Abstract: A sailboat, and a variable foil appendage, ie., keel or rudder for a sailboat, the appendage having a central core portion, and movable skin portions on either side thereof, the skin portions being connected to the core portion essentially along the forward and trailing edges of the skin portions, at least one of such connections being movable, and a fluid-operated skin movement control system being provided in the core portion, operation of the control system producing outward bulging or flattening of one or other of the skin portions to form a bulged generally convex profile extending upwardly and downwardly along one side of the core portion, while the skin portion on the opposite side of the core may be bulged or flat, the control system being selectively operable thereby enabling the foil section of the appendage to be changed, as the sailboat comes about from one tack to the other.