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MODERN ENGINEERING PRACTICE - 17


































THE engineer is called upon to minimize, if he cannot eliminate, that dreaded bugbear of the traveller, sea-sickness. This malady is most frequently brought about by the rolling of a vessel rather than by her pitching, and the problem of reducing the rolling motion has been tackled in several radically different ways.


Sir Henry Bessemer tried to solve it by suspending a saloon in a cross-Channel steamer so that it would keep stationary while the vessel rolled round it. Later attempts have comprised the alternate emptying and filling of water tanks in the sides of the vessel in time with the wave action so as to steady her, or the use of a gigantic gyroscope. The gyroscope has been fitted to large vessels such as the Italian liner Conte di Savoia. The bilge keel is a common device, but is only partly effective.


 One of the latest devices for minimizing rolling has been fitted to the Isle of Sark, a cross-Channel steamer of the Southern Railway Company plying between Southampton and the Channel Islands. It is known as the Denny-Brown Ship Stabilizer, and a view of that part of it outside the vessel's hull is given above. The stabilizer consists of an underwater fin on either side of the hull and situated about one-third of the vessel’s length from the bows. The fins turn round on an axis so that they can be set at an angle to the waterline of the vessel.


The fins function as follows. Suppose they are both set horizontally - that is, parallel to the waterline. In this position they would have no effect on the stability of the vessel as she moved forward. But, tilt one of the fins so that its trailing edge is downwards, then the flow of the water relative to the hull causes an upward pressure on that side of the ship and gives it a list to the opposite side. If the other fin is turned at the same time with its trailing edge upwards, the tilting effect is doubled. What happens in a seaway is this. A wave approaching the vessel endeavours to heel her over towards, say, the starboard side, but the fins are automatically set so as to give a heeling force to port. The two forces, that of the wave and that of the fins, tend to cancel each other out and the vessel remains steady.


As the wave passes under the ship her heeling tendency is in the other direction, but, in the meantime, the fins have been reversed as to their slope and, again, the ship is steadied. Rolling is not entirely eliminated, but is so reduced as not to worry most people. The tilting of the fins is effected by hydraulic cylinders, controlled, through electrical relays and other devices, by a tiny little gyroscope situated near the ceiling of the chart room, that is, at the highest practicable point of the ship and where the rolling motion would be most pronounced. A hand-regulating device in the chart room enables the automatic mechanism to be set to suit different periods of rolling, which vary with the state of the sea. The action of the stabilizer depends entirely upon the movement of the ship through the water, and it ceases when she is stationary.


[From part 27, published 31 August 1937]


You can read more on “Manufacture of a Rotor Shaft”, “The Oil-Engined Ship” and “The Principle of the Gyroscope” on this website.


You can read more on “The Conte di Savoia” in Shipping Wonders of the World

Ship Stabilizing Fin