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Fin stabilizers and stabilizing systems

Fin arrangements
Figure 10,8 :Fin arrangements

Active-fin stabilisers The reduction of rolling provides better fuel/speed performance on rough seas, enhances crew safety and possibly their efficiency. The roll amplitude can be reduced by passive devices, such as bilge keels, or by active devices, such as hydrofoilshaped fins usually located near amidships, port side and starboard. In a seaway, hydraulic tilting gear continuously varies the angle of attack of the fins, to produce heeling moments reducing the ship tendency to roll. Active-fin stabilisers require ship forward motion in order to develop lift. They are available as folding types or non-retractable construction. For many years active-fin stabilisers have been applied on cruise ships and passenger vessels as a means of reducing excessive rolling. Today, owners of other types of vessels, especially large containerships, are increasingly specifying fin stabilisers.

Fin stabilizers:

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.

Downward tilt of the forward edge of the fin, inverts the effect, so that a drop in pressure occurs at the underside and increased pressure at the top to give a downward force. Without a reasonable rate of forward movement of the ship, the small size of the fins makes them ineffective. Thus active fin stabilizers are fitted to the faster types of ship, operating at perhaps fifteen or more knots.

The hydrofoil section may be all-movable, with or without flaps or partly fixed, partly movable, (Figure 10.8). These fins are tilted, usually hydraulkally, in phase with the roll at long wave periods, 90 out of phase at resonance and in phase with roll acceleration at short periods.

Non retractable stabiliser
Fig: Non retractable stabiliser

Non-retractable fins are commonly used where space within the hull is limited. They are usually fitted at the turn of the bilge and do not project beyond the vertical line from the ship's side or below the horizontal line of the ship's bottom, to minimize the risk of contact with a quay wall or the dock bottom. The fin shaft, to which the fin is rigidly attached, passes through a sea gland in a mounting plate welded or bolted to the hull and is supported by two substantial bearings, A double-ended lever keyed to the inner end of the finshaft is actuated by two hydraulic rams supplied from an electrically driven variable delivery pump (Figure 10.9).Fin arrangements
Figure :Fin arrangements
Control of fin movement is automatic and is usually derived from gyroscopic sensing gear which, in its simplest form velocity control - is based on one small, electrically driven gyroscope mounted horizontally with its axis athwartships.

Non-retractable fin assembly
Figure : Non-retractable fin assembly

The angular velocity of roll of the ship causes the gyroscope to precess against centralizing springs to an amount proportional to the velocity and it generates a small force which is hydraulically amplified by a hydraulic relay unit to provide power sufficient to operate the controls of the variable delivery pump via suitable linkage. Part of the linkage is coupled to the fin shaft to transmit a cancelling signal to the pump control and to bring the fin to rest at the angle of tilt demanded by the sensing unit.

This type of control is often fitted in small installations, usually for economic reasons, and is most effective against resonant rolling. Ships seldom roll in a purely resonant mode; the sea state is often highly confused. More elaborate, and more expensive, control systems are required to deal with suddenly applied roll, rolling at periods off resonance and rolling in conditions arising from the combination of several wave frequencies.

A sensing unit based on a vertical-keeping gyroscope and a velocity gyroscope coupled into differentiating and summation units enables fin movement to be controlled by a composite function derived from roll angle, roll velocity and roll acceleration. By adding a 'natural list' unit, stabilization is achieved about the mean point of roll and so reduces both propulsion and stabilizing power demand. This is known as a compensated control system (Figure 10.10) and is generally used in large installations.

Non-retractable fin assembly
Figure : principles of multra control system

Roll reduction in excess of 90%, typically 30 deg out-to-out reduced to less than 3 deg out-to-out, can be achieved at resonance and low residual rolls can be maintained over a wide range of frequencies. However, since the stabilizing power varies as the square of the ship's speed, fins are least effective at low or zero speed where they function only as additional bilge keels.

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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