Rule 33 (COLREG-72) - equipment for sound signals
(A) A vessel of 12 meters or more shall be equipped with a whistle and a bell and a vessel length of 100 m or more must also be provided with a gong, the tone and sound of which could not be mistaken for the sound of the bell. The whistle, bell and gong shall comply with the requirements of Annex III to this Regulation. Bell and (or) the gong can be replaced by other equipment having the same respective sound characteristics, and should always be capable of providing the required signal manually.
(B) A vessel of less than 12 meters are not required to have a sound-signaling appliances prescribed in paragraph (a) of this Rule, but if such a vessel does not have them, it must be provided with other means of making an efficient sound signal.