Sound signals: the whistle and the bell

A short blast lasts about one second, a prolonged blast four to six seconds (Rule 32). In sight of one another, vessels signal their manoeuvres; in restricted visibility they sound identity signals at intervals of not more than two minutes, and a vessel at anchor rings her bell at intervals of not more than one minute (Rule 35). You can play every one of them here: dot = short, dash = prolonged. Further down: the attention signals (Rule 36) and the full list of distress signals (Rule 37 and Annex IV).

Manoeuvring signals (Rule 34)

Altering course to starboard · One short blast: I am altering my course to starboard.

Altering course to port · · Two short blasts: I am altering my course to port.

Operating astern propulsion · · · Three short blasts: I am operating astern propulsion. The engines run astern; the ship herself may still be moving ahead.

I intend to overtake on your starboard side – – · Two prolonged and one short blast, in a narrow channel or fairway: I intend to overtake you on your starboard side (Rule 9e).

I intend to overtake on your port side – – · · Two prolonged and two short blasts, in a narrow channel or fairway: I intend to overtake you on your port side.

Agreement to be overtaken – · – · One prolonged, one short, one prolonged and one short blast, in that order: the vessel about to be overtaken agrees to the manoeuvre.

The doubt signal · · · · · At least five short and rapid blasts: I do not understand your intentions, or I doubt whether you are taking sufficient action to avoid collision.

Bend or obstructed fairway – One prolonged blast: I am approaching a bend or a stretch where other vessels may be hidden from view. Answered with one prolonged blast.

The same signals may be repeated with light flashes (Rule 34b): one flash for starboard, two for port, three for astern propulsion. A flash lasts about one second, the interval between flashes about one second, and the interval between successive signals not less than ten seconds. The light, if fitted, is an all-round white light visible at not less than five miles. The doubt signal has a light version too: at least five short and rapid flashes (Rule 34d).

Signals in restricted visibility (Rule 35)

Power-driven vessel making way – One prolonged blast at intervals of not more than 2 minutes: a power-driven vessel underway and making way through the water.

Power-driven vessel, underway but stopped – – Two prolonged blasts at intervals of not more than 2 minutes: a power-driven vessel underway but stopped and making no way through the water.

NUC, RAM, sailing, fishing, towing – · · One prolonged and two short blasts at intervals of not more than 2 minutes: among others a vessel not under command, restricted in her ability to manoeuvre, constrained by her draught, sailing, fishing or towing.

Vessel being towed – · · · One prolonged and three short blasts, when the tow is manned: sounded right after the towing vessel’s signal.

Fishing or RAM at anchor – · · A vessel fishing at anchor and a RAM working at anchor sound one prolonged and two short blasts instead of the anchor bell (1981 amendment).

Pilot vessel identity signal · · · · Four short blasts: the identity signal of a pilot vessel on duty, added to her signal under Rule 35 a, b or g.

Vessel at anchor: the bell Rapid ringing of the bell for about five seconds, at intervals of not more than one minute. From 100 m of length, the gong answers from aft.

Vessel aground: the bell Three separate and distinct strokes, rapid ringing for about five seconds, then three strokes again.

The sounds are synthesized in your browser (Web Audio): the low whistle matches a big ship. At sea the pitch depends on the size of the vessel, but the rhythm of the signal never changes, and the rhythm is what you listen for.

Attention signals (Rule 36)

To attract the attention of another vessel, any vessel may make light or sound signals that cannot be mistaken for any signal of these Rules, or direct the beam of her searchlight towards the danger in a way that does not embarrass any vessel. The light must not resemble an aid to navigation, and high-intensity strobe lights are to be avoided (a sentence added by the 1981 amendment).

Distress signals (Rule 37 and Annex IV)

A vessel in distress and requiring assistance uses one or more of the signals of Annex IV to the Convention, together or separately. The full list:

a gun or other explosive signal fired at intervals of about a minute,
a continuous sounding with any fog-signalling apparatus,
rockets or shells throwing red stars, fired one at a time at short intervals,
the signal SOS in Morse code (· · · – – – · · ·), made by any signalling method,
the spoken word "Mayday" by radiotelephony,
the International Code Signal of distress: NC,
a square flag with a ball, or anything resembling a ball, above or below it,
flames on the vessel, as from a burning tar or oil barrel,
a rocket parachute flare or a hand flare showing a red light,
a smoke signal giving off orange-coloured smoke,
slowly and repeatedly raising and lowering arms outstretched to each side,
a distress alert by digital selective calling (DSC),
a ship-to-shore distress alert transmitted by the ship’s satellite communication equipment,
signals transmitted by emergency position-indicating radio beacons (EPIRB),
approved signals transmitted by radiocommunication systems, including survival craft radar transponders (SART).

Using any of these signals except to indicate distress, or using signals that may be confused with them, is prohibited. Source: COLREG Annex IV as amended.

COLREG lights: the learning path Lights trainer VHF radio module