Fog Horn / Sound Signal

What Makes This Type

A whistle or fog horn is the vessel's audible signal under the International Regulations for Preventing Collisions at Sea (COLREGs), used both for manoeuvring signals in clear visibility and for fog signals when visibility is restricted. Unlike navigation lights, which only work if the other vessel can see them, sound carries around headlands, through fog and to small craft with no radar reflector at all, which is why the rule keeps it mandatory even on vessels dense with electronic navigation aids. The distinguishing feature of this equipment type against other signalling gear on the bridge is that COLREGs fixes its minimum audible range and frequency band by vessel length, not by owner preference, so the unit fitted has to match the ship's length category, not just its power supply.

Main Components

• Sound-producing unit: an electric or pneumatic diaphragm horn, or a reed-type whistle on some smaller or older installations, mounted high on the mast or funnel structure for the widest unobstructed spread.
• Air or electrical supply: pneumatic horns draw from the ship's compressed air system through a dedicated reducing valve; electric horns run off a diaphragm driven by a motor or solenoid.
• Remote control panel: a bridge-mounted switch or pedal that lets the officer of the watch sound prolonged and short blasts without leaving the console, required to be operable from the main conning position.
• Bell and, on vessels 100 m and over, a gong: COLREGs requires these as a separate fog signal appliance, sounded from forward (bell) and aft (gong) on vessels at anchor or aground, independent of the horn.

Selection and Sizing

COLREGs Annex III sets the required audibility range and frequency band by vessel length: roughly 2 nautical miles for vessels 200 m and over, down through intermediate bands to about 0.5 nautical mile for vessels under 12 m carrying an appliance at all. Frequency range and sound pressure level at one metre are specified accordingly, so a horn rated for a coaster cannot simply be fitted, unmodified, to a vessel in a larger length band. Mounting height and any obstruction from funnel casings or masts in the horn's radiation path are also part of the selection, since a technically compliant horn poorly placed can still fail to meet the effective range in practice.

Regulations and Class

COLREGs Rule 33 requires every vessel 12 m or more in length to carry a whistle and a bell, and vessels 100 m or more to also carry a gong, tuned so it cannot be confused with the bell. Annex III sets the technical performance standard for range, frequency and duration of blasts. Class and flag administration surveys check the appliance is operable from the conning position and audible at the required range, and SOLAS carriage requirements for the equipment intersect with the ship's safety equipment certificate, meaning a defective horn can hold up a survey even though the item itself is mechanically simple.

Typical Faults

What to Look for in a Supplier

• Type approval documentation showing the unit meets the frequency and range band for the vessel's length category.
• Availability of the specific diaphragm, reed or air valve spares rather than a generic replacement that changes the tone or output level.
• A mounting bracket and remote control interface matching the existing bridge wiring or air run, to avoid a full re-installation for a like-for-like replacement.

Test the horn from the actual conning position during the weekly test, not from the manufacturer's local panel - a cable fault between bridge and horn is the failure that shows up in port state control, not a bad diaphragm.