Sound signals

COLREG Rules 32–37 govern the use of whistle, bell, and gong signals on the water. The whistle is the primary sound-signalling device; the bell and gong are required additionally on vessels of 100 m and over for anchored signals (Annex III). All signals follow a strict timing convention: a short blast (•) is about 1 second, a prolonged blast (—) is 4 to 6 seconds.

Manoeuvring & warning (Rule 34)

Used by power-driven vessels in sight of one another and manoeuvring as authorised or required by the Rules.

• •I am altering my course to starboard.Play
• ••I am altering my course to port.Play
• •••I am operating astern propulsion.Play
• •••••I am unsure of your intentions, or doubt you are taking sufficient action to avoid collision (the "wake-up" signal).Play

Narrow channels (Rule 34(c))

• ——•Overtaking on your starboard side.Play
• ——••Overtaking on your port side.Play
• —•—•I agree to your overtaking.Play
• —A vessel nearing a bend or obstruction where other vessels may be obscured (every minute or so).

Restricted visibility (Rule 35)

Sounded at intervals of not more than 2 minutes when in or near an area of restricted visibility, day or night.

• —Power-driven vessel making way through the water.Play
• ——Power-driven vessel underway but stopped, making no way.Play
• —••Vessel NUC, RAM, constrained by draft, sailing, fishing, or towing/pushing.Play
• —•Towed vessel (last vessel of the tow if manned), sounded after the towing vessel.Play
• •Pilot vessel on pilotage duty (optional, in addition to the appropriate signal above).Play

At anchor & aground (Rule 35(g)–(h))

• Vessel at anchor — rapid ringing of the bell for about 5 seconds at intervals of not more than 1 minute.
• Vessel ≥ 100 m at anchor — bell forward, then a separate gong sounded aft for about 5 seconds.
• Vessel at anchor (warning) — may additionally sound one short, one prolonged, one short blast to warn an approaching vessel.
• Vessel aground — three separate bell strokes, then 5 seconds of bell ringing, then three separate bell strokes (and gong if ≥ 100 m).
• Pilot vessel on station — may sound an identity signal of four short blasts (••••).

Distress (COLREG Annex IV)

Any of the following, used or exhibited together or separately, indicate distress and need of assistance.

• · Continuous sounding of any fog-signalling apparatus.
• · Gun or other explosive signal at intervals of about 1 minute.
• · Radiotelephony spoken "MAYDAY".
• · SOS in Morse — see the Morse page.
• · Rockets or shells throwing red stars fired one at a time at short intervals.
• · Orange-coloured smoke by day; red hand flare or parachute flare by night.
• · Slow and repeated raising and lowering of arms outstretched to each side.
• · The international code signal NC (November–Charlie) flown together.
• · A square flag with anything resembling a ball above or below it.
• · A piece of orange canvas with a black square and circle (visual signal for aircraft).
• · A dye marker.
• · Cospas-Sarsat EPIRB or AIS-SART activation, DSC distress alert.

Equipment standards (Annex III)

• · Whistle audibility: 2 nm for vessels ≥ 200 m, 1.5 nm 75–200 m, 1.0 nm 20–75 m, 0.5 nm under 20 m.
• · Whistle fundamental frequency: 70–200 Hz for vessels ≥ 200 m, 130–350 Hz 75–200 m, 250–700 Hz 20–75 m.
• · Bell minimum diameter: 300 mm for vessels ≥ 20 m. Bell and gong sound pressure level ≥ 110 dB at 1 m.

See also

• · Navigation lights — visual companion to these sound signals.
• · Distress procedures — Mayday voice format.
• · Distress frequencies — VHF / MF / HF channels.