A whistle from 12 m, a bell as well from 20 m, and a gong as well from 100 m.
Paragraph (a): Whistle, bell and gong
A vessel of 12 m or more in length shall be provided with a whistle; a vessel of 20 m or more shall in addition be provided with a bell; and a vessel of 100 m or more shall in addition be provided with a gong whose tone and sound cannot be confused with the bell. All shall comply with Annex III. The bell or gong (or both) may be replaced by other equipment having the same sound characteristics, provided manual sounding of the required signals is always possible.
Sound signals
Played at true length: a short blast lasts about 1 second and a prolonged blast 4 to 6 seconds (Rule 32). Here a prolonged blast plays for 5 seconds.
📢Whistle
Whistle: required on every vessel of 12 m or more. It produces the prescribed short and prolonged blasts (Annex III).
🔔Bell
Bell: required in addition on vessels of 20 m or more.
Gong
Gong: required in addition on vessels of 100 m or more; its tone must not be confused with that of the bell.
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Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
Read the whole of Rule 33 on one page
Paragraph (a): Whistle, bell and gong
A vessel of 12 m or more in length shall be provided with a whistle; a vessel of 20 m or more shall in addition be provided with a bell; and a vessel of 100 m or more shall in addition be provided with a gong whose tone and sound cannot be confused with the bell. All shall comply with Annex III. The bell or gong (or both) may be replaced by other equipment having the same sound characteristics, provided manual sounding of the required signals is always possible.
Whistle: required on every vessel of 12 m or more. It produces the prescribed short and prolonged blasts (Annex III).
Bell: required in addition on vessels of 20 m or more.
Gong: required in addition on vessels of 100 m or more; its tone must not be confused with that of the bell.
Paragraph (b): Vessels under 12 m
A vessel of less than 12 m in length is not obliged to carry the appliances in paragraph (a), but if she does not, she shall be provided with some other means of making an efficient sound signal.
Some other means of making an efficient sound signal, for example a portable horn, sufficient to be heard by other vessels.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.