Hydrofoil watercraft

A hydrofoil watercraft having a normal pitch attitude, the 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.

A hydrofoil watercraft comprising:

• 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;
• wherein the hydrofoil has:
• 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
• wherein both the strut and hydrofoil are provided with
• ventilation means for atmospheric communication from above the surface of the water to the discontinuity in the upper surface of the hydrofoil.