A boat's horn or whistle communicates through two things: how long each blast lasts and how many you hear. A short blast is about a second; a prolonged blast runs four to six seconds. When vessels are in sight, a power-driven vessel underway uses one short blast for altering course to starboard, two for port, and three for operating astern. Five or more short, rapid blasts mean doubt or danger. In restricted visibility, a power-driven vessel making way sounds one prolonged blast at intervals of no more than two minutes; a sailing vessel adds two short blasts. An anchored vessel required to carry a bell rings it, while smaller vessels may use an efficient alternative signal. On US inland waters, one and two short blasts work differently — verify which rule set applies.

You can't always see another boat's next move, and a headsail or a hull in the way can block a hand signal from ever reaching you. Sound travels around all of that. But a blast only means something if both boats agree on what it means, and on some waters, they don't quite agree the way you'd expect. This guide walks through the blast vocabulary, what the counts mean while boats are maneuvering in sight of each other, the one signal everyone treats the same way everywhere, and what changes once visibility drops.

The Vocabulary: Short and Prolonged Blasts

Every sound signal is built from two building blocks: a short blast, lasting about a second, and a prolonged blast, lasting four to six seconds — several times longer, so the two are never mistaken for each other. Everything else in this guide is just different combinations and counts of those two blasts.

None of it works without a way to make the sound in the first place. Every vessel either carries a whistle (and, past certain lengths, a bell, and past a larger length, a gong too) or has some other efficient way to signal. A smaller boat without the prescribed appliances still needs an efficient way to make a sound signal — the vocabulary below only matters once you can actually produce it.

Two glyphs on a pale cream background, connected by short leader lines to labeled plates. On the left, a small filled navy circle is labeled SHORT. On the right, a much longer rounded navy-outlined bar, filled rust-orange and roughly five times as wide as the circle, is labeled LONG.

What the Blasts Mean When You're Maneuvering

When vessels are in sight of one another, a power-driven vessel underway uses the blast counts to announce what's already happening: one short blast means "I am altering my course to starboard," two short means "I am altering my course to port," three short means "I am operating astern propulsion." Each is a statement about action already under way, not a request — the other vessel isn't expected to answer back.

US inland waters work differently. There, one and two short blasts aren't a statement — they're a proposal. One short means "I intend to leave you on my port side," two short means "I intend to leave you on my starboard side," and the other vessel has to answer: the same signal back if she agrees, or the five-short danger signal if she doesn't. Three short blasts mean the same thing in both systems. If you're on US inland waters, verify which rule set is in force before you rely on it — waiting for a reply that the international rule never asks for is exactly the mistake this difference sets up.

Three rows on a pale cream background, each showing a small boat hull heading right, an orange accent arrow, and a cluster of small navy dots connected to a labeled plate. Top row: the arrow curves down and right from the bow; one dot; labeled STARBOARD. Middle row: the arrow curves up and right from the bow; two dots; labeled PORT. Bottom row: a straight arrow points left from the stern; three dots; labeled ASTERN.

Five or More Blasts: The One Everyone Must Act On

However the one- and two-blast signals are worded where you're sailing, five or more short, rapid blasts always mean the same thing: the sounding vessel doesn't understand what you're doing, or doesn't think you're doing enough to avoid a collision. It's the one signal in this guide that reads identically in every system.

The response doesn't depend on who sounded it or why. Back off the throttle, take stock of both vessels' positions, and get clear on the situation before you commit to anything else. This isn't the moment to work out who was technically right — it's the moment to remove the doubt.

A boat hull heading right on a pale cream background, followed by five small navy dots spaced tightly together and labeled DANGER. Below, a simple semicircular dial with a needle pointing toward its low end is labeled SLOW DOWN.

Signals for Fog and Restricted Visibility

Once visibility drops, the maneuvering signals above stop applying — they're only sounded when vessels are in sight of one another. Restricted visibility has its own set, repeated on a clock rather than tied to a specific maneuver. A power-driven vessel making way through the water sounds one prolonged blast at intervals of no more than two minutes. A sailing vessel — grouped with a vessel not under command, restricted in her ability to maneuver, constrained by her draft, fishing, or towing another vessel — sounds one prolonged blast plus two short, at the same interval.

An anchored vessel required to carry a bell rings it rapidly for about five seconds at intervals of no more than a minute. Vessels under 20 meters are not obliged to give that prescribed bell signal; if they do not, they make another efficient sound signal at intervals of no more than two minutes. An anchored vessel may also sound one short, one prolonged, one short blast to warn an approaching vessel, so “at anchor” never means “no whistle at all.”

Three rows on a pale cream background, separated from each other by a horizontal dashed grey line above the bottom row. Top row: a boat hull heading right, followed by a long rust-orange bar, labeled POWER. Middle row: the same hull with a short vertical mast line above it, followed by a rust-orange bar and two small navy dots, labeled SAILING. Bottom row, below the dashed line: the same hull with a short anchor chain and anchor hanging from it, followed by a navy bell icon with two small orange curved motion lines beside it, labeled ANCHORED.

None of these signals require memorizing a rulebook cover to cover — they require knowing the vocabulary, recognizing which situation you're in, and never letting a five-blast signal go unanswered. SailStarter's lesson on sound signals walks through this same chart as an interactive diagram, tied back to the collision-avoidance and fog-response skills it builds on. If the water you sail includes a rule set with its own local wording, this is exactly the kind of thing worth confirming before you need it mid-maneuver rather than after.