Stabilising Devices

Stabilising systems

A vessel will roll due to the force of the waves .The actual effect of a wave system is dependent upon wave period , magnitude, ships metacentric height and displacement and others. All ships have a natural roll period depending upon; metacentric height, and the radius of gyration about the longitudinal polar axis. A large metacentric height will produce a short roll period which can be very uncomfortable whilst a small metacentric height produces slow roll periods with large amplitude .Both conditions can cause problems and reducing the roll amplitude is seen as advantageous.

Rolling causes severe racking stresses in the hull and additional loads due to gravity on the superstructure. It is unpleasant for passengers and crew and can lead to damage of cargo. The fitting of roll reduction systems reduces the effects without a supposed increase in racking stresses, due to the ship rising with the wave reducing its effect.

Bilge keels

These form the simplest method of controlling roll they consist of narrow steel strips extending along a portion of the length of the hull.

The are mounted at the turn of the keel and project no further than the breadth and depth of the ship thereby preventing contact damage.

They are attached to the hull by a relatively weak joint, say by riveting to a second fixing which is welded to the hull, or by stitch welding allowing the keel to be torn off without further hull damage.

Size of bilge keels depend upon the ship , but two conditions should be satisfied;

  1. The web must be long enough to penetrate the boundary layer of water travelling with the hull.
  2. The web should not be so deep so as to sustain damage due to force of water during rolling.

In addition the ends of the keel should be tapered or well rounded so that they blend smoothly with the lines of the hull,this reduces eddies which could lead to vibration and/or erosion damage.

Stabilising tanks

Two forms have been in general use but the passive tank system is the most common .

Passive system

In basic terms the system consists of a "U" shaped tank positioned across the ship with the main body of water being contained in the two side tanks .These are connected at he top by means of an air channel which contains one or more valves. This controls the rate of air flow and hence the rate of water flow between the tanks, baffle plates prevent surge.

Maximum rolling occurs when the time interval between wave crests is the same as the time taken for a complete roll and that time depends upon a particular ship. Under these conditions the roll of the ship lags behind the wave crests by exactly one quarter time period. The ship will be vertical midway between crest and trough , reaching maximum roll angle at trough or crest.Anti rolling tanks employ a third oscillating system, namely water held in the "U" tube;t he First and second being the waves and the ship.

Careful design of the tank in terms of its shape , water capacity and vertical positioning in the ship allows control to be exercised with respect to rolling. With correct design of tank the water oscillating period will equal the roll period of the ship but its motion will lag behind that of the ship by one quarter of the roll period and behind the wave by half of the roll period. Water in the tank thus opposes the wave action producing the roll. Water movement between the tanks is regulated to some extent by the air valves. With the valves closed the system is put out of action. With this arrangement, known as the controlled passive system, the mass of water to about 2 to 2.5% of the ships displacement.

Care must be taken when retrofitting this system not to place the tanks to high or a critical loss of GM results .In an emergency it must be possible to dump the water very quickly thus large dump v/v's must be fitted.

Flume tank

This is a passive system which whilst simple in design remains very effective in its action. A restriction in the form of a perforated plate reduces the flow of water from on side to another during ships roll. In this way a righting moment is given automatically to the vessel. The amount of righting moment is dependent on the ship roll angle and speed.

Active system

Active tanks employ the same basic principle of water counteracting the wave induced rolling but water flow is produced by means of a pump. In order to ensure rapid water flow in the desired direction the unidirectional pump impeller is kept running at all times and valves are actuated in order to direct the flow to and from particular tanks. Valves activation is by means of a control system employing gyroscopes.

In the neutral position all the flaps are open

Fin stabilisers

These work very much like aircraft wing in that they provide lift, positive or negative depending upon their aspect relative to water flow. Fins are of aerofoil cross section and are provided with tail flaps which can be moved relative to the main fin .This is accomplished automatically as the main fin is rotated . Main fins usually have a maximum movement of 20 degrees up or down whilst the tail can move a further 30 degrees relative to the main flap.

The magnitude determined by means of a control system employing gyroscopes .One of the gyro's reacts to change in position from vertical and is mounted vertically, whilst the other mounted horizontally reacts to rate of roll. Rapid acting control systems , rotate the fins by electro hydraulic systems similar to that used on the steering gear .Early arrangements were made to slide into fin boxes but recent systems employ hinged arrangement .The latter requires less space within the hull.

Very careful design of the fins and control systems is required in order to achieve optimum effect. The size of fin area and length of arm to ships center line must be taken into account as must the normal operating speed. At reduced speeds the fin may not operate effectively and in certain sea conditions even increase roll. Fin stabilisers require power to operate and increase drag slightly .Some larger vessels are fitted with twin sets operating independently giving total roll control.

Moving Weight

This system consists of a large mass (1% of ships displacement) which is able to be driven athwartships by electric motors to provide a righting moment
The advantage of this system is that it is able to cope with variations in roll period and is active even with the ship stopped