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Are used to minimize the rolling of a ship. They are normally fitted to cruise ships and ferries, where passenger comfort is essential, and to large container ships, where it is necessary to minimize the rolling forces on the containers stacked on deck. They should not be confused with anti-heeling systems (see last paragraph of this entry) which are used to correct small angles of heel.

Anti-rolling tanks were introduced by a German inventor, Dr H. Frahm, and one of his first systems was fitted in the ocean liner Laconia of 1912, his Frahm tanks being a development of an earlier system first used in the liner City of New York in 1889. The Frahm arrangement consists of two large tanks at the sides of the ship, joined by a connecting tank at the bottom, forming a large U-shaped tank; an air venting line joins the tanks at the top. As the ship rolls, water in the tanks moves from one side of the ship to the other and the effect of the moving water is to minimize the roll. As no operating system was used, the Frahm tanks are classed as passive tanks. The active tank stabilizing system, which is generally used on smaller ships, operates in the same way except that a pump is used to move water from one side of the ship to the other.

A disadvantage of a tank system is that it takes up valuable space inside the hull and so other stabilizing systems have been developed. A gyroscopic stabilizer unit was installed in the Italian liner Conte di Savoia during the late 1920s. Although it was effective in certain wave conditions, it was not successful and the experiment was not repeated. However, the gyroscope was later used at sea in Elmer Sperry's gyroscopic compass. The fin stabilizer was first patented in 1898, and the Japanese introduced a fin stabilizer system in 1925. But it was the one introduced by Denny-Brown in the 1930s, which used gearing to operated the fins, that became the most successful for all types of ship.

A fin stabilizer system has two fins, one on each side of the ship; larger ships may have two sets of fins. If a fin is turned to the nose-up attitude as the ship moves through the water, there is a resulting upward force on the fin just like the effect on an aircraft wing. If the fin on the opposite side of the ship is given a nose-down attitude there is a downward force on the fin. The total effect of the forces on the two fins is to exert a turning moment on the ship which opposes the rolling effect on the ship caused by a wave.

An essential feature of this system is the monitoring and control arrangements. These have to react to the angle of roll and the rate at which that angle is changing, and must react quickly enough to move the fins to the correct position before the angle of roll becomes too large. Nowadays, electronic control systems are used and the fin actuators are similar to the hydraulic systems used for steering gear, generally of the rotary vane type as this is compact. When the ship is not rolling, and when it enters port, the fins must be drawn into the hull. Early systems had retractable fins but modern ones use hinged fins.


Subjects: Maritime History.

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