HYDROFOIL WATERCRAFT
A hydrofoil section comprises first and second faces that create, in operation at speeds above a ventilation speed, a ventilated cavity defined by a first cavity face which departs from the first hydrofoil face and a second cavity face which departs from the second hydrofoil face. Each cavity face represents a free surface and each face separating from the said free surface at a discontinuity on that surface, the separated faces forming a continuation of the faces of arbitrary shape and enclosed by the free surfaces without contacting the said free surfaces. Below the speed at which full ventilation occurs the arbitrarily shaped portion of each face is configured to provide a modified flow configuration resulting in changed lift and or drag and or pitching moment under partial, or unventilated operation.
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 INVENTIONThe 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 INVENTIONHydrofoils 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 INVENTIONThe 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
- smoothly curved and shaped:
- 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.
- 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:
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.
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:
By reference to
Referring to
The invention provides a practical foil shape shown in
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
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,
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.
Claims
1-27. (canceled)
28. A hydrofoil watercraft having a normal pitch attitude, the watercraft comprising: wherein the hydrofoil is adapted to plane when the watercraft is driven above a planing speed and has:
- 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;
- 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.
29. The hydrofoil watercraft according to claim 28, wherein the strut is provided with ventilation means for atmospheric air communication to the discontinuity.
30. The hydrofoil watercraft according to claim 28, wherein the discontinuity is an aft facing step.
31. The hydrofoil watercraft according to claim 28, wherein:
- the fore portion of the second upper surface is substantially flat,
- the aft portion of the second upper surface is substantially flat
- the aft portion is angled down with respect to the fore portion from the discontinuity to the trailing edge.
32. The hydrofoil watercraft according to claim 28, wherein the aft portion of the second upper surface has at least one second discontinuity dividing the aft portion into facets.
33. The hydrofoil watercraft according to claim 32, wherein:
- the fore portion of the second upper surface is substantially flat,
- a fore one of the facets of the aft portion is substantially flat and is angled down with respect to the fore portion and
- an after one of the facets is substantially flat and is angled down with respect to the fore one of the facets.
34. The hydrofoil watercraft according to claim 32, wherein the strut is provided with ventilation means for atmospheric air communication to the second discontinuity.
35. The hydrofoil watercraft according to claim 32 where the second discontinuity is an aft facing step.
36. The hydrofoil watercraft according to claim 28, wherein the hydrofoil has a swept profile such that the discontinuity has a spanwise component.
37. The hydrofoil watercraft according to claim 28, wherein the aft facing step has a depth tapering towards a tip of the hydrofoil.
38. The hydrofoil watercraft according to claim 29, wherein the strut is substantially vertical and has
- a forward strut portion
- an aft strut portion, and
- an intersection between the forward and aft strut portions extending from the buoyant body to the hydrofoil, the intersection providing the ventilation means for atmospheric air communication to the discontinuity.
39. The hydrofoil watercraft according to claim 38, wherein the intersection is an aft facing step.
40. The hydrofoil watercraft according to claim 28, wherein:
- the aft portion of the second upper surface has at least one second discontinuity dividing the aft portion into facets,
- the strut is substantially vertical and has a forward strut portion an aft strut portion, an intersection between the forward and aft strut portions extending from the buoyant body to the hydrofoil, the intersection providing ventilation means for atmospheric air communication to the discontinuity and a second intersection between fore and aft facets of the after strut portion, the second intersection providing ventilation means for atmospheric air communication to the second discontinuity.
41. The hydrofoil watercraft according to claim 40, wherein the second intersection is an aft facing step.
42. The hydrofoil watercraft according to claim 28, wherein the leading edge of the hydrofoil has an armour surface.
43. The hydrofoil watercraft according to claim 28, including flow fences on the second upper surface.
44. The hydrofoil watercraft according to claim 28, including:
- a rudder and
- a stabilising, submerged hydrofoil is attached to the rudder.
45. The hydrofoil watercraft according to claim 28, including:
- a rudder and
- a secondary, surface running, ventilated hydrofoil is attached to the rudder at a foil-borne waterline.
46. The hydrofoil watercraft according to claim 28, wherein the hydrofoil is located close to the longitudinal centre of gravity of the watercraft and the watercraft includes a submerged stabilising foil positioned aft, the combination forming a stable, surface following combination.
47. The hydrofoil watercraft according to claim 28, wherein the hydrofoil is located close to the longitudinal centre of gravity of the watercraft and the watercraft includes a surface-running stabilising foil positioned fore, the combination forming a stable, surface following combination.
48. The hydrofoil watercraft according to claim 47, wherein the surface-running stabilising foil has an aspect ratio of less than two.
49. The hydrofoil watercraft according to claim 47, wherein the surface-running stabilising foil is swept by more than 45 degrees.
50. The hydrofoil watercraft according to claim 28, wherein the first lower surface of the hydrofoil has a dihedral angle when viewed in a vertical plane transverse to the longitudinal axis of the watercraft.
51. The hydrofoil watercraft according to claim 28, wherein the strut extends below the hydrofoil to provide stabilisation against leeway.
52. The hydrofoil watercraft according to claim 51, wherein the strut extends behind the hydrofoil as well as below the hydrofoil to provide stabilisation against leeway.
53. The hydrofoil watercraft according to claim 28, wherein the watercraft is a sail board.
54. The hydrofoil watercraft according to claim 28, wherein the watercraft is a sail boat.
55. The hydrofoil watercraft according to claim 28, wherein the watercraft is a motor boat.
56. A hydrofoil watercraft comprising: wherein the hydrofoil has: wherein both the strut and hydrofoil are provided with
- a buoyant body,
- a strut extending below the buoyant body,
- a hydrofoil secured to the strut beneath the buoyant body for lifting the buoyant body above the surface of the water when travelling at a surface running speed and
- means for driving the watercraft forwards;
- a first, lower surface shaped to generate a distributed pressure for supporting the watercraft with the hydrofoil running at the surface of the water at surface running speed and
- a second, upper surface having: at least one discontinuity between a leading edge and a trailing edge, dividing the upper surface into fore and aft portions and being shaped at the fore portion to generate negligible lift and at the aft portion to provide lift tending to raise the watercraft when travelling at speeds at which the aft portion is wetted by water flow in contact with it and to allow ventilation at intermediate speeds higher than the wetted speeds and below surface running speeds and
- ventilation means for atmospheric communication from above the surface of the water to the discontinuity in the upper surface of the hydrofoil.
57. The hydrofoil watercraft according to claim 56, wherein the ventilation means are aft facing steps in the strut and the second, upper face of the hydrofoil.
58. The hydrofoil watercraft according to claim 56, wherein:
- the fore portion of the second upper surface is substantially flat,
- the aft portion of the second upper surface is substantially flat
- the aft portion is angled down with respect to the fore portion from the discontinuity to the trailing edge.
59. The hydrofoil watercraft according to claim 57, wherein the aft portion of the second upper surface has at least one second discontinuity dividing the aft portion into facets.
60. The hydrofoil according to claim 59, wherein:
- the fore portion of the second upper surface is substantially flat,
- a fore one of the facets of the aft portion is substantially flat and is angled down with respect to the fore portion and
- an after one of the facets is substantially flat and is angled down with respect to the fore one of the facets.
61. A hydrofoil watercraft having lift inducing lower surface and a top surface which is substantially flat from a leading edge to a first discontinuity and substantially flat and angled down from the first discontinuity to a trailing edge.
62. The hydrofoil watercraft according to claim 61, wherein the discontinuity is an aft facing step.
63. The hydrofoil watercraft according to claim 61, wherein the strut is provided with ventilation means for atmospheric air communication to the discontinuity.
64. The hydrofoil watercraft according to claim 61 further comprising a second discontinuity, wherein the top surface which is substantially flat from the leading edge to the first discontinuity and further is substantially flat and angled down from the first discontinuity to a second discontinuity and substantially flat and angled further down from the second discontinuity to the trailing edge.
65. The hydrofoil watercraft according to claim 64, wherein the first and second discontinuities are aft facing steps.
66. The hydrofoil watercraft according to claim 64, wherein the strut is provided with ventilation means for atmospheric air communication to the discontinuities.
Type: Application
Filed: Sep 23, 2014
Publication Date: Mar 26, 2015
Inventor: Jonathan Sebastian Howes (Bolney)
Application Number: 14/494,318
International Classification: B63B 1/24 (20060101); B63H 25/38 (20060101);