Underwater appendages for vessels

A fin assembly of the type having mounting means connectible to the underside of a vessel and a support structure fixed to and extending outwardly from said mounting means for supporting a foil shaped outer casing in a position such that the incidence plane of the foil in symmetrical shape is substantially parallel to the fore and aft axis of the vessel and the sides of the casing are spaced from said support structure, and wherein the casing is formed of flexible material whereby the respective opposite sides of said casing may move towards or away from said support structure to alter shape to an asymmetrical foil, characterised in that said casing is supported between normally freely pivotable fore and aft pivot assemblies connected pivotably about outwardly extending pivot axles disposed substantially parallel to said incidence plane and adjacent the respective leading and trailing edges of said support structure and wherein said aft pivot assembly is pivoted adjacent its leading edge to permit its trailing edge to be displaced laterally either side of its normally symmetrical position, whereby the incidence plane of the asymmetrical foil may be disposed at a positive angle of attack with respect to the said fore and aft axis of the vessel thus increasing the lift/drag ratio of said asymmetrically disposed casing, and torque transferring means comprising a first pivot lever operatively fixed to said fore pivot assembly, an oppositely arranged second pivot lever fixed to said aft pivot assembly and a transfer linkage interconnecting said levers such as to cause the fore and aft pivot assemblies to pivot oppositely to one another, whereby said casing articulates automatically to a respective asymmetrical attitude due to water pressure against a respective side of said casing, the respective sides of the trailing portions of said fore and aft pivot assemblies abutting said support structure when said casing is disposed in a respective said asymmetrical attitude.