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Ship Stabilizing

A ship at sea does rolling and pitching movements about its longitudinal and its transverse axis respectively (Fig.1). Rolling in particular is disagreeable to crew and passengers because of its relatively large amplitude, besides presenting problems with regard to the storage of the cargo.

The rolling motion depends on various factors: the wave movement according to the state of the sea, the vessel’s moment of inertia with respect to the rolling axis, the damping moment due to friction between the hull and the water, and the stability moment, determined by the horizontal distance between center of gravity and center of buoyancy.

Ship stabilizers are fins mounted beneath the waterline and emerging laterally. In contemporary vessels, they may be gyroscopically controlled active fins, which have the capacity to change their angle of attack to counteract roll caused by wind or waves acting on the ship.

Different kinds of devices, known as stabilizers, have been developed for the purpose of reducing the rolling motion of ships. In general these appliances are of the passive or of the active type. The action of a passive stabilizer can be obtained by the rolling itself, i.e., such a device responds to the motion and takes corrective action. On the other hand, an active stabilizer has present control whereby the corrective action in the form of a counteracting movement is programmed to take place simultaneously with the occurrence of the disturbing movement that causes the rolling of the ship.

The wave movements, in particular, are never quite regular, but it is nevertheless possible, by means of appropriately designed active stabilizers, to reduce rolling by at least 75%. The greatest effect is obtained when the stabilizer operates at the natural frequency of the ship, but with a phase difference of 90 degrees in relation to the ship’s motion.

The simplest stabilizing device is the bilge keel (Fig.2), this is one such keel which is fitted on each side and extending about 30-50% of the ship’s length. Bilge keels develop considerable resistance to the rolling motion and thus reduce it. The stabilizing effect achieved by these keels depends to a great extent on the speed of the ship. They have the drawback that they present a not inconsiderable resistance and thus slow down the vessel. Instead of being a continuous keel, the stabilizing device may take the form of a series of short fins having a streamlined shape in section so as to reduce the resistance.

Stabilizing (or antiroll) tanks are situated on each side of the ship and are inter-connected by two pipes (Fig.3). The tanks are about half filled with water, oil or some other suitable liquid. Water flows through the lower pipe from the upper to the lower side when the ship heels over, while the upper pipes serves to equalize the air pressure in the tanks. The upper pipe contains a throttle which is adjusted to regulate the airflow and thus control the flow of water from one tank to the other in accordance with the rolling conditions.

One of the well-known stabilizing devices is the gyrostabilizer (Fig.4). It consists of a large and heavy steel rotor located on the center line of the ship and mounted in horizontal transverse gudgeon bearings. When the ship is on an even keel, the rotor axis is vertical. A sensitive small control gyroscope responds immediately to any rolling motion of the ship and transmits a counteracting motion to the gyrostabilizer, which thus exerts a righting force against the action of the wave which tends to roll the vessel over.

The simplest form of active stabilizer is the antiroll device shown schematically in Fig.5. It comprises a heavily ballasted truck or trolley which travels on a track extending transversely and is so propelled by an electric motor that the moment developed by the trolley’s weight counteracts the wave moment that causes rolling. The motor is under the direction of a control gyroscope. This installation can alternatively be used to give the ship a rolling motion in calm water, as is sometimes necessary for experimental and testing purposes.

Activated antiroll tanks (Fig.6) differ from the ordinary “passive” type, already described, in that the movement of the water from one side of the ship to the other is now not made dependent on the action of a throttle valve, but is controlled by a pump installed in the lower pipe or by a blower controlling the airflow and pressure in the upper pipe and air-filled space above the liquid in the tanks. Again the operation of the pump or blower is under the direction of a control gyroscope. This installation can likewise be used to produce rolling motion in calm water.

A fin stabilizing system (Fig.7) contains a set of retractable fins mounted approximately amidships on each side of the vessel. These fins can be pivoted in opposite directions about axes extending transversely to the vessel’s longitudinal center line. They reduce the rolling motion by developing a counteracting effect which depends on the angle at which the fins are set, the size of the fins, and the speed of the vessel.

Control equipment ensures that the fins are at all times swiveled to the appropriate angle for most effectively counteracting the wave action tending to cause rolling. Great performance is achieved only within a certain speed range, and careful design of the stabilizing system is necessary to make it as effective as possible. When not in use, the fins can be retracted into the hull or swung back into recesses provided for them.