Types [Video Lesson]

Roll motion stabilization can be achieved in conventional ships by changing their hull forms, however, reduction in roll amplitudes is possible by other means as well. Stabilization systems can be broadly classified into –

1. Passive Systems: In which no separate source of power is required and no special control system like the Bilge keel, anti-rolling tanks (passive), fixed fins & passive moving weight system.

2. Active Systems: In which the moment opposing roll is produced by moving masses or control surfaces by means of power like the active fins, Anti – rolling tanks (active), active moving weight & the gyroscope.

Bilge Keels

Bilge Keels are the most popular and fitted to the great majority of ships. They are plates projecting from the turn of the bilge and extending over the middle half to two-thirds of the ship’s length. To avoid the damage they do not normally protrude beyond the ship’s side or keel lines, but they need to penetrate the boundary layer around the hull.

They cause a body of water to move with the ship and create turbulence thus dampening the motion and causing an increase in period and reduction in amplitude. Although relatively small in dimension, they have large levers about the rolling axis and the forces on them produce a large moment opposing the rolling.

Their effect is generally enhanced by ahead speed. They are aligned with the flow of water past the hull in still water to reduce their drag in that state. When the ship is rolling the drag will increase and slow the ship a little.

Anti–Rolling Tanks (Active)

They are similar to the principle of a passive tank system but the movement of water is controlled by pumps or by the air pressure above the water surface. The tanks on either side of the ship may be connected by a lower limb or two separate tanks can be used. The air duct contains valves operated by a roll-sensing device. This concept uses an axial flow pump to force the water in the tank from one side of the ship to the other, rather than to have it slosh under the natural roll, sway, and yaw forces, as happens in a passive tank.

Active Fins

With active fins, a sensitive gyro system senses the rolling motion of the ship and sends a signal to the actuating system which, in turn, causes the fins to move in a direction such as to cause forces opposing the roll. The actuating gear is usually electrohydraulic.

The fins, which may be capable of retraction into the hull, are placed about the turn of the bilge in order to secure maximum leverage for the forces acting upon them. A flap from the trailing edge may be used to enhance the lift force generated.

The capacity of a fin system is usually expressed in terms of the steady angle of the heel it can cause with the ship moving ahead in still water at a given speed. The force on a fin varies in proportion to the square of the ship’s speed, whereas the GZ curve for the ship is independent of speed. However, a fin system is not likely to be very effective at speeds below about 10 knots.