Deformable shrouded water foil

An underwater appendage for a sailboat of the type having a hull and at least one underwater appendage, said appendage comprising:

• a core portion being substantially keel-shaped having mounting means to said hull, a shell-like foil-shaped flexible outer shroud placed around the said core which shroud has a rounded front portion descending parallel to the front edge of the core appendage, with free space between the core and the rounded front of the shroud, and with the two sides of said shroud enveloping the opposite sides of said core, with the upper and lower edges of said shroud being essentially horizontal and with the upper edge being close to the bottom of said hull, and the lower edge being nearer to the bottom of said core, with said shroud being disposed such that the sides of the shroud are essentially parallel to the core, and spaced outward from it, such that the distances of the outer shroud sides from said core is substantially constant along a line parallel to the descending axis of said core but which distance gradually diminishes toward the rear descending edge of said core such that the opposite sides of the shroud join their respective sides of the core near the rear descending edge of the said core with there being mounting means for both rear edges of said foil shroud to the said core where said shroud joins said core, forming an outer foil shaped shroud in a position such that the incidence of said foil is substantially parallel to the fore/aft axis of the boat in the undeformed symmetrical condition of said shroud and cross sections of said foil in any horizontal plane having a rounded front incidence portion tapering toward the rear edge such that a hydrofoil shape is formed by said shroud and a space exists between said shroud and the core at all points on the inside surface of said shroud except where it has mounting means along its rear edge where it joins the said core, said mounting means being the only attachment of said shroud such that said spaces provide space for displacing the forward round descending portion of the shroud to either side of said core, and space for the corresponding side of the shroud to displace in the same direction, except for the rear fixed edge of such side, and space for the opposite side of said shroud to displace inward toward said core, creating an incidence angle between the axis of said foil shroud and the fore/aft axis of said boat, and said space receiving variable actuating means for said displacing and for bulging of said shroud on the side which has been displaced outward, and variable actuating means for said bulging and said displacing of said shroud and variable actuating means for flattening the opposite side of said shroud such that an asymmetrical foil shape may be formed, creating lift in the direction in which the foil is both displaced and bulged and reversing means for such actuation in the opposite direction such that the desired hydrodynamic lift may be created on the opposite side of the appendage, said variable means also adjusting the shroud to a symmetrical foil when no lift is desired.