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Ship stabilizers and stabilizing systems

Stabilizers and stabilizing systems A ship at sea has six degrees of freedom, i.e. roll, heave, pitch, yaw, sway and surge (Figure 10.5). Of these, only roll can effectively be reduced in practice by fitting bilge keels, anti-rolling tanks or fin stabilizers. A combination of fins and tanks has potential advantages in prime cost and effective stabilization at both high and low speeds.

Since a ship is a damped mass elastic system, it has a natural rolling period and large rolling motions may be induced by resonance with relatively small wave forces. Large resonant rolls can be avoided by generating forces equal and opposite to the impressed sea force.

Figure 10.6 shows that the roll amplitude at resonance is much greater than that at long wave periods. The ratio of these amplitudes is the dynamic amplification factor which is limited by the inherent damping of the ship, i.e. viscous damping and the action of bilge keels.

Ship motions
Figure : Ship motions

Summarized below ship stabilizing system detail guideline:
  1. Ships bow thrusters-Electric drives, diesel drives, hydraulic drives & white Gill type

  2. Safety is increased when berthing in adverse weather conditions provided that the required thruster capacity has been correctly estimated. Transverse thrusters are installed to facilitate the positioning of some types of workboats. More .....

  3. Fin stabilizers and stabilizing systems

  4. The stabilizing power of fins is generated by their movement through the sea and lift' created by the flow of water above and below the 'aerofoil' or hydrofoil shape. When the front edge of the fin is tilted up, water flow across the top of the profile produces lift due to a drop in pressure while a lifting pressure is provided by flow along the underside.More.....

  5. Folding fin stabilizer & Retractable fin stabilizers-

  6. Housing and extending the fin is achieved by a double acting hydraulic cylinder connected to the upper part of the trunnion. Power units, control and sensing equipment are generally similar to other types of stabilizer except that feed-back of fin angle is accomplished electrically by synchros.More....

  7. Tank stabilizers

  8. Tank stabilizers (like bilge keels) are virtually independent of the forward speed of the vessel. They generate anti-rolling forces by phased flow of appropriate masses of fluid, usually water, in tanks installed at suitable heights and distances from the ship's centre line.More....

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