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 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 Rule 33(a) but if she does not, she shall be provided with some other means of making an efficient signal.
Notice in the rule they make reference to a Bell and a Gong for larger vessels, this will be for anchoring in restricted visibility as you will see in the next rule. A vessel under 20 meters (40 feet) just needs some way of making and efficient signal. Kind of vague but you vessels horn will suffice, as well as the air cans, I have a trumpet type manual horn etc.