Boat Sound Signals — Horn and Fog Signals Explained

A short blast lasts about one second and a prolonged blast four to six seconds. Between vessels in sight of each other: one short blast means 'I am altering course to starboard', two 'to port', three 'I am operating astern propulsion' and five or more 'I don't understand your intentions'. In fog a power-driven vessel making way sounds one prolonged blast every two minutes; a sailing vessel sounds one prolonged and two short.

Two large ship's horns mounted on a mast against a blue sky — Ship's horns. Large vessels must carry a whistle that can be heard up to 2 miles away. Photo: Drw25 at English Wikipedia, CC BY-SA 3.0.

Manoeuvring and warning signals (Rule 34)

Short and long blast patterns for manoeuvring signals and for fog signals in restricted visibility — The signals you will hear most often. A short blast is about 1 second, a long one 4 to 6.

Vessels in sight of one another.
Signal	Meaning
1 short	I am altering my course to starboard
2 short	I am altering my course to port
3 short	I am operating astern propulsion
5 or more short	I am in doubt about your intentions
1 prolonged	Approaching a bend or an area hidden by an obstruction
2 prolonged, 1 short	I intend to overtake you on your starboard side (narrow channel)
2 prolonged, 2 short	I intend to overtake you on your port side (narrow channel)
Prolonged, short, prolonged, short	Agreement to be overtaken

A weathered foghorn on Ballycotton Island lighthouse, Ireland — A shore foghorn on Ballycotton Island. Vessels make their own fog signals too. Photo: Podstawko, CC BY-SA 4.0.

Restricted visibility (Rule 35)

Sounded at intervals of not more than 2 minutes unless stated.
Signal	Vessel
1 prolonged	Power-driven vessel making way
2 prolonged	Power-driven vessel underway but stopped
1 prolonged, 2 short	Sailing, fishing, towing, NUC, RAM or CBD vessel
1 prolonged, 3 short	Last manned vessel of a tow
Rapid bell for 5 s, every minute	At anchor (gong aft as well if 100 m or more)
1 short, 1 prolonged, 1 short	At anchor: optional warning to an approaching vessel
3 bell strokes, rapid bell, 3 strokes	Aground

A vessel under 12 m does not have to give these signals but must make some other efficient sound signal at least every two minutes. A handheld horn counts.

Hear and practise them in the sound signals section of the COLREGs trainer, or print the sound signals cheat sheet.

The sound signal rules in full

Related guides

Frequently asked questions

How long is a prolonged blast?

Four to six seconds. A short blast is about one second.

What sound signal does a sailing boat make in fog?

One prolonged blast followed by two short blasts, at intervals of not more than two minutes.

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