Foghorn

The foghorn, or whistle in COLREGs language, is the sound-signaling appliance required by Rule 33 and specified in Annex III by fundamental frequency and audible range tied to vessel length: a vessel of 200 m or more uses 70 to 200 Hz, audible 2 nautical miles. In restricted visibility (Rule 35) it gives the prescribed signals: one prolonged blast every two minutes for a power-driven vessel making way, two prolonged for one stopped, and the cardinal-rhythm groups for vessels not under command, restricted in ability to maneuver, or towing.