Patent Publication Number: US-2015083034-A1

Title: Hydrofoil watercraft

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of Ser. No. 12/935,065 filed on Dec. 10, 2010, which is a U.S. National Phase application under §371 for International Application No. PCT/GB2009/000615 having an international filing date of Mar. 6, 2009, and from which priority is claimed under all applicable sections of Title 35 of the United States Code including, but not limited to, Sections 120, 363 and 365( c ), and which in turn claims priority under 35 USC §119 to U.K. Patent Application No. 0806523.6 filed on Mar. 28, 2008 and to U.K. Patent Application No. 0813286.9 filed on Jul. 21, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improved hydrofoil watercraft. More particularly, the present invention relates to the design, configuration and construction of improved wind and motor driven watercraft having ventilated hydrofoils. 
     BACKGROUND OF THE INVENTION 
     Hydrofoils are widely used in both motor and wind powered water craft to reduce drag and/or improving passenger comfort by lifting the hull of the craft out of the water. However, it can be difficult to lift and maintain the body of the craft at a specified distance above the water surface, known as ride height. Hydrofoils can suffer inconsistencies in performance resulting from cavitation around the surface of the hydrofoil. Further, hydrofoils often have a very narrow operating speed range unless moving parts such as flaps are introduced to maintain optimum conditions. 
     It is known to control ride height by use of a ladder of hydrofoil lifting surfaces. As the speed of the craft increases, lift is generated and the craft rises. As lift increases, the upper lifting surface is lifted clear of the water as the water craft rises. The operational loss of a lifting surface results in reduced lifting area, producing a reduction in total lift. This continues as the craft rises until equilibrium is reached and the craft rises no further. In addition, each of the plurality of foils produces additional drag at low speed in a fully immersed condition. 
     An alternative known solution is use of an inclined hydrofoil that pierces the surface of the water. Again, as the speed of the craft increases, lift is generated and the water craft rises. As the craft rises, a portion of the inclined foil rises out of the water thus resulting in a reduced lifting area and a reduction in total lift. A further consequence of the lifting surface rising above the surface of the water is the resulting unwanted ventilation and spray drag. 
     In the case of the inclined hydrofoils, sections that are optimal when fully immersed, are sub-optimal at the water surface and produce undesirable characteristics as they pass through it—such as unwanted ventilation and spray drag. For ladder foils as well as the above difficulties the multiple small hydrofoils and junctions produce additional drag at low speed in a fully immersed condition. 
     An alternative approach to height control is use of fully immersed hydrofoils that control ride height by varying the amount of lift generated via a mechanical or electrical surface sensor. However, such systems struggle to control height accurately in the presence of large waves and varying loads resulting from variations in operating conditions. Further, such systems add complexity and require long vertical legs between the buoyant body and the lifting hydrofoils. 
     It is difficult for mechanical systems to control height accurately in the presence of large waves and varying loads. Additionally sections with high lift to drag ratios cavitate at high speeds so reducing their lift. 
     Super ventilating surface running hydrofoils have been used to control ride height directly as they run on the surface. However, such hydrofoils tend to have high drag and low lift at low speed and undesirable pitching moment characteristics. The transition from unventilated to ventilated operation is often associated with undesirable non-linear lift behaviour with hydrofoil sections with high lift to drag ratios tending to cavitate at high speeds thus reducing the amount of lift generated. Since ventilated foils often require sharp or very thin leading edge sections they are also vulnerable to damage and erosion. This mitigates against the use of simple fibre-composite construction adding both expense and complexity. 
     The present applicants have identified the need for a simple robust hydrofoil system together with constructional techniques and configurations for deploying it advantageously on water craft such that the said craft inherently maintains an appropriate ride height, has good lift-drag characteristics at low speed and transitions smoothly between non-ventilated and ventilated operation and further, does not suffer any adverse effects when the hydrofoils ventilate and does not suffer any significant degradation of performance at high speed due to cavitation. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a robust hydrofoil of simple construction configured for use with water craft such that the craft inherently maintains an appropriate ride height, has good lift-to-drag characteristics at low speed, transitions smoothly between non-ventilated and ventilated operation and does not suffer significant degradation of performance at high speed due to cavitation. 
     In accordance with the present invention there is provided a watercraft comprising:
         at least one buoyant body,   at least one strut extending below the or each buoyant body,   a hydrofoil secured to the or each strut beneath the buoyant body, the hydrofoil being for lifting the buoyant body of the watercraft above the surface of the water and   means for driving the watercraft forwards;
 
wherein the hydrofoil is adapted to plane when the watercraft is driven above a planing speed and has:
   a leading edge,   a trailing edge,   a first, lower surface between the leading edge and the trailing edge, the lower surface being:
           smoothly curved and shaped:
               to plane on the surface of the water when the watercraft is driven forwards at or above the planing speed with the lower surface wetted at normal pitch attitude and   to generate lift when submerged with the watercraft being driven at less than planing speed at normal pitch attitude and   
               
           a second, upper surface between the leading edge and the trailing edge, the upper surface having:
           at least one discontinuity between the leading edge and the trailing edge, providing a fore portion between the leading edge and the discontinuity and an aft portion between the discontinuity and the trailing edge and being shaped:
               at the fore portion to generate negligible lift in comparison with that of the lower surface when submerged at normal pitch attitude and   at the aft portion to slope down towards the trailing edge when submerged at normal pitch attitude, whereby:
                   the aft portion generates lift when the watercraft is driven at a speed lower than a super-ventilation speed with the aft portion wetted and   the aft portion generates less lift when the watercraft is driven at a speed above the super-ventilation speed to allow formation of super-ventilation over at least part of the aft portion.   
                   
               
               

     Preferably, the buoyant body is a sailing boat or ship but can also be a sailboard or other vessel or body having a preferred ride height. 
     Non-active surfaces of the hydrofoil are allowed to ventilate and hence, as long as a supply of air (or other gas) is available, hydrofoil behaviour is consistent across a very wide speed range. The hydrofoil can either run fully submerged with air delivered via a channel or along the exterior of a suitably designed strut or other foil, or at the surface in which case it planes at the water surface. Performance degradation caused by impact with waves is not significant since air is entrained on immersion and hydrofoil behaviour is largely unaffected. 
     When applied to a sailing craft the ventilated hydrofoil may serve as the primary lifting foil which runs at the water surface, a conventional foil may then be applied aft to operate as a stabilising foil. In this way the height control of the vessel is provided by the surface following tendency of the main ventilated surface and the aft foil finds a natural level of submersion at which to operate. Optionally, the stabilising foil may also be of surface running form in which case both surfaces will plane on the surface. 
     In another form the aft stabilising foil may be mounted on the rudder. In yet another form the aft stabilising hydrofoil may comprise two hydrofoils, one of ventilated and surface running form and a conventional, non-ventilated, or ventilated hydrofoil positioned below the surface running hydrofoil. In this way ventilation down the rudder may be controlled by the presence of the surface running hydrofoil resulting in more reliable rudder operation. The surface running foil may also provide a discontinuity of lift with immersion depth and so provide a reference for maintenance of the correct running angle for the vessel, and hence the primary lifting hydrofoil angle of attack. 
     A ventilation path may be provided by a strut or struts that attach the hydrofoil to the vessel by making the strut or struts of wedge cross section such that the base of the wedge forms the trailing edge of the strut or struts. In this way the pressure on the base (base pressure) will, in operation, be reduced below that of the free stream and will entrain air from the water surface and conduct it down to the low pressure regions on the second face of the hydrofoil and so provide an air source for ventilation of the hydrofoil. 
     Preferably, the strut is substantially vertical. 
     If the attachment strut base is configured to coincide with the aftmost second face discontinuity the ventilation air flow will first reach the aftmost facet and air will then reach the facets ahead of the aftmost discontinuity in a sequential manner with increasing speed. 
     In another embodiment, the attaching struts may be of conventional i.e. non-cavitating or non-ventilating hydrofoil cross section with the trailing edge truncated to provide a base area. In this way the pressure on the base (base pressure) will, in operation, be reduced below that of the free stream and will entrain air from the water surface and conduct it down to the low pressure regions on the second face of the hydrofoil and so provide an air source for ventilation of the hydrofoil. 
     In yet another embodiment, the attaching struts may carry a second base area in the form of an aft facing step positioned ahead of the strut trailing edge and meeting the second face of the hydrofoil ahead of the strut trailing edge. In operation this allows an additional air path to more forwardly located facets. If the top of this aft facing step is below the point at which the strut meets the surface of the hull the step may be prevented from conduction air to the more forwardly located facets until the hull has been lifted some distance above the static rest waterline. This allows a higher degree of ventilation to be established before the hydrofoil reaches the water surface resulting in a smaller change in performance as surface running is established. 
     In another configuration the primary lifting hydrofoil may be placed behind the stabilising, secondary foil in which case it will be beneficial for both surfaces to be of ventilated form. A configuration where both hydrofoils are of similar size and of ventilated form may also be found to be beneficial in that it will give a wide, stable centre of gravity position range. Although the board may be rolled to generate a lateral component of force to resist the lateral rig loads, a vertical hydrofoil would be beneficial in a similar manner to the vertical fins normally used under sailboards to ensure that lateral resistance is always available to react the rig loads. This vertical fin may either be attached directly to the board or to the primary lifting hydrofoil. If necessary, for the purposes of directional balance against sail loads, the vertical fin may be positioned ahead of, or behind the main lifting hydrofoil by means of a boom extending ahead or behind the hydrofoil. 
     If the primary lifting hydrofoil is positioned behind the secondary hydrofoil the secondary hydrofoil may be configured to provide some directional stiffness by means of dihedral, i.e. the tips are raised above the root. This dihedral may take the form of a vee foil, which may then be surface piercing, or a highly tapered planform such that the tip section is significantly thinner than the root and the dihedral is then on the lower surface only. The dihedral then provides a small keel area to the secondary hydrofoil which generates some lateral force in response to side slip. 
     In another embodiment the lateral resistance of the secondary hydrofoil may be provided by a fin or fins below the hydrofoil. 
     The secondary hydrofoil may have a section in accordance with the present invention. It may also be of low aspect ratio, typically less than two, to provide a high stalling angle and make the board less prone to uncontrollable divergences in pitch due to stalling, particularly in rough water. 
     The hydrofoil of the watercraft is in the fully ventilated condition. The load is carried by the first lower pressure face and the second face is designed to carry a zero pressure differential. Air is admitted to the flow around the foil such that the aft portion of the upper surface are geometrically defined by a free surface, i.e. if the foil surface was locally removed, the flow pattern would match the removed surface and hence other than the first face, all surfaces are defined by the natural free-surface of the fluid. 
     As all load is carried on the first lower face flow separation is rarely a concern and the maximum mean pressure coefficient can approach unity although the pressure drag in this case would be excessively high. Having selected a pressure distribution, a camber line is developed to produce a desirable chordwise load distribution. 
     A symmetrical thickness distribution produces, on both first lower and second upper surfaces, half the pressure loading for the chordwise load distribution on each surface. 
     Adding the thickness distribution to the camber line produces a cambered section with a zero pressure coefficient on the second upper face and the designed pressure coefficient distribution, and hence full chordwise loading on the first face. The second surface takes the form of a free surface. 
     A practical way to design hydrofoil sections of this form is to define an array of vorticity across the chord of the foil where the vortex strengths are set to develop the intended chordwise loading at free stream velocity across the chord. This is sufficiently accurate for a thin, lightly loaded foil although corrections to the free stream velocity will become necessary if very high pressure coefficients are sought as the camber will be increased and streamwise direction velocity increments induced by the vorticity become significant. Effective designs have been developed using this method up to positive pressure coefficient values of around 0.5. Solving the flow vectors across the chord in the presence of this array of vorticity provides the slope of the camber line across the chord which, in turn, allows the camber line to be developed. The thickness distribution is developed using a chordwise array of sources, the strengths of these sources being solved to develop half the intended chordwise loading across the chord, in this case, symmetrically and on both faces. The addition of the thickness form to the camber line results in the pressure loading on each face being additive, hence the second face becomes zero-loaded and the first face then carries the full load at the design condition. 
     As sections with very sharp, thin leading edges are vulnerable to damage and can have handling risks the section is preferably modified by the addition of slight thickening, or armour, at the leading edge. This will give a small rounding to the leading edge and will result in increased strength in the region of the main force production and positive load on the first face. The applicant has been surprised to ascertain that a small thickening, typically one percent of section chord or lower, will not affect the overall performance of the section to any significant degree despite some localised cavitation. 
     The aft section of the second upper face of the hydrofoil section may be truncated between the discontinuity and the trailing edge. In this way the low speed (unventilated or partially ventilated) characteristics of the foil may be modified. This may also be used to allow adjustment of the structural capabilities of the section. 
     To ensure clean separation of the free surface from the second face the second face must diverge from the free surface at a discontinuity, this discontinuity may take the form of a sharp chine, i.e. a local, sudden, angular change in direction away from the free surface, or an aft facing step. Under lower speed operation, i.e. operation in which the reduction in pressure coefficient after the chine or step is insufficient for ventilation to overcome hydrostatic pressure at the level of immersion of the hydrofoil, the flow will now remain attached to the second face and, if the second face is so configured, will result in a greater deflection of the flow and a negative pressure coefficient on the second face. In this way the lift coefficient of the section may be increased at lower speeds without any significant change in geometric incidence and without the addition of moving parts (e.g. flaps). 
     Further, the aft portion of the second upper face is divided into a series of facets by further discontinuities. Each facet may be defined by a straight or curved line when considered as a two-dimensional section, the precise profile being defined by the desired flow characteristics when operating with the flow attached to that facet. 
     A progressive ventilation may be achieved with increasing speed such that the aftmost facet ventilates first, followed by ventilation of the next most aft facet until the second free surface departs the second face at the most forward discontinuity and fully ventilated operation is established. This results in a series of lift coefficient steps with increasing or decreasing speed as each facet ventilates and the flow geometry is modified providing a progressive reduction in lift coefficient with increasing speed and a corresponding progressive increase in lift coefficient with decreasing speed. The first lower surface generates useful force at lower speeds as the water craft accelerates such that a very wide operational speed may be used. 
     Advantageously, since all load is carried by surfaces with a positive pressure coefficient, cavitation is entirely eliminated or reduced to a limited area adjacent the leading edge. 
     Advantageously, since the hydrofoil requires ventilation in use, the foil will naturally tend toward a running position at the water surface when sufficient speed is achieved simultaneously raising the craft to a corresponding level. In a suitable foil configuration this gives a craft so fitted a natural surface following capability. 
     Non-active parts of the hydrofoil are allowed to ventilate and hence, as long as a supply of air (or other gas) is available, the foil behaviour is consistent across a very wide speed range. The foil can either run fully submerged with air delivered via a channel or along the exterior of a suitably designed strut or other foil, or at the surface in which case it planes at the water surface. Impact with waves is not significant since air is entrained on immersion and the foil behaviour is largely unaffected. 
     When applied to a sailing craft the ventilated foil may serve as the primary lifting foil which runs at the water surface, a conventional foil may then be applied aft to operate as a stabilising foil. In this way the height control of the vessel is provided by the surface following tendency of the main ventilated surface and the aft foil finds a natural level of submersion at which to operate. Optionally, the stabilising foil may also be of surface running form in which case both surfaces will plane on the surface. 
     In another form the aft stabilising foil may be mounted on the rudder. In yet another form the aft stabilising hydrofoil may comprise two hydrofoils, one of ventilated and surface running form and a conventional, non-ventilated, or ventilated hydrofoil positioned below the surface running hydrofoil. In this way ventilation down the rudder may be controlled by the presence of the surface running hydrofoil resulting in more reliable rudder operation. The surface running foil may also provide a discontinuity of lift with immersion depth and so provide a reference for maintenance of the correct running angle for the vessel, and hence the primary lifting hydrofoil angle of attack. 
     A ventilation path may be provided by a strut or struts that attach the hydrofoil to the vessel by making the strut or struts of wedge cross section such that the base of the wedge forms the trailing edge of the strut or struts. In this way the pressure on the base (base pressure) will, in operation, be reduced below that of the free stream and will entrain air from the water surface and conduct it down to the low pressure regions on the second face of the hydrofoil and so provide an air source for ventilation of the hydrofoil. 
     If the attachment strut base is configured to coincide with the aftmost second face discontinuity the ventilation air flow will first reach the aftmost facet and air will then reach the facets ahead of the aftmost discontinuity in a sequential manner with increasing speed. 
     In another embodiment, the attaching struts may be of conventional i.e. non-cavitating or non-ventilating hydrofoil cross section with the trailing edge truncated to provide a base area. In this way the pressure on the base (base pressure) will, in operation, be reduced below that of the free stream and will entrain air from the water surface and conduct it down to the low pressure regions on the second face of the hydrofoil and so provide an air source for ventilation of the hydrofoil. 
     In yet another embodiment, the attaching struts may carry a second base area in the form of an aft facing step positioned ahead of the strut trailing edge and meeting the second face of the hydrofoil ahead of the strut trailing edge. In operation this allows an additional air path to more forwardly located facets. If the top of this aft facing step is below the point at which the strut meets the surface of the hull the step may be prevented from conduction air to the more forwardly located facets until the hull has been lifted some distance above the static rest waterline. This allows a higher degree of ventilation to be established before the hydrofoil reaches the water surface resulting in a smaller change in performance as surface running is established. 
     The hydrofoil may be provided with sweep such that the hydrofoil tips are positioned behind the hydrofoil root. If sufficient sweep is provided and ventilation paths are provided to the hydrofoil root area, the flow over the hydrofoil will have a component along each second face discontinuity from root to tip. This can assist the spanwise spread of ventilation along each discontinuity. 
     The hydrofoil may also be provided with sweep such that the hydrofoil tips are positioned ahead of the hydrofoil root. If sufficient sweep is provided and ventilation paths are provided to the hydrofoil tip area, the flow over the hydrofoil will have a component along each second face discontinuity from tip to root. This can assist the spanwise spread of ventilation along each discontinuity. 
     Another means of controlling the spanwise development of ventilation is by means of upper surface fences as is well known in the art of conventional hydrofoils, however, their application to ventilated hydrofoil is not found in the art. This is advantageous if, for example, the tip sections are designed to ventilate at a higher speed than the root sections or that ventilation must be inhibited on a part of the hydrofoil until surface running is established, or that the tips may break the water surface first as ride height is increased and the additional ventilation path resulting from this breaking the surface must be limited to avoid a sudden loss of lift. 
     If the second face discontinuities are configured as aft facing steps some control of spanwise ventilation rate may also be achieved by varying the step depth across the span, for example, if root ventilation is desired the steps may be configures to be of greater depth close to the root and lesser depth towards the hydrofoil tips. In another embodiment the step may be tapered out to zero depth at a partial span location and the discontinuity may then continue as a simple chine. 
     A ventilated hydrofoil that may achieve a surface running condition may also be furnished with a second, conventional hydrofoil positioned beneath the ventilated hydrofoil. In this way the ride height of the assembly may be set by the position of the surface running hydrofoil whereas the conventional hydrofoil may provide a substantial part of the total lift. This will be found advantageous in that ride height may then be controlled without moveable components or surface following mechanisms or sensors. 
     If applied to a sailboard, the main lifting hydrofoil may be positioned ahead of, but close to the centre of gravity. A conventional trailing submerged foil, or another surface running ventilated foil may then be attached to the rear of the board as a stabilising surface. 
     In another configuration the primary lifting hydrofoil may be placed behind the stabilising, secondary foil in which case it will be beneficial for both surfaces to be of ventilated form. A configuration where both hydrofoils are of similar size and of ventilated form may also be found to be beneficial in that it will give a wide, stable centre of gravity position range. Although the board may be rolled to generate a lateral component of force to resist the lateral rig loads, a vertical hydrofoil would be beneficial in a similar manner to the vertical fins normally used under sailboards to ensure that lateral resistance is always available to react the rig loads. This vertical fin may either be attached directly to the board or to the primary lifting hydrofoil. If necessary, for the purposes of directional balance against sail loads, the vertical fin may be positioned ahead of, or behind the main lifting hydrofoil by means of a boom extending ahead or behind the hydrofoil. 
     If the primary lifting hydrofoil is positioned behind the secondary hydrofoil the secondary hydrofoil may be configured to provide some directional stiffness by means of dihedral, i.e. the tips are raised above the root. This dihedral may take the form of a vee foil, which may then be surface piercing, or a highly tapered planform such that the tip section is significantly thinner than the root and the dihedral is then on the lower surface only. The dihedral then provides a small keel area to the secondary hydrofoil which generates some lateral force in response to side slip. 
     In another embodiment the lateral resistance of the secondary hydrofoil may be provided by a fin or fins below the hydrofoil. 
     The secondary hydrofoil may have a section in accordance with the present invention. It may also be of low aspect ratio, typically less than two, to provide a high stalling angle and make the board less prone to uncontrollable divergences in pitch due to stalling, particularly in rough water. 
     Construction of hydrofoils in accordance with the present invention may be of any suitable material, however, as the leading edges tend to be extremely thin they can be vulnerable to damage, accordingly it may be found to be beneficial to place a metallic amour around the leading edge. This may be applied within a moulding process such that the armour becomes a part of the mould, or it may be attached after moulding. The aft facing steps arising from the edges of the armour do not adversely affect the performance of the foil since the first face operates under a highly stable, positive pressure coefficient environment and the edge on the second face will act as a natural break point for the free surface to separate the flow from the surface of the foil. 
     In a further embodiment, fences may be applied to the upper surface of the hydrofoil. Fences are small fins placed to prevent ventilation air from migrating along a hydrofoil. The fences are attached to the hydrofoil running in a fore-aft orientation to be parallel to the direction of fluid flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  shows an example hull with a typical hydrofoil installation; 
         FIG. 2  shows a hydrofoil section designed according to the basic principles of the present invention; 
         FIG. 3  shows a hydrofoil section designed according to the principles of the present invention with indicative streamlines representing ventilated and non-ventilated operation; 
         FIG. 4  shows a hydrofoil section designed according to the principles of the present invention with the second surface discontinuity in the form of an aft facing step; 
         FIG. 5  shows a hydrofoil section designed according to the principles of the present invention with sequential second-surface discontinuities and associated indicative streamlines; 
         FIG. 6  shows a hydrofoil section designed according to the principles of the present invention with sequential second-surface discontinuities configured as aft-facing steps and associated indicative streamlines; 
         FIG. 7  is a table of co-ordinates for a hydrofoil; 
         FIG. 8  is a perspective view of a hydrofoil; 
         FIG. 9  is a perspective view of a hydrofoil assembly with multiple second surface discontinuities; 
         FIG. 10  is a three-view drawing of a foil installation on a sail board; 
         FIG. 11  is a three view drawing of the forward foil of the board; 
         FIG. 12  is a pressure plot of an advantageous pressure distribution for an hydrofoil for a watercraft; 
         FIG. 13  is an hydrofoil cross-sectional shape generating the pressure distribution of  FIG. 12 ; 
         FIG. 14  is an hydrofoil cross-sectional shape for a watercraft of the invention; 
         FIG. 15  is a pressure plot for the hydrofoil of  FIG. 14  at slow, non-ventilated, fully wetted speed; 
         FIG. 16  is a cross-section similar to  FIG. 14  showing the hydrofoil operating at ventilated speed; 
         FIG. 17  is a pressure plot for the hydrofoil of  FIG. 14  at ventilated speed of  FIG. 16 ; 
         FIG. 18  is an alternative hydrofoil cross-sectional shape for a watercraft of the invention; 
         FIG. 19  is a pressure plot for the hydrofoil of  FIG. 18  at slow, non-ventilated, fully wetted speed; 
         FIG. 20  is a cross-section similar to  FIG. 18  showing the hydrofoil operating at partially ventilated speed; 
         FIG. 21  is a pressure plot for the hydrofoil of  FIG. 18  at partially ventilated speed of  FIG. 20 ; 
         FIG. 22  is a cross-section similar to  FIG. 18  showing the hydrofoil operating at fully ventilated speed; 
         FIG. 23  is a pressure plot for the hydrofoil of  FIG. 18  at fully ventilated speed of  FIG. 22 ; 
         FIG. 24  is a cross-section similar to  FIG. 18  showing the hydrofoil operating at surface running/planing speed; and 
         FIG. 25  is a pressure plot for the hydrofoil of  FIG. 18  at surface running/planing speed of  FIG. 24 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     By reference to  FIG. 1  a typical installation of a hydrofoil in accordance with the present invention may be described. From the hull means ( 1 ) depends a substantially vertical strut ( 4 ) to which, at the foil-borne water surface is attached a substantially horizontal hydrofoil means ( 2 ), below this hydrofoil ( 2 ) is a further substantially vertical surface ( 7 ) which provides lateral resistance when the vessel is foil-borne with the foil ( 2 ) at the water surface. A further foil( 6 ) is mounted on the rudder ( 3 ) to act as a stabilising surface. This may be of conventional non-ventilating and non-cavitating form and configured to operate fully submerged. To provide a means of keeping the primary foil ( 2 ) at the correct incidence a further foil ( 5 ) may be attached to the rudder at a height setting coinciding with the foil-borne water surface. This foil ( 5 ) may advantageously be of ventilated form to allow consistent surface running operation. The primary foil ( 2 ) and the rudder surface running foil ( 5 ) have a section designed in accordance with the present invention. A cross section of the primary foil ( 5 ) is shown on  FIG. 1  as a section through A-A ( 8 ). 
       FIG. 2  illustrates the significant features of a hydrofoil section ( 8 ) designed in accordance with the present invention. There is provided a hydrofoil section comprising a first face ( 9 ) and a second face ( 10 ) which creates, in operation at speeds above the ventilation speed, a ventilated cavity( 11 ) defined by a first cavity face ( 12 ) which departs from the first hydrofoil face ( 9 ) and a second cavity face ( 13 ) which departs from the second hydrofoil face ( 10 ), each cavity surface representing a free surface and each face ( 9 , 10 ) separating from the said free surfaces at a discontinuities on those faces ( 15 ), the separated faces forming a continuation of the first and second faces of arbitrary shape ( 14 ) and enclosed by the free surfaces ( 12 , 13 ) without contacting the said free surfaces ( 12 , 13 ). Below the speed at which full ventilation occurs the arbitrarily shaped portion of each face ( 14 ) is configured to provide a modified flow configuration resulting in changed lift and or drag and or pitching moment under partial, or unventilated operation. 
       FIG. 3  illustrates the two flow conditions representing ventilated and unventilated operation. The two states of operation are governed by behaviour at the second surface discontinuity ( 16 ). If the flow remains attached after this point a negative pressure coefficient will be generated over the aft region ( 14 ) of the second face ( 10 ) and the flow will generate the stagnation streamlines shown in the  FIG. 17 ). If the pressure over this region ( 14 ) falls below the hydrostatic pressure arising from the depth of foil immersion and an air path is provided (e.g. by a duct or other means) the aft region ( 14 ) will ventilate and flow will separate at the discontinuity ( 16 ), the free surfaces ( 12 , 13 ) will then become established around the ventilation cavity ( 11 ). The ventilated cavity surfaces are similar to those shown in  FIG. 1  ( 12 ,  13 ). In non-ventilated operation the leading and trailing stagnation streamlines are shown ( 17 ) and the greater deflection of, particularly the trailing stagnation streamline, indicates that the lift coefficient will be significantly increased over that of the ventilated state. 
       FIG. 4  shows the discontinuity ( 16 ) replaced by an aft facing step ( 18 ) to ensure more positive separation of the free-surfaces ( 12 ,  13 ) when ventilation conditions are present. The streamline patterns are only minimally affected by this shape and are shown as being similar to  FIG. 3  in both the ventilated condition with the free surfaces ( 12 ,  13 ) and the non-ventilated condition with the stagnation streamlines ( 17 ) depicted. 
       FIG. 5  illustrates the subdivision of the continuation of the second face ( 14 ) into a series of facets ( 20 ,  21 ), each with a discontinuity ( 16 ,  22 ) at the leading edge of the facet. When in non-ventilated operation the stagnation streamlines ( 17 ) apply and the maximum lift coefficient occurs. As speed increases the pressure on the aftmost facet ( 20 ) reduces until ventilation occurs and flow separation is established at the aft discontinuity ( 22 ) resulting in cavity surfaces ( 19 ) deflected through a lesser angle than the stagnation streamlines ( 17 ) and a reduced lift coefficient. Further increase in speed results in the pressure reducing over the next most forward facet ( 21 ) which may then draw air forwards from the cavity defined by the two free surfaces ( 19 ) to the discontinuity at the leading edge of the facet ( 16 ). This results in a further reduction in lift as evidenced by the deflection angles associated with the new cavity shape defined by cavity surfaces ( 12 ,  13 ). A facetted foil is illustrated below, for operation at three different lift coefficients, namely, 0.2 fully ventilated, 0.48 with ventilation initiating from the aft discontinuity ( 22 ) and 0.77 for non-ventilated operation. 
       FIG. 6  shows a similar situation to  FIG. 5  but with the discontinuities replaced by aft facing steps ( 18 ). There is only a minimal impact on the flow patterns associated with this replacement, however, separations at the discontinuities become more positive and reliable due to the more severe discontinuity caused by the steps. 
       FIG. 7  presents a table of co-ordinates for a hydrofoil with a design lift coefficient of 0.2. The “upper surface” is the second surface ( 10 ), the “lower surface” is the first surface ( 9 ). The upper surface follows the free surface cavity shape ( 13 ) and discontinuities may be placed at any location on this surface, the lower surface ( 9 ) is the force generation surface and the lower cavity surface ( 12 ) continues from this face at the 100% chord location. 
       FIG. 8  provides a perspective view of a primary lifting hydrofoil assembly. The substantially vertical strut ( 4 ) comprises a forward portion ( 27 ) and an aft portion ( 26 ). The forward portion may be of a straight sided wedge section or with cambered faces, the aft portion may also be of a straight sided wedge section or with cambered faces. The intersection between the forward and aft portions is a discontinuity such as a sharp change in angle or an aft facing step as shown. In operation this discontinuity ( 25 ) generates a local drop in pressure which entrains air from the surface to feed the discontinuity ( 18 ) of the substantially horizontal hydrofoil ( 2 ). The substantially horizontal hydrofoil ( 2 ) is shown with an aft facing step at the second surface discontinuity. This is an example of a single discontinuity, more discontinuities may be beneficial if more steps in lifting performance are desired. This step is tapered in depth from the root ( 23 ) to the tip ( 24 ) with the depth of the step being less at the tip than the root. This feature provides an air path at the discontinuity in operation of greater cross-sectional area at the root than the tip. Since air reaching the tip ( 24 ) must first travel from the root ( 23 ) but must also ventilate the root it is clear that the spanwise flow rate must be higher at the root than the tip and so a greater spanwise flow path cross-section will be advantageous. A small amount of sweepback of the hydrofoil ( 2 ) is shown this is advantageous to ventilation as the water flow across the foil is, by this means, given a spanwise component along the discontinuity ( 18 ) and this assists the airflow in the spanwise direction and hence aids the establishment of full ventilation. Generally greater sweep will give a greater benefit in this regard although the drag performance of the hydrofoil may then be impaired and hence an engineering compromise is implicit. 
       FIG. 9  illustrates a foil assembly with multiple second surface discontinuities ( 18 ,  29 ), in this case two discontinuities are shown although more could be used. The substantially vertical strut ( 4 ) comprises a forward ( 27 ) and an aft part ( 26 ) separated by a discontinuity ( 25 ) in the form of an aft facing step. This discontinuity ( 25 ) meets at its lower end the aftmost discontinuity ( 29 ) on the substantially horizontal hydrofoil ( 2 ). Since the aftmost region of the hydrofoil ( 2 ) ventilates first with increasing speed the cavity so created is a source of air for ventilation of the forward discontinuity ( 18 ) as the pressure around this discontinuity ( 18 ) decreases with increasing speed. As speed increases, with no change in ventilation lift will also increase and the vessel may then be lifted above the water surface by the hydrofoil. Ventilation of the forward discontinuity ( 18 ) may then occur when the vessel is partially lifted and so there is provided a second discontinuity ( 28 ) on the vertical strut ( 4 ) which extends partially over the length of the vertical strut ( 4 ) and communicates with the water surface only when the foil has lifted sufficiently to expose the upper end of the discontinuity ( 28 ). By this means an additional air path is created for ventilation of the forward hydrofoil ( 2 ) discontinuity ( 18 ) as speed and lift are increased ensuring that when the hydrofoil ( 2 ) reaches the water surface substantially full ventilated operation has been established and no sudden lift loss occurs. 
       FIG. 10  presents a three view drawing of a hydrofoil assembly configured for a sail board. Two substantially vertical struts ( 31 ,  33 ) are attached below the board ( 30 ) in forward ( 31 ) and aft ( 33 ) locations and carry at their respective lower ends two substantially horizontal hydrofoils ( 32 ,  34 ). Below the aft hydrofoil ( 34 ) is a further substantially vertical surface ( 36 ) which resists lateral forces when in operation and provides directional stability. This surface ( 36 ) is attached to the aft hydrofoil ( 34 ) via a boom ( 35 ) extending in an aftwards direction from the root of the hydrofoil ( 34 ). The primary lifting surface is the aft hydrofoil ( 34 ) and pitch stability and a small amount of lift is provided by the forward hydrofoil ( 32 ) which also provides tactile surface reference feedback to the operator to assist in keeping the board trimmed at the optimum incidence angle to the water surface. The forward foil ( 32 ) is of low aspect ratio (typically less than two) and may also incorporate significant leading edge sweepback (typically greater than 45 degrees), both features leading to high stalling angles of attack and rendering the behaviour of the board more consistent in rough water and when manoeuvring. Another feature of the forward foil ( 32 ) is that the lower surfaces form a shallow vee when viewed from the front. This provides a small keel effect, i.e., some lateral resistance is created when the foil, and hence the board, is side slipped. This, in combination with the substantially vertical surface ( 36 ) provides some directional stiffness and damps directional stability. 
       FIG. 11  shows a three view of the forward foil ( 32 ). The front view (right hand side of figure) shows the shallow vee angle in the lower surface ( 37 ) which provides a small keel effect. The upper surface has two discontinuities ( 38 ) which operate in the same manner as already described. These discontinuities assist with lifting the front of the board at lower speeds and help to avoid a nose-diving tendency as the board accelerates. 
     Referring to  FIG. 12 , its graph shows an advantageous pressure distribution for a hydrofoil. For a typical leading to trailing edge chord, the upper surface has a substantially zero pressure distribution  101 , generating negligible lift, whereas and substantially constant for the lower surface has a substantially constant pressure distribution  102 , generating substantially evenly distributed lift for supporting the weight of the watercraft to be supported by the hydrofoil. Normally this will be the entire weight of a hydrofoil watercraft of the invention. In other words, this graph indicates zero lift suction over the upper surface and substantially constant lift pressure over the lower surface. 
       FIG. 13 , is a typical hydrofoil shape capable of exhibiting this pressure distribution. Its upper surface  103  is substantially flat and horizontal in use, whereby flow over the top surface is flat and aligned with free stream flow past the hydrofoil. Such flow generates no suction on the upper surface. Its lower face  104  is angled down and slightly concave from the leading edge  105  at a steadily increasing angle. Flow along the lower surface is progressively deflected downwards giving rise to lift pressure on the lower surface. Whilst the leading edge of this foil shape is sharp, its trailing edge  106  is a vertical face. This latter creates vortices and reduced pressure. It results in drag on the hydrofoil, which is disadvantageous. 
     The invention provides a practical foil shape shown in  FIG. 14  benefiting from advantageous pressure distribution, in that a front portion  1031  of the top surface remains substantially flat from the leading edge  105  to a discontinuity step  107  beyond which an aft portion  1032  of the top surface slopes down to the trailing edge in the same position as that of  FIG. 13 . The pressure plot for this shape at slow speed,  FIG. 15  has a top surface suction hump  108  from the discontinuity back to the trailing edge, whilst the lower surface retains the same plot for the same lower surface shape and speed. It should be noted that the quantum of upper surface suction and lower surface lift is less than that of  FIG. 12  and not enough to support the full weight of the watercraft which still requires some buoyancy support at this speed. When speed increases, the top surface flow detaches at the discontinuity as result of ventilation show diagrammatically as  109 , resulting in an air bubble  110  below the water surface  111  forming over the rear portion  1032  and a substantial reduction of the top surface hump  108 ′, as shown in  FIGS. 16 and 17 . However the lower surface pressure increases, with result that the watercraft rises slightly. 
     It should be noted that the pitch attitude of the watercraft has an effect on the lift of the hydrofoil. The Figures show the lift for the normal pitch attitude of the watercraft, namely parallel to its normal floating attitude. 
     As shown in  FIG. 18 , the hydrofoil shape can be further improved by provision of two discontinuities  1071 , 1072 . The pressure coefficient plot for this shape of hydrofoil at slow speeds has two humps,  FIG. 19 . As the speed increases to the point where the flow detaches from the rear discontinuity, the pressure coefficient plot changes with the reduction of the rear hump,  FIGS. 20 and 21 . On further speed increase, the flow detaches at the front discontinuity, with reduced suction behind the front discontinuity,  FIGS. 22 and 23 . 
     In this situation, the lift pressure generated by the lower surface is nearly enough to support the full weight of the watercraft. A small increase in speed causes the hydrofoil to rise to the surface,  FIG. 24 . At this point the hull of the watercraft, also referred to as its buoyant body, is clear of the water and the lower surface pressure supports the entire weight of the watercraft, as shown in  FIG. 25 . A trough  112  in the water surface extends behind the foil immediately behind it. 
     The advantages of these two foils are that at lower speed through the water, they generate increasing lift as speed increases to come close to being able to support the weight of the craft as the flow detaches at the discontinuities. Below the detachment speed, the rear portion of the upper surface generates suction lift, without significant drag. A small further increase in speed generates added lift from the lower surface alone for the hydrofoil to lift the craft for the foil to rise to the surface. 
     The absence of suction lift from the front portion of the upper surface results in there not being a reduction in overall lift as the foil breaks the surface, with the absence of the foil hunting above and below the surface. 
     The twin discontinuity foil has advantage over the single discontinuity in that the reduction of lift on flow detachment at the single discontinuity is smoother with increase in speed. 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.