Astern propulsion

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Astern propulsion (as applied to a ship) is a maneuver in which a ship's

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In a sailing ship astern propulsion can be achieved by the appropriate manipulation of the sails. In square-rigged ships 'backing the sails', that is, aligning the sails so that the wind impinged on the bow surface, could provide sufficient retrograde thrust to slow or reverse the ship. This maneuver had to be carried out with care as the rigging of masts and yards was principally designed to accept and transmit thrust in the forward direction.

In a ship with a

180 degrees has the same effect. As the efficiency of traditional rudders is greatly reduced when not located in the propeller wash, only propulsion systems with steerable thrust provide adequate maneuverability during astern operation.

A marine vessel is required to signal that she is operating astern propulsion by either blowing three short, easily audible blasts or by flying the Sierra signal flag (white border with one central blue square).

Some aircraft are also able to develop astern propulsion. Airships such as the

.

Some propeller-driven aircraft using controllable pitch propellers can change the blade pitch sufficiently to provide astern propulsion. This facility is sometimes used to control aircraft speed in steep descents, or to taxi backwards when on the ground. Most jet airliners and some transport aircraft

and shortening the landing rollout. Helicopters can develop thrust in any direction, including astern.

Most mechanically driven land vehicles can develop astern propulsion, although in this case the ability is more usually termed '

arrangements.

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