Stabilizer, Ship

a device designed to reduce the oscillatory motions of a ship in a seaway. Such devices provide more comfortable conditions for the crew and passengers on board the ship and improve the operating environment for equipment and instruments; they also improve the speed-to-power ratio and maneuverability of ships in rough seas and increase the effectiveness of armaments on warships. Most stabilizers are used to diminish rolling.

The operation of hydrodynamic stabilizers is based on the resistance to ship oscillations afforded by rigid plates or fins mounted on the hull, such as controllable roll-resisting fins and bilge keels. Stabilizing rudders located on either side of the ship are automatically deflected by a special mechanism when the ship heels, thus developing a force opposing the inclination. With tank stabilizers, the needed force is produced in rough seas by shifting solid or liquid masses within special tanks located on opposite sides of the ship. The tanks are usually filled with water that flows in the necessary direction from one tank to the other, either through a connecting pipe or an open transfer channel. Some tanks communicate with the open sea through a series of holes. Gyrostabilizers are also used to stabilize ships.

The effectiveness of a stabilizer is judged by the reduction in oscillations effected, that is, by the ratio of the oscillation amplitude without the stabilizer to the amplitude with the stabilizers. The roll reduction ratio is usually approximately 1.5 for bilge keels and tank stabilizers and 5 or more for roll-resisting fins.