Sierra Flag

Sierra, letter S of the International Code of Signals, means “I am operating astern propulsion,” that is, my engines are going astern. The flag is white with a blue rectangle at its center. Hoisted singly it warns other vessels the ship is backing down, and it is read with the COLREGs sound signal of three short blasts indicating astern propulsion. It is common in close-quarters maneuvering and berthing.