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.