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Rule 34: Maneuvering and Warning Signals (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 34 sets whistle signals for vessels in sight of one another. Under International rules, one short blast means "I am altering my course to starboard," two means port, and three means astern propulsion. Under Inland rules, one and two short blasts propose a passing that the other vessel answers. Five or more short blasts is the danger signal.

The rule text

one short blast to mean “I am altering my course to starboard”;

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 34

One short blast to mean “I intend to leave you on my port side”

Inland Rules, 33 CFR Part 83 (eCFR), Rule 34

the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle.

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 34

What it means on the water

  • Only when vessels are in sight of one another. In fog you use Rule 35 instead.
  • International 34(a) signals announce an action you are taking; no reply is required.
  • Inland 34(a) applies to power-driven vessels meeting or crossing within half a mile, and the other vessel answers with the same signal if she agrees.
  • At least five short and rapid blasts is the danger or doubt signal in both (Rule 34(d)).
  • One prolonged blast near a bend or obstruction, answered by one prolonged blast (Rule 34(e)).

International vs Inland

Big differences. International 34(a) signals mean "I am altering my course to starboard/port"; Inland 34(a) signals mean "I intend to leave you on my port/starboard side" and require agreement. Overtaking (34(c)): International uses two prolonged plus one or two short in a narrow channel, with agreement of prolonged, short, prolonged, short; Inland uses one or two short. Light signals (34(b)): International all-round white, 5 miles; Inland white or yellow, 2 miles. Inland adds one prolonged blast when leaving a dock or berth (34(g)) and lets vessels that agree by radiotelephone skip whistle signals (34(h)).

Common exam traps

  • Using Rule 34 signals in fog. They're for vessels in sight of one another.
  • Giving the Inland meaning to an International one-blast signal, or the reverse.
  • Expecting a reply to an International 34(a) signal.

Try 3 questions on Rule 34

  1. International waters, in sight: what does one short blast mean?

    1. I intend to leave you on my port side
    2. I am altering my course to starboard
    3. I am operating astern propulsion
    4. Danger
    Show answer

    B. I am altering my course to starboard International Rule 34(a).

  2. Inland waters, meeting within half a mile: what does two short blasts mean?

    1. I intend to leave you on my starboard side
    2. I am altering my course to port
    3. I am operating astern propulsion
    4. I intend to overtake you on your port side
    Show answer

    A. I intend to leave you on my starboard side Inland Rule 34(a)(i).

  3. What's the danger signal?

    1. One prolonged
    2. Three short
    3. At least five short and rapid
    4. Prolonged, short, prolonged, short
    Show answer

    C. At least five short and rapid Rule 34(d).

Frequently asked questions

What does three short blasts mean?

"I am operating astern propulsion" in both International and Inland Rule 34(a).

Do I have to answer a one-blast signal?

Under Inland Rule 34(a), yes, with the same signal if you agree, or the danger signal if in doubt. International 34(a) signals don't call for a reply.

What signal do you sound leaving a dock?

Inland Rule 34(g): one prolonged blast. There's no equivalent in International Rule 34.

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