Astern propulsion

The Sierra signal flag used to convey, "I am operating astern propulsion."

Astern propulsion is a ship maneuver in which the ship's propeller is faced towards the bow. In other words, the ship is pushing itself in the opposite direction it normally would. Astern propulsion does not necessarily mean the ship is moving astern (in reverse); it could simply mean the ship is slowing itself down by applying force in the opposite direction it is going.

A boat is required to signal it is operating aster propulsion by either blowing three short easily audible blasts or by waving the Sierra signal flag (white border with one central blue square).