COLREGS international ....you no longer need a bell under 20M, but must still sound an appropriate signal...whatever that is on can only guess.
Inland still shows bells for 12M to 20M...but the USCG has accepted the international change and is not enforcing the currently published Inland rule.
From the Navrules under International...
RULE 33
Equipment for Sound Signals
(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 lIl 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 sound signal.
..........
(i) A vessel of 12 meters or more but less than 20 meters in length shall
not be obliged to give the bell signals prescribed in paragraphs (g) and (h) of
this Rule. However, if she does not, she shall make some other efficient
sound signal at intervals of not more than 2 minutes.
(j) A vessel of less than 12 meters in length shall not be obliged to give the
above-mentioned signals but, if she does not, shall make some other
efficient sound signal at intervals of not more than 2 minutes.