When ships sail on the open ocean, they are subjected to a large number of forces in all
directions. Be it wave loads, structural stresses, wind forces or even turning forces, these
vessels must be built to withstand all such loads and stresses.
It also leads to ships moving in different degrees of freedom. You must have seen many
videos of ships tilting to one side, coming back to centre, and then tilting to the other
side. This happens because of one of the three rotational degrees arising out of ship motions:
1. Yawing which is Vertical rotation at Z-axis 2. Rolling which Longitudinal rotation at
X-axis 3. Pitching which is Transverse rotation at
Y-axis
You can learn more about the ship’s degree of freedom in our youtube video. Also, Please
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Back to the topic - Rolling is indeed the biggest problem among all the other motions
in the sea. Technological advancements in the design of ships have already given us
naval architects the advantage to study the wave motions around the hull of the ship and
give the best possible design to minimize such effects & ensure a comfortable and safe
voyage for the passengers and the crew.
Roll motion stabilization can be achieved in conventional ships by changing their hull
forms, however, reduction in roll amplitudes are possible by other means as well. Stabilization
systems can be broadly classified into – Passive Systems: In which no separate source
of power is required and no special control system like the Bilge keel, anti-rolling tanks
(passive), fixed fins & passive moving weight system
Active Systems: In which the moment opposing roll is produced by moving masses or control
surfaces by means of power like the active fins, Anti – rolling tanks (active) etc.
Bilge Keels Bilge Keels are the most popular and fitted
to the great majority of ships. They are plates projecting from the turn of the bilge and
extending over the middle half to two-thirds of the ship’s length.
To avoid the damage they do not normally protrude beyond the ship’s side or keel lines, but
they need to penetrate the boundary layer around the hull.
They cause a body of water to move with the ship and create turbulence thus dampening
the motion and causing an increase in period and reduction in amplitude.
Although relatively small in dimension, they have large levers about the rolling axis and
the forces on them produce a large moment opposing the rolling.
Their effect is generally enhanced by ahead speed.
Anti – Rolling Tanks (Active) They are similar to the principle of the passive
tank system but the movement of water is controlled by pumps or by the air pressure above the
water surface. The tanks either side of the ship may be connected by a lower limb or two
separate tanks can be used. The air duct contains valves operated by a
roll-sensing device. This concept uses an axial flow pump to force the water in the
tank from one side of the ship to the other, rather than to have it slosh under the natural
roll, sway and yaw forces, as happens in a passive tank.
in a simplified version of an active system, an accelerometer senses the rolling motions,
and signals are sent from this roll-sensing device to a variable pitch pump, which controls
the liquid flow between the tanks. The device can be either a simple accelerometer
or a complicated gyroscopic sensing system that detects even a small angle of the roll
by the gyroscopic precession. Thus, the device can be used to control ship
motion due to every single wave. Depending on the sophistication of the system active
tank stabilizers have been found to leave an efficiency of 80% or more in motion stabilization.
Active Fins With active fins, a sensitive gyro system
senses the rolling motion of the ship and sends a signal to the actuating system which,
in turn, causes the fins to move in a direction such as to cause forces opposing the roll.
The actuating gear is usually electrohydraulic. The fins, which may be capable of retraction
into the hull, are placed about the turn of bilge in order to secure maximum leverage
for the forces acting upon them. A flap from the trailing edge may be used
to enhance the lift force generated. The capacity of a fin system is usually expressed in terms
of the steady angle of heel it can cause with the ship moving ahead in still water at a
given speed. By using Active fins, there can be a huge reduction in the rolling compared
to ships with no active fins whose rolling angle will be large.
The force on a fin varies in proportion to the square of the ship speed, whereas the
GZ curve for the ship is independent of speed. However, a fin system is not likely to be
very effective at speeds below about 10 knots.
it can be concluded that each and every stabilization system has got its own advantages & disadvantages.
Therefore, more effort has to be given to the hydrostatics and the hydrodynamics of
the ship hull motion through the waves, so as to avoid the need for kind of roll stabilization
system or to use it to its minimum.