Abstract: A sidewall gas-cushion vehicle is provided with movable buoyancy members, each comprising a buoyant portion of relatively large volume and a web support portion of relatively little volume. The buoyancy members are pivotally connected to the bottoms of the sidewalls by way of their web support portions to the sidewalls, the members being movable about axes extending substantially parallel to the longitudinal axis of the vehicle. In fully submerged operative positions extending below the sidewalls for low speed operation, sufficient hydrostatic forces are applied to the fully submerged buoyant portions of the members to provide buoyancy lift for the vehicle and to counter unwanted movements of the vehicle. In inoperative positions retracted alongside the sidewalls for high speed operation the buoyant portions lie substantially above the operational waterline.