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COLREG

Rule 33: Equipment for Sound Signals

In COLREG, length sets the sound-signalling equipment: whistle, bell, and, from 100 metres, a gong with a distinguishable sound.

Rule 33 of 41

Decide

Decision 1 of 2

Listen. Which appliance is sounding?

The rule in plain words

  1. Which equipment applies at 12 metres?From 12 metres, a vessel must carry a whistle; from 20 metres, it must also carry a bell. The thresholds are inclusive: exactly 12 and 20 metres count.
  2. What is added at 100 metres?From 100 metres, add a gong that cannot be confused with the bell. Bell and gong may be replaced by equivalent equipment if manual sounding always remains possible.
  3. What about a vessel under 12 metres?It need not carry the appliances in paragraph (a); if it does not, it needs another means of making an efficient sound signal.
International equipment thresholds: whistle from 12 m, bell from 20 m, and an additional gong from 100 m. Below 12 m, an efficient means of sound signalling is required.
1 of 2

Before you read on: what would you do?

Question 1/1

What equipment must a vessel of 12 metres or more carry?

Listen to it

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

Frequent mistakes

  • Wrong: "At 12 metres I need no whistle."

    Right: At 12 metres or more, the vessel must carry a whistle.

  • Wrong: "The gong may sound the same as the bell."

    Right: From 100 metres, the gong's tone and sound cannot be confused with those of the bell.

  • Wrong: "A vessel under 12 metres may have no sound-signalling means."

    Right: If it lacks the paragraph (a) appliances, it needs another means of making an efficient sound signal.

Exam keys & interpretation

  1. "12 metres or more" includes exactly 12, 20 and 100 m; "less than 12" excludes exactly 12 m.
  2. If the gong appears, look for 100 metres and a tone and sound not confused with the bell's.
  3. Replacing equipment does not remove manual sounding: check "manual sounding ... always possible".

How to read it

Read the limits as cumulative steps: at 20 m the bell is added, and at 100 m the gong is added.

For a vessel under 12 m, the exemption concerns paragraph (a) appliances, not every efficient sound signal.

Key points

  • ≥12 m → whistle · ≥20 m → additional bell.
  • ≥100 m → gong with sound distinguishable from the bell.
  • All three appliances → Annex III specifications.
  • <12 m → another efficient means if paragraph (a) appliances are absent.

Test your knowledge

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

Question 1/3

At exactly 20 metres, which combination applies?

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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) A vessel of 12 meters or more in length shall be provided with a whistle, a vessel of 20 meters or more in length shall be provided with a bell in addition to a whistle, and a vessel of 100 meters or more in length shall, in addition, be provided with a gong, the tone and sound of which cannot be confused with that of the bell. The whistle, bell and gong shall comply with the specifications in Annex III to these Regulations. The bell or gong or both may be replaced by other equipment having the same respective sound characteristics, provided that manual sounding of the prescribed signals shall always be possible. (b) A vessel of less than 12 meters in length shall not be obliged to carry the sound signaling appliances prescribed in paragraph (a) of this Rule but if she does not, she shall be provided with some other means of making an efficient sound signal.
Source:

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

Frequently asked questions about COLREG Rule 33

Which vessels must carry a whistle, a bell and a gong?

A vessel of 12 metres or more must be provided with a whistle, a vessel of 20 metres or more with a bell in addition, and a vessel of 100 metres or more with a gong as well (Rule 33(a)). The gong's tone must not be confusable with the bell's.

From what length is a bell required on board?

20 metres. A whistle is required from 12 metres and a gong only from 100 metres, so assuming a 12-metre vessel needs a bell is a common mistake.

What must a vessel of less than 12 metres carry for sound signals?

She is not obliged to carry the whistle, bell and gong, but if she does not she shall be provided with some other efficient means of making a sound signal (Rule 33(b)).