Anti-rolling gyro

Ship stabilizing gyroscopes are a technology developed in the 19th century and early 20th century and used to stabilize roll motions in ocean-going ships. It lost favor in this application to hydrodynamic roll stabilizer fins because of reduced cost and weight. However, more recently (since the 1990s) a growing interest in the device has reemerged for low speed roll stabilization of vessels. The gyroscope does not rely on the forward speed of the ship to generate a roll stabilizing moment and therefore has shown to be attractive to motor yacht owners for use whilst at an anchorage.

One of the most famous ships to first use an anti-rolling gyro was the 1930 Italian passenger liner the SS Conte di Savoia which had three huge gyros to control roll.

The ship gyroscopic stabilizer typically operates by constraining the gyroscope's roll axis and allowing it to "precess" either in the pitch or the yaw axes. Allowing it to precess as the ship rolls causes its spinning rotor to generate a counteracting roll stabilizing moment to that generated by the waves on the ship's hull. Its ability to effectively do this is dependent on a range of factors that include its size, weight and angular momentum. It is also affected by the roll period of the ship. Effective ship installations require approximately 3% to 5% of a vessel's displacement.

Unlike hydrodynamic roll stabilizing fins, the ship gyroscopic stabilizer can only produce a limited roll stabilizing moment that may be exceeded as the wave height increases. Otherwise, it is not unusual for the manufacturer to recommend that the unit not be used at sea in large waves.

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Wikipedia