Sierra (S)
maritime signalsmaritimesignal flagnavigation
The Sierra flag shows a blue rectangle centered on a white background. It means 'I am operating astern propulsion,' indicating the vessel's engines are in reverse. This is vital information for nearby ships, as a vessel going astern behaves unpredictably and may not respond to its rudder normally.
Large ships can take over a mile to stop even with full astern propulsion. The Sierra flag gives critical warning because a vessel going astern can swing unpredictably, and its propeller wash can create dangerous currents for smaller craft nearby.
Adopted: 1857
