Sound signals

Short blast = about 1 second. Prolonged blast = 4–6 seconds.

Manoeuvring (Rule 34)

• • (one short) — I am altering my course to starboard.
• • • — I am altering my course to port.
• • • • — I am operating astern propulsion.
• • • • • • (five or more) — I am unsure of your intentions, or I doubt you are taking sufficient action to avoid collision.
• ─ • (overtaking, narrow channel) — I intend to overtake on your starboard side.
• ─ • • — I intend to overtake on your port side.
• ─ • ─ • — I agree to your overtaking.

Restricted visibility (Rule 35)

• ─ (one prolonged, every 2 min) — Power-driven vessel making way.
• ─ ─ (every 2 min) — Power-driven vessel underway, stopped, not making way.
• ─ • • (every 2 min) — Vessel not under command / restricted manoeuvre / constrained by draft / sailing / fishing / towing or pushing.
• Bell ring for 5 seconds, each minute — at anchor (vessels ≥100m add a gong aft).
• ─ • (towed vessel, last if more than one) — every 2 minutes, immediately after the towing vessel's signal.

Distress (Annex IV)

• Continuous sounding of any fog-signalling apparatus.
• Signal made by radiotelephony of MAYDAY.
• Gun or other explosive signal at ~1-minute intervals.
• SOS in Morse.
• Rockets / flares showing red.
• Orange smoke by day; red hand flare or parachute flare by night.