Sound signals

Last verified 3 months ago

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

Narrow channels (Rule 34(c))

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

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.

FAQ

What is the difference between a short blast and a prolonged blast?

A short blast lasts about one second; a prolonged blast lasts from four to six seconds. The distinction is timing, not pitch — the manoeuvring and fog signals in COLREG Rules 34 and 35 are defined entirely by the pattern and length of blasts, so getting the duration right is what makes a signal unambiguous.

Which signal means “I do not understand your intentions”?

At least five short and rapid blasts on the whistle (•••••) — the Rule 34(d) doubt or “wake-up” signal. It is sounded by a power-driven vessel in sight of another when she is in doubt whether the other vessel is taking sufficient action to avoid collision, and may be supplemented by an equivalent light signal.

What sound signal does a vessel at anchor make in fog?

Under Rule 35(g) a vessel at anchor rings the bell rapidly for about five seconds at intervals of not more than one minute. A vessel of 100 m or more also sounds a gong for about five seconds in the after part of the ship immediately after the bell, so the two ends of a long vessel can be distinguished.

Primary source

· IMO — COLREG 1972 (Convention on the International Regulations for Preventing Collisions at Sea), Rules 32–37 and Annex III.