Abstract: A boat stabilizer is disclosed for substantially eliminating "porpoising" or up and down movement of the bow portion of a boat as the boat is moving through the water. The stabilizer includes a wedge-shaped member formed on an adjustably mounted trim tab located on the underside of an anti-cavitation plate of an outboard motor. The wedge-shaped member has a downwardly inclined lower surface extending generally laterally from and perpendicularly to a fin-shaped member of the trim tab. Movement of the boat through the water imparts a force against the downwardly inclined surface to effect a generally vertical torque on the boat substantially eliminating porpoising of the boat in a given speed range. The force imparted against the downwardly inclined surface is proportional to the angle of attack of the downwardly inclined surface so that upward displacement of the bow of the boat moving through the water is opposed by a proportionately greater force on the downwardly inclined surface.