Originally Posted by Jim Quibell
Every vessel less than 39.4 ft (12 meters) in length must carry an efficient sound producing device. Every vessel 39.4 ft (12 meters) or larger in length must carry a whistle and a bell. The whistle must be audible for 1/2 nautical mile. The mouth of the bell must be at least 7.87 inches (200mm) in diameter
It has changed in the last few years. Inland rules still requires a bell for vessels > 12 meters, International rules require a bell > 20 meters.
Rule 32 & 33: Definitions and Equipment for Sound Signals
(a) A vessel of 12 meters or more in length
shall be provided with a whistle and a bell [INLD
], 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 signal.