Astern propulsion is a maneuver in which a ship's propelling mechanism is used to develop thrust

Thrust

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

 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 stern

Stern

The stern is the rear 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 of the bow, the foremost part of a ship. Originally, the term only referred to the aft port section...

 of the ship, instead of the bow

Bow (ship)

The bow is a nautical term that refers to the forward part of the hull of a ship or boat, the point that is most forward when the vessel is underway. Both of the adjectives fore and forward mean towards the bow...

.

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

Rigging is the apparatus through which the force of the wind is used to propel sailboats and sailing ships forward. This includes masts, yards, sails, and cordage.-Terms and classifications:...

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

In mechanically driven vessels astern propulsion may be achieved by reversing the direction of rotation of a fixed blade propeller, changing the pitch of a controllable pitch propeller, reversing the direction of paddle wheel

Paddle wheel

A paddle wheel is a waterwheel in which a number of scoops are set around the periphery of the wheel. It has several usages.* Very low lift water pumping, such as flooding paddy fields at no more than about height above the water source....

 rotation in a paddle-driven vessel, redirecting the thrust of a water jet driven craft, changing the cyclic pitch of a Voith-Schneider

Voith-Schneider

The Voith Schneider propeller , also known as a cycloidal drive is a specialized marine propulsion system . It is highly maneuverable, being able to change the direction of its thrust almost instantaneously...

 propulsor, or rotating an azimuth thruster

Azimuth thruster

An azimuth thruster is a configuration of ship propellers placed in pods that can be rotated in any horizontal direction, making a rudder unnecessary...

 180 degrees. 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

International maritime signal flags

The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships...

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

International maritime signal flags

The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships...

 (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

Mooring

Mooring may refer to:* Mooring , any device used to hold secure an object by means of cables, anchors, or lines* Mooring mast, a structure designed to hold airships and blimps securely in the open when they are not in flight....

.

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 use astern propulsion (more commonly termed 'reverse thrust') 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

Reverse

Reverse may refer to:*The reverse side of currency or a flag; see Obverse and reverse*A change in the direction of:**the movement of a motor or other prime mover; see Transmission **an engineering design: see Reverse engineering...

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

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

arrangements.