Abstract: Corrective stabilizing motions are applied to a sea vessel hull during seawater travel in response to stern flap displacement by hydrodynamic forces induced at the buttock of the vessel hull and by the lower flap surfaces at the stern end of the sea vessel hull in response to angular displacement of flap elements from a deployed position in either in-phase or out-of-phase relation to each other in rough seas. An angle of attack range for limiting angular displacement of the flaps is selected so as to minimize resistance to travel and optimize fuel saving during propulsion of the vessel hull at different speeds under different seawater conditions.