In U.S. Inland waters, at least five short, rapid blasts mean that a vessel is unsure of another vessel’s intentions or doubts that it is taking enough action to avoid a collision. It is a warning signal of doubt—not, by itself, a distress call.
What the five-blast signal means
In an encounter where two vessels are in sight of one another and approaching, a vessel that does not understand the other’s intentions or actions—or doubts that the other is doing enough to prevent a collision—must immediately signal that doubt with at least five short, rapid blasts on its whistle. That is the meaning set out in U.S. Coast Guard Inland Rule 34(d).
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“At least five” means five is the minimum. The rule defines a short blast as about one second long. It also allows the doubt signal to be supplemented by at least five short, rapid flashes.
What to do if you hear five blasts
If you can see the vessels, treat the signal as a warning that one of them is uncertain about the developing encounter. Keep clear, pay attention to the vessels’ movements, and avoid assuming that the other boat understands your intentions.
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If you hear the blasts but cannot see the encounter, the sound alone does not identify the vessel, explain what prompted the signal, or establish the exact circumstances. Treat it as a warning and heed the situation around you rather than guessing what happened.
How five blasts differ from other boat signals
| Signal or context | Meaning |
|---|---|
| One short blast | Under the International Rule, indicates a starboard course alteration. |
| Two short blasts | Under the International Rule, indicates a port course alteration. |
| Three short blasts | Under the International Rule, indicates operation of astern propulsion. |
| At least five short, rapid blasts | Signals doubt about another vessel’s intentions or collision-avoidance action in an approaching, in-sight encounter under Inland Rule 34(d). |
| Sound signal in restricted visibility | Governed separately by Rule 35; five blasts are not a generic fog signal. |
| Distress signal | Listed separately in Annex IV. The spoken radiotelephony distress signal is “Mayday.” |
The one-, two- and three-blast meanings above are International Rule maneuvering signals. Inland Rule 34(a) has additional context-specific wording for passing signals, so the meaning of a blast pattern depends on the applicable rule and situation. The Coast Guard’s Navigation Rules FAQ provides an official explanatory reference.
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Why five blasts are not automatically a distress call
Rule 34(d) uses five or more short blasts to communicate doubt in a specific vessel-to-vessel encounter. Distress signals are addressed separately in Annex IV of the Coast Guard’s Navigation Rules, which includes the spoken word “Mayday” sent by radiotelephony. Hearing five blasts alone is not enough to conclude that a boat is declaring distress.
Which rules apply?
The meaning above describes U.S. Inland Rule 34(d). The Coast Guard’s cited reference contains both Inland and International Navigation Rules, but the applicable rules can depend on the waterway and jurisdiction. State or local requirements, or rules outside the United States, may add context. Without knowing where the sound occurred, it is not possible to make a waterway-specific determination.
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