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A ship can be subjected to heavy disturbances in rough weather, giving rise to movement such as rolling and pitching which may lead to drastic loss in the ship’s stability. No matter how flawless the shape and design of the ship might be, the forces of the waves often tend to shift the ship's centre of gravity, causing havoc in its overall stability. A slight change in the ship's centre of gravity is not a big deal, but if the movement goes beyond a certain limit, it might lead to dangerous conditions.
Maintaining the stability of a ship is a daunting task, especially during heavy rolling and pitching. Ships are provided with stabilizers or ship stability apparatus to provide them with the needed stability. Fin stabilizer is one such type of ship stabilizers which resembles the fins of marine animals.
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Construction of Stabilizer Fins
In order to maintain the ship's sane equilibrium condition, fin stabilizers are attached to the sides of ship. Boat stabilizer fins are generally rectangular in shape and are attached in pair on each side to provide the necessary balance. The size of the fins depends on various factors such as the ship's breadth, length, displacement etc. However, compared to the size of the ship, the size of the fins is relatively quite small.
Fin stabilizers are generally of retractable type, pivoted or mounted on a sliding mechanism in the ship's form. The operating system of the fin stabilizer is similar to that of a steering gear system. The movement of the fins occur when a signal from the control unit is sent. A hydraulic power unit comprising of a variable displacement pump receives the signal and provides the necessary fin movement.
However, the effectiveness of the system depends on the speed with which the fins can operate. According to international regulations the fins should rapidly move from one extreme end to the other smoothly and without any hindrances. The shape and design of the ship should be rectangular and streamlined respectively. Moreover, most of the modern ships are provided with movable fins to facilitate a greater restoring movement to the ship.
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Sensing the Motion
The sensing control of the system determines the angle of tilt of the fins and the resulting movement of the ship. For this, the control system of the stabilizers makes use of two gyroscopes, one that senses the movement from the vertical and other that identifies the rolling velocity. After assessing a ship's roll angle, roll velocity, roll acceleration and natural list the control system sends the signal for the right inclination angle needed for the fins.
Thus, the fin stabilizers act rapidly to apply a righting movement to the ship, as and when it is inclined by a wave or force on either side of the ship. The forward speed of the ship allows the fins to generate a thrust which results in the righting movement. However, it is to note that at low speeds the stabilizing power of the fins falls and when stationary, no stabilization is possible. In such conditions, tank stabilizers are used.
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Perez, T. (2005) Ship Motion Control: Course Keeping and Roll Stabilisation Using Rudder and Fins (Advances in Industrial Control). London: Springer Verlag