Foghorns
Foghorns, or ship's horns, and ship's whistles, are required in restricted visibility to warn other vessels that may be in your proximity, or to warn about hazards that, in good visibility, would normally be avoided due to e.g. the light from a lighthouse, or channel marker beacon. A vessel underway must give one prolonged horn blast every two minutes. Again,
as for the ship's bell, the ship's horn must be of a type that complies
with the International Regulations for Preventing Collisions at Sea.
