Official rule text as published in the U.S. Coast Guard Navigation Rules
(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 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 (a), but if she does not, she shall be provided with some other means of making an efficient signal.
MyCOLREGS explanation
Summary
What sound equipment you must carry depends on your vessel’s length: 12 meters or more, a whistle; 20 meters or more, a bell in addition; 100 meters or more, a gong too, with a tone that will not be confused with the bell. Vessels under 12 meters do not have to carry this equipment, but if they skip it, they still need some other efficient way to make a sound signal.
Real World Example
The gong required on vessels 100 meters and up exists because a bell alone, rung at the bow, may not carry the length of a ship that size back to crew standing watch aft - a redundancy that has nothing to do with tradition and everything to do with a signal actually being heard where it needs to be heard.
Practice Questions
Q1. Your vessel is 22 meters long. Rule 33 already requires her to carry a whistle - does she also need a bell aboard?
A. No - the bell requirement doesn't begin until 50 meters.
B. Yes - Rule 33 requires a bell in addition to a whistle once a vessel reaches 20 meters in length.
C. No - bells are only required on vessels that also carry a gong, which doesn't apply until 100 meters.
D. Yes, but only if she operates on inland waters; a 22-meter vessel offshore needs only a whistle.
Q2. A 9-meter vessel does not carry a whistle. Is that a violation of Rule 33?
A. No, vessels under 12 meters have no sound-signal obligation at all.
B. Yes, unless she is at anchor.
C. Yes, every vessel must carry a whistle regardless of length.
D. No, but she must have some other means of making an efficient sound signal.
Practice Question Answers
Q1.
B. 20 meters Rule 33(a) requires a bell in addition to a whistle at 20 meters or more in length, and a gong as well at 100 meters or more. A overstates the threshold well beyond where the bell requirement kicks in. B and D each undershoot or overshoot it - Rule 33 sets the bell requirement at 20 meters and above.
Q2.
D. No, but she must have some other means of making an efficient sound signal. Rule 33(b) excuses vessels under 12 meters from carrying the prescribed appliances, but only if they are provided with some other means of making an efficient signal. A wrongly assumes vessels under 12 meters have no obligation at all - they still need some way to signal efficiently. B invents an anchoring-based exception. C overstates the requirement into needing a literal whistle, when Rule 33 allows any efficient sound-signal means for smaller vessels.