Abstract: A keel assembly or similar fin-like underwater appendage for a boat is provided which is adjustable to adapt its characteristics to the operating conditions of the boat, for example to optimize hull performance of a sailing boat on opposite tacks. The assembly comprises a support and a pair of curved flexible side plates carried by the support and connected at their leading and trailing edges, the curvature of the side plates being adjustable to render the assembly asymmetrical. The leading and trailing portions of the plates are provided by blade assemblies interconnected so as to be oppositely pivotable about axes parallel to leading and trailing edges of the support, the trailing blade assembly having its pivoted axis adjacent its leading edge.