There are two basic stabilising systems used on ships the fin and the tank. A stabilising system is fitted to a ship in order to reduce the rolling motion. This is achieved by providing an opposite force to that attempting to roll the ship.
1. Fin stabilise
2. Tank stabiliser
1. Fin stabiliser
One or more pairs of fins are fitted on a ship, one on each side. The size or area of the fins is governed by ship factors such as breadth, draught, displacement, and so on, but is very small compared with the size of the ship. The fins may be retractable, i.e. pivoting or sliding within the ship's form, or fixed.
They act to apply a lighting moment to the ship as it is inclined by a wave or force on one side. The angle of tilt of the fin and the resulting moment on the ship is determined by a sensing control system. The forward speed of the ship enables the fins to generate the thrust which results in the righting moment.
The operating system can be compared to that of the steering gear in that a signal from the control unit causes a movement of the fin which, when it reaches the desired value, is brought to rest. The fin movement takes place as a result of a hydraulic power unit incorporating a type of variable displacement pump.
The effectiveness of the fins as stabilisers depends upon their speed of movement, which must be rapid from one extreme point to the other. The fins are rectangular in shape and streamlined in section. The use of a movable flap or a fixed and movable portion is to provide a greater restoring moment to the ship for a slightly more complicated mechanism.
The control system which signals the movement of the fins utilises two gyroscopes, one which senses movements from the vertical and the other the rolling velocity. As a result of this control system, fin movement is a function of roll angle, roll velocity, roll acceleration and natural list.
Fin stabilisers provide accurate and effective roll stabilisation in return for a complex installation, which in merchant vessels is usually limited to passenger ships. It is to be noted that at low ship speeds the stabilising power falls off, and when stationary no stabilisation is possible.
2.Tank stabiliser
A tank stabiliser provides a righting or anti-rolling force as a result of the delayed flow of fluid in a suitably positioned transverse tank. The system operation is independent of ship speed and will work when the ship is at rest. Consider a mass of water in an athwartships tank. As the ship rolls the water will be moved, but a moment or two after the ship.
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