IALACOLREG

Sound & Light Signals

COLREG Part D (Rules 32–37) standardises the whistle, bell and gong signals used to communicate intent, warn of danger, and indicate presence in restricted visibility.

Sound signals — the three families you need to know

COLREG sound signals split into three families. Manoeuvring and warning signals (Rule 34) — short whistle blasts used when two vessels are in sight of one another to announce a turn or alert the other vessel. Fog signals (Rule 35) — prolonged blasts and bell/gong patterns used in or near an area of restricted visibility, day or night. Distress signals (Rule 37, Annex IV) — continuous fog-horn sounding and SOS by any method.

Two durations: a SHORT blast is about one second; a PROLONGED blast is four to six seconds. Combine them and you've covered most of Rule 34: one short = "I am altering course to starboard", two short = "to port", three short = "my engines are operating astern", five or more short = "I do not understand your intentions" or "I am in doubt". The exam loves the last one — most candidates forget it.

Fog signals (Rule 35) follow a strict cadence. Power-driven vessel making way: one prolonged blast every 2 minutes. Power-driven vessel underway but stopped: two prolonged blasts. Sailing, fishing, NUC, RAM, CBD, towing or being towed: one prolonged + two short. At anchor under 100 m: rapid 5-second bell every minute. Aground: extra three strokes of the bell before and after the anchor pattern. Memorise the cadences — they decide far more exam marks than the wording of the rule.

Whistle signals at a glance

SignalMeaningRule
1 short blast (•)I am altering course to starboard34(a)
2 short blasts (• •)I am altering course to port34(a)
3 short blasts (• • •)I am operating astern propulsion34(a)
5+ short blasts (• • • • •)I do not understand your intentions / I am in doubt34(d)
1 prolonged blast (—)Power-driven vessel making way through the water (fog)35(a)
2 prolonged blasts (— —)Power-driven vessel underway but stopped (fog)35(b)
1 prolonged + 2 short (— • •)Sailing, fishing, NUC, RAM, CBD, towing (fog)35(c)

Practise the signals

Frequently asked questions

What is the difference between a short blast and a prolonged blast?
A short blast is a blast of about one second. A prolonged blast is four to six seconds. Rule 32(b) defines both. The distinction is critical: one short blast means "altering course to starboard" (Rule 34), but one prolonged blast every two minutes is the fog signal for a power-driven vessel making way (Rule 35). Same single sound, two completely different meanings — context (visibility) decides which.
What does five short blasts mean?
Five or more short and rapid blasts on the whistle mean "I do not understand your intentions" or "I am in doubt that sufficient action is being taken by the other vessel to avoid collision" (Rule 34(d)). They may be supplemented by a light signal of at least five short and rapid flashes. It is the only "interrogative" sound signal in the COLREGs.
What sound signal must a vessel at anchor make in fog?
A vessel at anchor must ring the bell rapidly for about five seconds at intervals of not more than one minute (Rule 35(g)). If she is 100 m or more in length, the bell is sounded in the forepart and a gong is sounded in the after part for five seconds. A vessel aground adds three separate and distinct strokes of the bell immediately before and after the rapid bell ringing.
When must I sound a fog signal?
In or near an area of restricted visibility, whether by day or by night (Rule 35). "Restricted visibility" is defined in Rule 3(l) as any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or any other similar causes. The signals are made AT THE PRESCRIBED INTERVALS — not on demand — and continue until conditions clear.
Is Morse code SOS still a valid distress signal?
Yes. Rule 37 and Annex IV(a)(xiv) explicitly list "the radiotelegraph alarm signal" and SOS made by any signalling method as recognised distress signals. The full Annex IV list includes 17 visual and audible signals from rockets and flames to the orange smoke and the upraised-and-lowered arms signal — all still valid, all still expected to be recognised.