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Rule 33: Equipment for Sound Signals (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 33 sets sound equipment by length. A vessel of 12 meters or more carries a whistle; 20 meters or more adds a bell; 100 meters or more adds a gong that can't be confused with the bell. Under 12 meters, she needs some other means of making an efficient sound signal.

The rule text

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

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 33

she shall be provided with some other means of making an efficient sound signal.

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 33

What it means on the water

  • 12 m+: whistle. 20 m+: whistle and bell. 100 m+: whistle, bell, and gong (33(a)).
  • Bell or gong may be replaced by equipment with the same sound characteristics if manual sounding remains possible (33(a)).
  • Under 12 m: some other efficient sound signal (33(b)).

International vs Inland

Same in both. Inland cites Annex III as 33 CFR Part 86.

Common exam traps

  • Requiring a bell at 12 meters. The bell starts at 20 meters.
  • Requiring a gong under 100 meters.

Try 3 questions on Rule 33

  1. At what length is a bell required?

    1. 12 m
    2. 20 m
    3. 50 m
    4. 100 m
    Show answer

    B. 20 m Rule 33(a).

  2. A gong is required at:

    1. 50 m
    2. 75 m
    3. 100 m or more
    4. 150 m
    Show answer

    C. 100 m or more Rule 33(a).

  3. A 10-meter boat must have:

    1. Whistle and bell
    2. Some other means of making an efficient sound signal
    3. Nothing
    4. A gong
    Show answer

    B. Some other means of making an efficient sound signal Rule 33(b).

Frequently asked questions

Does a small boat need a whistle?

Under 12 meters, not the Rule 33(a) whistle, but some other means of making an efficient sound signal (Rule 33(b)).

When is a gong required?

At 100 meters or more (Rule 33(a)).

Same Inland?

Yes.

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