Boat Horns and Sound Signals: Requirements, Testing and the Signals Every Skipper Should Know

With the boat at the dock, give a quick tap (a brief tap is enough in a quiet marina). It should sound loud and clear right away. A weak, raspy or delayed sound means low voltage, corrosion or a failing horn. Check it again before running at night or in fog.

When power boats are in sight of each other and meeting or crossing within half a mile:

• 1 short: "I intend to leave you on my port side" (pass port to port). When overtaking: "I intend to pass on your starboard side."
• 2 short: "I intend to leave you on my starboard side." When overtaking: "I intend to pass on your port side."
• 3 short: "I am operating astern propulsion" (backing).
• 5 or more short and rapid: Danger or doubt. "I don't understand your intentions" or "I don't think your maneuver is safe."
• 1 prolonged: leaving a dock or berth, or approaching a blind bend where boats on the other side can't see you.

If you agree with a one- or two-blast signal, answer with the same signal. If you don't, sound the danger signal and both boats slow down until it's sorted out. If you've already agreed on the radio, horn signals become optional (33 CFR 83.34); if the radio call doesn't produce a clear agreement, exchange horn signals in time, and the horn signals are what count.

Three more details from Rule 34 that confuse owners:

• At a blind bend, your prolonged blast should be answered with a prolonged blast by any boat coming the other way that can hear it. If you hear one coming around a bend, answer it, slow down and keep to the starboard (right) side of the channel.
• Flashing lights on big boats: a vessel may back up its whistle signals with an all-round white or yellow light that flashes in step with the horn: one flash, two flashes, three flashes, and five or more for danger. If you see a ship or tug flashing a light as it sounds, it means the same as the blasts.
• Danger signal goes both ways: any boat in doubt, including the one being overtaken, gives the five short blasts. Don't wait for the bigger boat to do it. BoatUS stresses that these passing signals are only for boats that can see each other; in fog, use the fog signals instead.

In or near fog, rain or any restricted visibility, day or night (33 CFR 83.35):

• Power boat moving through the water: 1 prolonged blast at least every 2 minutes
• Power boat underway but stopped (drifting): 2 prolonged blasts, about 2 seconds apart, at least every 2 minutes
• Sailboat under sail, boat fishing with nets or lines that restrict maneuvering, boat towing, or a boat that can't maneuver normally: 1 prolonged and 2 short at least every 2 minutes
• At anchor: ring a bell rapidly for about 5 seconds every minute. A boat under 20 m (65.6 ft) doesn't have to do this in a Coast Guard-designated special anchorage. An anchored boat may also sound 1 short, 1 prolonged, 1 short on the horn to warn a boat that seems to be coming too close.

Boats under 12 m (39.4 ft) aren't required to give these exact signals, but if they don't, they must make some other efficient sound signal at least every 2 minutes. In practice, a small-boat skipper should still give the standard signals with a horn.

BoatUS (2022) reminds owners that "restricted visibility" isn't only fog: heavy rain, night, dust and even wildfire smoke count. And sounding your own signal is only half the job. Mercury suggests listening for other boats between your blasts, and the Navigation Rules (33 CFR 83.19, Rule 19) say that if you hear another boat's fog signal apparently ahead of your beam, slow to the minimum speed that keeps you on course, stop if you need to, and go with extreme caution until the danger is past.

Most horns stop working because of the circuit, not the horn.

1. Check the fuse or breaker for the horn circuit (see our fuse and breaker panel guide). Replace a fuse only with the same rating; if it blows again, find the short before going out.
2. Make sure the battery switch is on and the battery is charged.
3. With a helper pressing the horn button, measure the voltage at the horn's terminals with a multimeter. Full battery voltage while pressed means the problem is the horn itself. No or low voltage means the problem is upstream.

• Horn button: these sit in the weather and corrode. With the circuit fused and the battery on, check for voltage going in and coming out when pressed. A button that's crunchy, sticky or intermittent should be replaced with a marine switch rated for the horn's current.
• Relay: larger horns (especially dual-trumpet horns) use a relay so the small button doesn't carry the full current. Listen and feel for a click when the button is pressed. No click: check the relay's power and the button signal. A click but no horn: the relay contacts or the heavy wire to the horn may be bad.
• Ground: a loose or corroded ground wire is a classic cause of a weak or dead horn. Follow it to the negative bus or battery and clean both ends.

Disconnect power. Clean corroded terminals with a wire brush or fine pad until they're bright, reconnect them tightly and coat them with a corrosion inhibitor or dielectric grease. Replace any green, brittle wire ends with new tinned wire and heat-shrink crimp connectors. See our electrical corrosion guide.

Rinse salt and dirt off with fresh water and a soft brush. Check that the trumpet opening is clear of insect nests, spider webs, mud and water. Many horns are designed to be mounted with the opening angled slightly down so water drains out; if yours fills with water, check the maker's mounting instructions. Some electric horns have a tone adjustment screw on the back; only adjust it if the horn's instructions describe how, and in small steps.

If the horn gets full voltage and is still silent, weak or buzzy after cleaning, replace it. Pick a marine horn the maker rates for your boat's length. On boats 39.4 feet and longer, choose a horn or whistle advertised as meeting the Navigation Rules for your size. The rules' technical annex calls for the horn to point forward and be mounted as high as practicable so the sound isn't blocked (33 CFR 86.01). For boats under 20 m, the annex lists a typical audibility range of about half a nautical mile. Use wire sized for the horn's current draw and run length, a fuse of the rating the maker specifies close to the power source, and a relay if the instructions call for one.

A handheld canister air horn is a cheap, legal sound device for small boats and a good backup on any boat if the electric horn or battery fails.

• Test it with a short burst at the start of the season and after long storage. Canisters slowly lose pressure.
• Carry a spare canister. A long fog signal every 2 minutes uses a lot of air.
• Cold weather lowers canister pressure and loudness; keep it somewhere warmer in cold-weather boating.
• Follow the label for storage and keep it out of direct sun in a hot console.
• A whistle on each life jacket is a smart extra for people in the water.

Some yachts use compressor-fed air horns with a small tank. Water collects in the tank and lines, and salt corrodes valves. Follow the system maker's instructions for draining and servicing; if the compressor runs but the horn is weak, or the compressor won't run, have a marine electrician check it.

On a quiet day, practice counting a 1-second short blast and a 4–6 second prolonged blast so they come out right under stress. Before a trip in fog-prone waters, review the fog signals with your crew and assign someone to time them.