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. O ...

. 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 to ...

. 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 and control a sailing ship or sail boat's masts and sails. ''Standing rigging'' is the fixed rigging that supports masts including shrouds and stays. ''Running rigg ...

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 or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

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 (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 a working flu ...

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

paddle wheel A paddle is a handheld tool with an elongated handle and a flat, widened end (the ''blade'') used as a lever to apply force onto the bladed end. It most commonly describes a completely handheld tool used to propel a human-powered watercraft by p ...

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 A cyclorotor, cycloidal rotor, cycloidal propeller or cyclogiro, is a fluid propulsion device that converts shaft power into the acceleration of a fluid using a rotating axis perpendicular to the direction of fluid motion. It uses several blades w ...

propulsor, or rotating an

azimuth thruster An azimuth thruster is a configuration of marine propellers placed in pods that can be rotated to any horizontal angle (azimuth), making a rudder redundant. These give ships better maneuverability than a fixed propeller and rudder system. Type ...

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 A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

that she is operating astern propulsion by either blowing three short, easily audible blasts or by flying the Sierra

signal flag Flag signals can mean any of various methods of using flags or pennants to send signals. Flags may have individual significance as signals, or two or more flags may be manipulated so that their relative positions convey symbols. Flag signals allo ...

(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 A mooring is any permanent structure to which a seaborne vessel (such as a boat, ship, or amphibious aircraft) may be secured. Examples include quays, wharfs, jetties, piers, anchor buoys, and mooring buoys. A ship is secured to a mooring to ...

. 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 Reverse or reversing may refer to: Arts and media *Reverse (Eldritch album), ''Reverse'' (Eldritch album), 2001 *Reverse (2009 film), ''Reverse'' (2009 film), a Polish comedy-drama film *Reverse (2019 film), ''Reverse'' (2019 film), an Iranian cr ...

') to slow down after landing, reducing the load on the wheel brakes 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 arrangements.

References

{{DEFAULTSORT:Astern Propulsion Marine propulsion