Topic - 12 Stability systems

The stability systems are classified as
1.1.1        Passive systems

1.      Bilge keel: A bilge keel is a long fin of metal, often in a "V" shape, welded along the length of the ship at the turn of the bilge. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase the hydrodynamic resistance when a vessel rolls, thus limiting the amount of roll a vessel has to endure.

2.      Outriggers: Outriggers may be employed on certain vessels to reduce rolling. Rolling is reduced either by the force required to submerge buoyant floats or by hydrodynamic foils.

3.      Antiroll tanks: Antiroll Tanks are tanks within the vessel fitted with baffles intended to slow the rate of water transfer from the port side of the tank to the starboard side. The tank is designed such that a larger amount of water is trapped on the higher side of the vessel. This is intended to have an effect completely opposite to that of the free surface effect.

4.      Para vanes: Para vanes may be employed by slow-moving vessels (such as fishing vessels) to reduce roll.

1.1.2        Active systems

Active stability systems are defined by the need to input energy to the system in the form of a pump, hydraulic piston, or electric actuator. These systems include stabilizer fins attached to the side of the vessel or tanks in which fluid is pumped around to counteract the motion of the vessel.
1.      Stabilizer fins: Active fin stabilizers are normally used to reduce the roll that a vessel experiences while underway or, more recently, while at rest. The fins extend beyond the hull of the vessel below the waterline and alter their angle of attack depending upon heel angle and rate-of-roll of the vessel.
 2.      Gyroscopic internal stabilizers: Gyroscopes were used to control a ship's roll. Gyro stabilizers consist of a spinning flywheel and gyroscopic precession that imposes boat-righting torque on the hull structure. A gyroscope has three axes: a spin axis, an input axis, and an output axis. The spin axis is the axis about which the flywheel is spinning and is vertical for a boat gyro. The input axis is the axis about which input torques is applied. The principal output axis is the transverse (athwart ship) axis about which the gyro rotates in reaction to an input.