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COLREG

Rule 32: Definitions (Sound Signals)

Rule 32 gives names to three sound-signal definitions: the appliance and the two durations you need for COLREG study.

Rule 32 of 41

Decide

Decision 1 of 2

Listen to the signal. What did you hear?

The rule in plain words

  1. What is a whistle?It is any sound signaling appliance capable of producing the prescribed blasts and complying with Annex III to these Regulations.
  2. How long is a short blast?It is a blast of about one second’s duration. The word “about” does not set an exact figure.
  3. How long is a prolonged blast?It is a blast of from four to six seconds’ duration.
A ship's whistle and two blasts compared: a short blast (about one second) and a prolonged blast (four to six seconds).
1 of 2

Before you read on: what would you do?

Question 1/1

What can be considered a whistle under Rule 32?

Listen to it

Play each signal and listen: this is how it sounds at sea.

Frequent mistakes

  • Wrong: "Any device that makes a noise is a whistle."

    Right: It must produce prescribed blasts and comply with Annex III.

  • Wrong: "A short blast lasts exactly one second."

    Right: The duration is about one second, not an exact fixed time.

  • Wrong: "A prolonged blast always lasts exactly six seconds."

    Right: Its duration is from four to six seconds.

Exam keys & interpretation

  1. Only the short blast is "about" one second; the prolonged blast is simply four to six seconds.
  2. "Annex III" identifies the appliance specifications; making any sound is not enough.
  3. Separate short (about 1 s) from prolonged (4–6 s); one changed word can reverse the answer.

How to read it

Read "about" as written: COLREG gives a time reference, not an exact duration.

Apply the whistle definition with both checks: prescribed blasts and compliance with Annex III to these Regulations.

Key points

  • Whistle = sound signaling appliance complying with Annex III.
  • Short blast → about one second’s duration.
  • Prolonged blast → from four to six seconds’ duration.
  • Rule 32 separates the appliance, short blast and prolonged blast.

Test your knowledge

3 questions to check it. Pick an answer to see why.

Question 1/3

What is the duration of a short blast?

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Official text

IMO COLREG 1972 U.S. Coast Guard — International Navigation Rules (2024)

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:

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Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed:

Frequently asked questions about COLREG Rule 32

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.

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.