Sound Signals

Maneuvering Signals

When vessels are in sight of one another, COLREGS Rule 34 governs the sound signals used to communicate maneuvering intentions. These signals are made on the whistle (or horn) and must be short blasts of approximately one second each.

One short blast — I am altering my course to starboard. Under international rules, this signal announces the maneuver; under inland rules (Inland Navigation Rules), it means 'I intend to pass you on my port side' and requires an agreement signal from the other vessel.

Two short blasts — I am altering my course to port. Same distinction applies: international rules announce the action; inland rules require agreement.

Three short blasts — My engines are operating astern (propulsion is in reverse). This does not necessarily mean the vessel is moving astern — a vessel may be slowing down or stopping while still moving forward through the water. It is an engine signal, not a course signal.

Five or more short, rapid blasts — The doubt or danger signal. This signal means 'I am unsure of your intentions' or 'I believe a risk of collision exists.' It is the COLREGS equivalent of a horn honk in traffic. Any vessel that hears another vessel and cannot determine what it is doing, or sees a developing situation that is not resolving, should use this signal immediately. There is no penalty for using it; the risk of not using it is far greater.

The inland agreement signal (Inland Rules only): when a power vessel proposes a meeting or passing arrangement using one or two blasts, the other vessel must respond with the same signal to agree, or sound the danger signal if it disagrees. This mutual-agreement structure is one of the most important differences between International and Inland Rules.

Restricted Visibility Signals

When visibility is reduced by fog, mist, rain, snow, or any other condition, vessels must use fog signals under Rule 35, regardless of whether other vessels are detected. These signals are required at regular intervals and must be made on the whistle.

Power vessel underway and making way: one prolonged blast every 2 minutes. A prolonged blast lasts 4–6 seconds.

Power vessel underway but stopped (not making way): two prolonged blasts every 2 minutes with about a 2-second pause between them.

Sailing vessel underway: one prolonged blast followed by two short blasts (1 long + 2 short) every 2 minutes. This same signal is also used by vessels not under command, vessels restricted in ability to maneuver, vessels constrained by draft, fishing vessels, and vessels towing or pushing. Memorizing the sailing vessel signal gives you the RAM/NUC/CBD/fishing signal for free.

Vessel at anchor: rapid ringing of a bell for about 5 seconds, at intervals of not more than 1 minute. Vessels over 100 meters must also immediately follow the bell with a gong rung rapidly aft.

Vessel aground: three distinct strokes on the bell before and after the rapid bell ringing, at intervals of not more than 1 minute.

A vessel of less than 12 meters is not required to make these signals but must make some efficient sound signal at intervals of not more than 2 minutes.

Why fog signals matter beyond rule compliance: in restricted visibility you cannot see the lights of an approaching vessel. Sound signals give you bearing information — and if the signal is getting louder, the vessel is getting closer. If you can hear a fog signal but cannot determine its direction reliably, reduce speed, post a sound watch, and be ready to stop.

Doubt, Warning, and Distress Signals

Beyond the scheduled maneuvering and fog signals, COLREGS provides for urgent signals that any vessel may use at any time.

The doubt or danger signal (Rule 34(d)): five or more short, rapid blasts on the whistle. This signal is mandatory to know and mandatory to respond to. If you hear it, treat it as a collision risk signal and immediately determine whether the warning applies to you. If you are uncertain of another vessel's intentions, or believe risk of collision is developing, give this signal immediately. It requires no prior warning and no formality — it is the maritime equivalent of 'watch out.'

Whistle vs. horn: under COLREGS, 'whistle' is the technical term for any sound-producing device that meets Annex III specifications. On small vessels this is typically an air horn (compressed gas) or an electric horn. For vessels less than 12 meters, a whistle of any type is acceptable. For vessels 12 meters and above, the device must meet specific frequency and sound pressure level requirements. Carry a handheld compressed air horn as a backup regardless.

Distress signals (Annex IV): distress signals are separate from COLREGS sound signals and are governed by Annex IV. Relevant sound distress signals include: a continuous sounding of a fog signal apparatus (any fog signal device used continuously, not rhythmically), and the Mayday call on VHF Channel 16. Other distress signals include the orange smoke signal, parachute rocket flares, the EPIRB, and the dye marker. A vessel in distress is not required to follow COLREGS maneuvering conventions — collision avoidance takes a back seat to survival.

Bridge-to-bridge radio (VHF Ch 16): while not a sound signal in the COLREGS sense, Rule 34(d) in the Inland Rules also recognizes VHF radio as an alternative to whistle signals for establishing maneuvering agreements. Many commercial vessels now use Ch 16 or the appropriate working channel for passing arrangements. However, an agreement made on VHF does not replace the sound signal requirement — it supplements it.