Page 204 - Introduction to Marine Engineering

Deck machinery and hull equipment 189

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.

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. Thus when the
ship is finishing its roll and about to turn, the still moving water will
oppose the return roll. The water mass thus acts against the roll at each
ship movement. This athwartships tank is sometimes referred to as a
'flume'. The system is considered passive, since the water flow is
activated by gravity.
A wing tank system arranged for controlled passive operation is
shown in Figure 10.8. The greater height of tank at the sides permits a
larger water build-up and thus a greater moment to resist the roll. The
rising fluid level must not however fill the wing tank. The air duct
between the two wing tanks contains valves which are operated by a roll
sensing device. The differential air pressure between tanks is regulated
to allow the fluid flow to be controlled and 'phased' for maximum roll
stabilisation.
A tank system must be specifically designed for a particular ship by
using data from model tests. The water level in the system is critical and