Folding fin stabilizer & Retractable fin stabilizers- Ship propulsion system
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.
Fig 1 illustrates The Brown Brothers Neptun II folding stabilisers for large vessels. Fin area: 4.2 - 22.33 m2, total
weight: 36.5 -190 tonnes. The transatlantic liner QUEEN MARY 2 features four sets of 15.63 m2 Neptune folding
stabilisers.
For more information visit www.simplex-turbulo.com
Fig 1: Folding fin stabiliser
Image credit :Wärtsilä Encyclopedia of Ship Technology
The Denny-Brown-AEG hinged or folding fin stabilizer (Figure 10.13) rotates
to a fore and aft stowed position with the fins folded forward into the hull. The
finshaft (Figure 10.14) is rigidly fixed into the crux which has a heavy vertical
trunion that is housed in bearings top and bottom.
The fin is free to oscillate on
the finshaft and the tilting force is provided by a vane type servo-motor the
stator of which is secured to the crux with the rotor being keyed to the fin
through a flexible coupling. The vane servo-motor is housed in an oil-tight
casing which is secured to the fin. There is a stainless steel sleeve fitted to the
crux, tor the casing lip seal. The whole of the casing and the interior of the fin is full of oil under pressure, to prevent the ingress of sea water.
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.
Variable delivery pumps also used for stabilizer operation.
Figure 10.13: Arrangement of Denny-Brown-AEG forward folding fin
Retractable fin stabilizers
Activated fin stabilizers which extend and stow athwartships, require spaces
for fin boxes and guide bars of a length more than twice that of the fins, within
the ship. The sketch of the general arrangement of a retractable fin (Figure
10.11) shows the amount of space required and the support seatings.
The finshaft (Figure 10.12) has a tapered outboard end to which the fin is
keyed. The parallel inboard end passes through a sea gland on the inboard face
of the fin box and is supported by two bearings. One, close to the inboard end
of the fin, is carried in a heavy crosshead, arranged to slide in top and bottom
guides within the fin box and the other in a crosshead slidably mounted on the
extension guides, within the hull.
The hollow bore of the parallel section of the
finshaft houses a double acting piston to act as housing and extending gear,
Tilting of the fin is by two or four hydraulic cylinders which may be of the
simple oscillating type or arranged on the Rapson-slide principle as described
for steering gears. Power units, control and sensing equipment are as for
non-retractable fins.
Fig: Detail of retractable fin (R. C. Dean)
Figure : Arrangement of Denny-Brown athwartships retracting fin
Summarized below ship stabilizing system detail guideline:
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 .....
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.....
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....
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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