Sound signals, governed by COLREGs Rule 34 and Rule 35, use a vessel’s whistle to communicate maneuvering actions and warnings — short blast (about 1 second) and long/prolonged blast (4-6 seconds) combinations with specific, standardized meanings.
Maneuvering Signals (Rule 34) — Vessels in Sight of One Another
| Signal | Meaning |
|---|---|
| One short blast | ”I am altering my course to starboard” |
| Two short blasts | ”I am altering my course to port” |
| Three short blasts | ”I am operating astern propulsion” |
| Five (or more) short blasts | Danger signal — doubt about the other vessel’s intentions or actions |
These signals accompany an actual maneuver — they’re not requests for permission, but a real-time notification of what a vessel is actually doing.
Fog Signals (Rule 35) — Restricted Visibility
A separate set of signals applies specifically in fog or other conditions of restricted visibility, sounded at set intervals (not less than every 2 minutes) regardless of whether another vessel is actually in sight:
- Power-driven vessel underway, making way — one prolonged blast
- Power-driven vessel underway, stopped, making no way — two prolonged blasts
- Vessel not under command, restricted in ability to maneuver, or constrained by draft — one prolonged plus two short blasts
- Vessel at anchor — rapid ringing of a bell for about 5 seconds
Why the Distinction Matters
Maneuvering signals (Rule 34) communicate what you’re doing to a vessel you can see; fog signals (Rule 35) announce your presence and status to vessels you can’t see — genuinely different purposes, and mixing them up in an exam or in practice would be a real, testable error.
The Danger Signal
Five or more short, rapid blasts is specifically a warning that a vessel doesn’t understand or agrees with another vessel’s intentions or actions — a genuine, urgent “something is wrong here” signal, not a routine maneuvering notification.
Exam Relevance
COLREGs Rule 34 and Rule 35 sound signal patterns, and correctly distinguishing maneuvering signals from fog signals, are core, heavily-tested Navigation topics in deck officer competency exams at every level.