rearward position towards the stern. To retract the main hydrofoil assembly, the pivot arm  165  is moved to a forward position as illustrated in  FIG. 43 . When in the retracted configuration, the vertical foil  115  is recessed into the bay  163  of the housing  161 , which reduces drag. As discussed above in the other embodiments, main hydrofoil  17  is free to pivot to a neutral orientation to reduce drag. 
         [0126]    An alternative embodiment of the rudder hydrofoil assembly  111  is illustrated in  FIGS. 44 and 45 . In this embodiment, the rudder foil  123  is vertically and slidingly retractable through the support  119 . 
         [0127]    A further alternative is illustrated in  FIG. 46 . In this embodiment, a single wand  133  is mounted at the trailing rear of the vertical foil  15 . As the wand is centred on the trailing edge, only one wand is required to provide the same response when the vessel  1  is heeled in either direction. In this embodiment, the corresponding support  13  may have a single channel to accommodate the wand  133  near the trailing edge of the vertical foil  15 . 
         [0128]    In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 
         [0129]    It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.