Astern propulsion (as applied to a ship) is a maneuver in which a ship's propelling mechanism is used to develop

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

in a retrograde direction. Astern propulsion does not necessarily imply the ship is moving astern (''in reverse''); astern propulsion is used to slow a ship by applying a force in the direction of the bow of the ship, instead of the

stern The stern is the back or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite the bow, the foremost part of a ship. Or ...

. The equivalent concept for an airplane is

thrust reversal Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's thrust for it to act against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft t ...

. 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 Rigging comprises the system of ropes, cables and chains, which support a sailing ship or sail boat's masts—''standing rigging'', including shrouds and stays—and which adjust the position of the vessel's sails and spars to which they ar ...

of masts and yards was principally designed to accept and transmit thrust in the forward direction. In a ship with a

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...

engine and a variable-pitch propeller, astern thrust is simply a matter of changing the propeller pitch to a negative value. Most other

propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...

-driven ships will reverse the direction the prop spins. For a

paddle wheel A paddle wheel is a form of waterwheel or impeller in which a number of paddles are set around the periphery of the wheel. It has several uses, of which some are: * Very low-lift water pumping, such as flooding paddy fields at no more than a ...

ship, reversing the direction of the paddle will provide astern propulsion. Redirecting the thrust of a water jet driven craft, changing the cyclic pitch of a Voith-Schneider propulsor, or rotating an azimuth thruster 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 R-100 could reverse the direction of rotation of some engines, so reversing the direction of thrust of the attached propeller. This facility was used to slow down or stop the airship when mooring. 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"C-17 fact sheet."
''US Air Force'', 27 October 2004. Accessed: 15 March 2014. use astern propulsion (more commonly termed ' reverse thrust') to slow down after landing, reducing the load on the wheel

brakes A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Backgroun ...

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 ' reverse'. In land vehicles reverse propulsion is usually achieved through various

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...

arrangements.

References

{{DEFAULTSORT:Astern Propulsion Marine propulsion