A foghorn is the equipment aboard a vessel that produces the sound signals required in fog or other conditions of restricted visibility — the physical device that generates the blasts specified under COLREGs Rule 35.
What It Actually Produces
The foghorn sounds the prolonged blasts (4-6 seconds) and combinations that indicate a vessel’s status in restricted visibility — one prolonged blast for a power-driven vessel underway, two for one stopped and making no way, and other specific combinations for vessels not under command, anchored, or in other conditions (see the “Sound Signals” entry for the full set).
Historical vs Modern Equipment
Traditional foghorns were mechanical, often hand or steam-powered devices producing a distinctive low tone. Modern vessels typically use electric or compressed-air whistle systems, automated to some degree, but still required to meet SOLAS specifications for audible range and tone.
Why It’s Still Mandatory Despite Radar and AIS
Even with radar and AIS providing detection capability, sound signals remain a mandatory, independent safety layer — a vessel without functioning AIS or one not being actively tracked can still be warned of your presence and status audibly, and sound signals don’t depend on the receiving vessel having any particular electronic equipment switched on or correctly interpreted.
Testing and Reliability
Foghorn functionality is checked as part of routine equipment testing, particularly before entering areas of known or forecast reduced visibility — a foghorn that doesn’t work when fog actually develops is a real, avoidable safety gap.
Exam Relevance
Foghorn equipment requirements and its relationship to COLREGs Rule 35 fog signals are covered within Navigation and Ship Knowledge topics in deck officer competency exams.