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Ship stabilizers are fins or rotors mounted beneath the waterline and emerging laterally from the hull to reduce a ship's roll due to wind or waves. Active fins are controlled by a gyroscopic control system. When the gyroscope senses the ship roll, it changes the fins' angle of attack to exert force to counteract the roll. Fixed fins and bilge keels do not move; they reduce roll by hydrodynamic drag exerted when the ship rolls. Stabilizers are mostly used on ocean-going (blue water) ships.
Fins work by producing lift or downforce when the vessel is in motion. The lift produced by the fins should work against the roll moment of the vessel. To accomplish this, two wings, each installed underwater on either side of the ship, are used. Stabilizers can be:
- Retractable - All medium and large cruise and ferry ships have the ability to retract the fins into a space inside the hull in order to avoid extra fuel consumption when the use of the fins is not needed.
- Non-retractable - This is the case on very small ships, for example a yacht.
Stabilizer movement is similar to that of aircraft ailerons. Some types of fins, especially the ones installed on larger ships, are provided with flaps, that increase the fin lift by about 15%. Stabilizer control needs to consider numerous variables that change quickly: wind, waves, ship motion, draft, etc. Fin stabilizers are vastly more efficient at higher velocities and lose effectiveness when the ship is under a minimum speed. 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. The latter two systems are retractable, allowing for a thinner vessel profile when docking, and reducing drag while cruising.
Fuel consumption and carbon emission
Fin stabilizers are used to reduce the roll motion of the ship and improve the passengers' comfort; however, they can be a considerable hydro dynamic brake for the ship. Stabilizers can reduce a ship's speed due to an increase in hydrodynamic drag. This increases fuel consumption and CO2 emissions. Some ships employ systems to reduce the stabilizers energy dissipation by using computers to control their motion. This reduces their fuel consumption and CO2 emissions.
An earlier stabilization technology was gyroscopic stabilization. The World War I transport USS Henderson, completed in 1917, was the first large ship with gyro stabilizers (right). It had two 25 ton, 9 ft diameter flywheels mounted near the center of the ship, spun at 1100 RPM by 75 HP AC motors. The gyroscopes' cases were mounted on vertical bearings. When a small sensor gyroscope on the bridge sensed a roll, a servomotor would rotate the gyros about a vertical axis in a direction so their precession would counteract the roll. In tests this system was able to reduce roll to 3 degrees in the roughest seas. The ship Conte di Savoia, which first sailed in November 1932, had three flywheels which were 13 feet in diameter and weighed 108 tons. Gyroscope stabilization was replaced by fin stabilization due to its lower weight and bulk.
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.
|Wikimedia Commons has media related to Ship stabilizers.|
- "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.