Definitions (Sound Signals)

Short blast (~1 s) vs prolonged blast (4-6 s) — duration timeline.

Rule 32 sets out the vocabulary used by every sound signal in the COLREG. Without these three definitions, the rest of Part D is ambiguous.

The word "whistle" means any sound-signalling appliance capable of producing the prescribed blasts and complying with the specifications in Annex III — that includes diaphragm horns, electric horns, sirens and air-powered foghorns. Whistle frequency, audibility range and sound level all depend on vessel length per Annex III (audibility range 0.5 nm for small craft up to 2 nm for vessels ≥ 200 m).

The term "short blast" means a blast of about one second's duration.

The term "prolonged blast" means a blast of from four to six seconds' duration.

Why exact duration matters

The same number of blasts can mean very different things depending on whether they are short or prolonged. Get the duration wrong and the message changes:

Blasts	Short (~1 s each)	Prolonged (4-6 s each)
1	I am altering my course to starboard (Rule 34a)	Bend / blind-channel warning (Rule 34e); power-driven vessel making way in fog (Rule 35a)
2	I am altering my course to port (Rule 34a)	Power-driven vessel underway but stopped in fog (Rule 35b)
3	I am operating astern propulsion (Rule 34a)	—
2 + 1	—	Intend to overtake on your starboard (Rule 34c)
2 + 2	—	Intend to overtake on your port (Rule 34c)

Signal Workflow

Decide the context first: manoeuvring in sight, restricted visibility, or distress.

The same whistle can mean very different things in a different context.

Check both the pattern and the interval.

For fog signals, the time spacing is part of the rule, not just the blast sequence.

When in doubt about another vessel's intentions, use the prescribed warning signal early rather than waiting for the situation to deteriorate.

Exam Focus

Three short blasts are astern propulsion, not a fog signal.

In restricted visibility, think 'every two minutes' for underway signals and 'every one minute' for anchor bell signals.

Key Takeaways

Short blast = about 1 second; prolonged blast = 4 to 6 seconds

"Whistle" is a generic term for any Annex III-compliant horn, siren or foghorn

Same number of blasts means different things depending on duration — duration is part of the signal

1 short = altering course to starboard; 1 prolonged = fog signal making way OR bend warning — context decides

Annex III sets minimum sound levels and audibility ranges by vessel length (0.5-2 nm)

Common Mistakes

Making short blasts too long (2-3 s) — that is neither short nor prolonged, so it is wrong

Confusing 1 short (manoeuvring) with 1 prolonged (fog / bend) — duration is what tells them apart

Assuming "whistle" means a literal whistle — it covers any prescribed horn/siren

Forgetting that an electronic horn must still allow manual sounding

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