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Ship stabilizers are fins or rotors 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.
Fins work by producing lift or downforce when the vessel is in motion. Thus, fins are vastly more efficient at higher velocities. Stabilization solutions at anchor or at low speed include actively-controlled fins (such as the Stabilisation at rest system developed by Rolls Royce that oscillate to counteract wave motion), and rotary cylinders employing the Magnus effect (developed by Quantum Med Marine under the MagLift™ Zero Speed™ name). The latter two systems are also retractable, allowing for a thinner vessel profile when docking, and reducing drag while cruising.
The bilge keel is an early 20th-century predecessor. Although not as effective at reducing roll, bilge keels are cheaper, easier to install, and do not require dedicated internal space inside the hull.
In November 1932 the ship Conte di Savoia made her maiden voyage. She had three huge gyroscopes fitted low down in a forward hold. These rotated at high revolutions and were designed to eliminate rolling - a persistent problem that affected all shipping lines on the rough North Atlantic crossing. Each of the three flywheels was 13 feet in diameter and weighed 108 tons.
The first use of fin stabilizers on a ship was by a Japanese cruise liner in 1933. 
In 1934 a Dutch liner introduced one of the world's most unusual ship stabilizer systems, in which two large tubes were mounted on each side of the ship's hull with the bottom of the tubes open to the sea. The top of the tubes had compressed air or steam pumped in. As the ship rolled, the side it was rolling to would fill with water and then compressed air or steam would be injected to push the water down, countering the roll.
- "Quantum Rotary Stabilizers" (video). YouTube. 2 June 2009.
- "Stabilisation at rest". Products. Rolls-Royce Group PLC.
- "Fins Proposed For Big Liners To Prevent Rolling". Popular Mechanics. August 1932. p. 251.
- "Fins to Stop Ship's Rolling Governed by Gyro". Popular Mechanics. April 1933. p. 509.
- "Compressed Air Stabilizer Stops Roll of Ship". Popular Mechanics. October 1934. p. 573.