Foghorn

A sound-signalling appliance used to make a powerful warning signal in fog or other restricted visibility.

Foghorns may be carried by vessels or installed at fixed locations such as lighthouses, harbour entrances and offshore structures. Modern appliances can be pneumatic, electric or electronic; the familiar deep tone is intended to carry over distance, although its apparent direction can be difficult to judge.

The signals required of a vessel depend on its status and activity under the applicable collision regulations. Hearing a fog signal gives useful warning but does not establish an exact position, so speed, lookout and navigation must be managed for the prevailing visibility.