Ship's stabilizers are devices used to reduce the rolling motion of a ship in rough seas. BALLAST is a material, usually water or rocks, placed in the hull of a ship to increase its stability and reduce its tendency to roll. A gyrostabilizer, also known as a gyrostat, is a device that uses a spinning wheel to counteract the rolling motion of a ship. Both BALLAST and gyrostabilizers are important components of a ship's stabilizer system.