Able Bodied Seaman - Rules of the Road Practice Test

In restricted visibility, vessels underway must signal their presence at regular short intervals so nearby ships know someone is there and can gauge how close they are. The standard interval for this signal is not more than two minutes, using a single prolonged blast. This cadence keeps the signal frequent enough to be reliable in fog or haze, helping to prevent collisions.

Intervals longer than two minutes (five, ten minutes) could leave others without timely notice, increasing the risk of misunderstanding or collision. The one long blast itself is the recognized signal, typically lasting about 4 to 6 seconds, and is repeated at up to two-minute intervals.