How long is a prolonged blast under the COLREGs?

From four to six seconds (Rule 32(c)). A short blast, by contrast, is a blast of about one second's duration (Rule 32(b)).

What is a short blast in the COLREGs?

A blast of about one second's duration (Rule 32(b)). A blast of two or three seconds is neither short nor prolonged, so the signal is wrong rather than a softer version of either.

Definitions (Sound Signals)

Q: What counts as a whistle under COLREG Rule 32?

Any sound-signalling appliance capable of producing the prescribed blasts and complying with the specifications in Annex III — diaphragm horns, electric horns, sirens and air-powered foghorns all qualify.

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. The Rule 34(a) manoeuvring signals below apply to a power-driven vessel underway, in sight of another vessel. Rule 34(c) overtaking signals apply to vessels in sight in a narrow channel or fairway, using the Rule 9(e)

1procedure. Rule 35(a)/(b) signals apply in or near restricted visibility, subject to the special-vessel and small-vessel provisions of Rule 35. A bend warning under Rule 34(e) does not require visual contact.

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 through the water in restricted visibility (Rule 35a)
2	I am altering my course to port (Rule 34a)	Power-driven vessel underway, stopped and making no way through the water in restricted visibility (Rule 35b)
3	I am operating astern propulsion (Rule 34a)	—
2 prolonged + 1 short	Mixed duration	Intend to overtake on your starboard (Rule 34c)
2 prolonged + 2 short	Mixed duration	Intend to overtake on your port (Rule 34c)

IMO COLREG 1972Official text

Full wording of the rule, reproduced from the official source below.

(a) The word whistle means any sound signaling appliance capable of producing the prescribed blasts and which complies with the specifications in Annex III to these Regulations. (b) The term short blast means a blast of about one second’s duration. (c) The term prolonged blast means a blast of from four to six seconds’ duration.

Source:

U.S. Coast Guard — International Navigation Rules (2024)

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 vessels in sight are approaching and you do not understand the other vessel’s intentions or actions, or doubt its collision-avoiding action, immediately sound at least five short, rapid blasts (Rule 34(d)).

Exam Focus

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

Rule 35(a)–(f) signals have a maximum interval of 2 minutes; the anchor and aground signals in (g)–(h) have a maximum interval of 1 minute.

Paragraphs (i)–(j) provide small-vessel alternatives.

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

A short course-change signal applies to a power-driven vessel underway in sight; a prolonged blast may be a Rule 35(a) signal or a Rule 34(e) bend warning, depending on the circumstances

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

Confusing a compliant whistle with replacement equipment for a bell or gong under Rule 33

Test Your Knowledge

Practise this topic and review the explanation for each answer.

Last reviewed: September 9, 2026