From another list:

RULE 33(a) is amended to read as follows:

(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 required signals shall always be possible.

Please note that the bell is no longer required on a vessel 12 meters or
more but less than 20 meters in length.

http://www.navcen.uscg.gov/mwv/navrules/navrules2003.htm
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