Stabilizer (ship)

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:

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.

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.

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