COLREGs Rule 34: Maneuvering and warning whistle signals

Last reviewed: October 14, 2026

Rule 34 sets the whistle signals that vessels in sight of one another use to show maneuvers, overtaking intentions, doubt, and bend warnings. The International and Inland rules differ on what one and two short blasts mean and on when signals apply. The Inland rule adds a signal for leaving a dock or berth and a rule on agreements reached by radiotelephone.

Applies toInternational: power-driven vessels underway when maneuvering, and any vessel for light signals. Inland: power-driven vessels meeting or crossing within half a mile of each other, and other vessels as each paragraph states.

What it says

  1. International: A power-driven vessel underway that is maneuvering as authorized or required by these Rules and is in sight of another vessel must signal by whistle: one short blast for altering course to starboard, two for port, three for astern propulsion.

    COLREGs Rule 34(a)“one short blast to mean “I am altering my course to starboard”;”

  2. Inland: Power-driven vessels in sight of one another, meeting or crossing within half a mile of each other, must signal by whistle that one short blast means "I intend to leave you on my port side".

    COLREGs Rule 34(a)(i)(1)“One short blast to mean “I intend to leave you on my port side”;”

  3. Inland: A vessel that hears the one or two blast signal must, if in agreement, sound the same signal and take the steps necessary to effect a safe passing.

    COLREGs Rule 34(a)(ii)“Upon hearing the one or two blast signal of the other shall, if in agreement, sound the same whistle signal and take the steps necessary to”

  4. International: In a narrow channel or fairway, a vessel intending to overtake must signal by whistle, in compliance with Rule 9(e)(i): two prolonged blasts followed by one short blast means overtaking on the other vessel's starboard side.

    COLREGs Rule 34(c)(i)“two prolonged blasts followed by one short blast to mean “I intend to overtake you on your starboard side”;”

  5. A vessel in doubt about the other vessel's intentions or actions, or about whether enough action is being taken to avoid collision, must immediately give at least five short and rapid blasts on the whistle.

    COLREGs Rule 34(d)“the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle.”

  6. A vessel nearing a bend or an area of a channel or fairway where other vessels may be obscured must sound one prolonged blast.

    COLREGs Rule 34(e)“shall sound one prolonged blast.”

  7. If whistles are fitted on a vessel more than 100 meters apart, only one whistle must be used for maneuvering and warning signals.

    COLREGs Rule 34(f)“one whistle only shall be used for giving maneuvering and warning signals.”

  8. Inland: A power-driven vessel leaving a dock or berth must sound one prolonged blast.

    COLREGs Rule 34(g)“When a power-driven vessel is leaving a dock or berth, she shall sound one prolonged blast.”

Common questions

What does one short blast mean under the International rule?

Under the International rule, one short blast means "I am altering my course to starboard". Two short blasts mean altering course to port, and three mean operating astern propulsion.

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

What signal must a vessel give when leaving a dock under the Inland rule?

Under the Inland rule, a power-driven vessel leaving a dock or berth must sound one prolonged blast.

“When a power-driven vessel is leaving a dock or berth, she shall sound one prolonged blast.”

What is the danger or doubt signal?

A vessel in doubt must immediately give at least five short and rapid blasts on the whistle. The signal may be supplemented by a light signal of at least five short and rapid flashes.

“Such signal may be supplemented by a light signal of at least five short and rapid flashes.”

Must Inland vessels sound whistle signals if they agree by radiotelephone?

No. A vessel that reaches agreement in a head-on, crossing, or overtaking situation is not obliged to sound the whistle signals, but may do so. If agreement is not reached, whistle signals must be exchanged in a timely manner and prevail.

“is not obliged to sound the whistle signals prescribed by this Rule, but may”

The text of COLREGs Rule 34

U.S. Coast Guard, Navigation Rules and Regulations Handbook

Rule 34: Maneuvering and Warning Signals

International (COLREGs)

(a) When vessels are in sight of one another, a power-driven vessel underway, when maneuvering as authorized or required by these Rules, shall indicate that maneuver by the following signals on her whistle: − one short blast to mean “I am altering my course to starboard”; − two short blasts to mean “I am altering my course to port”; − three short blasts to mean “I am operating astern propulsion”.

(b) Any vessel may supplement the whistle signals prescribed in paragraph (a) of this Rule by light signals, repeated as appropriate, while the maneuver is being carried out:

(i) these light signals shall have the following significance: − one flash to mean “I am altering my course to starboard”; − two flashes to mean “I am altering my course to port”; − three flashes to mean “I am operating astern propulsion”;

(ii) the duration of each flash shall be about one second, the interval between flashes shall be about one second, and the interval between successive signals shall be not less than ten seconds;

(iii) the light used for this signal shall, if fitted, be an all-round white light, visible at a minimum range of 5 miles, and shall comply with the provisions of Annex I to these Regulations.

(c) When in sight of one another in a narrow channel or fairway:

(i) a vessel intending to overtake another shall in compliance with Rule 9(e)(i) indicate her intention by the following signals on her whistle: − two prolonged blasts followed by one short blast to mean “I intend to overtake you on your starboard side”; − two prolonged blasts followed by two short blasts to mean “I intend to overtake you on your port side”.

(ii) the vessel about to be overtaken when acting in accordance with Rule 9(e)(i) shall indicate her agreement by the following signal on her whistle: − one prolonged, one short, one prolonged and one short blast, in that order.

(d) When vessels in sight of one another are approaching each other and from any cause either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle. Such signal may be supplemented by a light signal of at least five short and rapid flashes.

(e) A vessel nearing a bend or an area of a channel or fairway where other vessels may be obscured by an intervening obstruction shall sound one prolonged blast. Such signal shall be answered with a prolonged blast by any approaching vessel that may be within hearing around the bend or behind the intervening obstruction.

(f) If whistles are fitted on a vessel at a distance apart of more than 100 meters, one whistle only shall be used for giving maneuvering and warning signals.

Inland Rules

Maneuvering and Warning Signals

(a) When power-driven vessels are in sight of one another and meeting or crossing at a distance within half a mile of each other, each vessel underway, when maneuvering as authorized or required by these Rules:

(i) Shall indicate that maneuver by the following signals on her whistle:

(1) One short blast to mean “I intend to leave you on my port side”;

(2) Two short blasts to mean “I intend to leave you on my starboard side”; and

(3) Three short blasts to mean “I am operating astern propulsion”.

(ii) Upon hearing the one or two blast signal of the other shall, if in agreement, sound the same whistle signal and take the steps necessary to effect a safe passing. If, however, from any cause, the vessel doubts the safety of the proposed maneuver, she shall sound the signal specified in paragraph (d) of this Rule and each vessel shall take appropriate precautionary action until a safe passing agreement is made.

(b) A vessel may supplement the whistle signals prescribed in paragraph (a) of this Rule by light signals:

(i) These signals shall have the following significance:

(1) One flash to mean “I intend to leave you on my port side”;

(2) Two flashes to mean “I intend to leave you on my starboard side”;

(3) Three flashes to mean “I am operating astern propulsion”;

(ii) The duration of each flash shall be about 1 second; and

(iii) The light used for this signal shall, if fitted, be one all-round white or yellow light, visible at a minimum range of 2 miles, synchronized with the whistle, and shall comply with the provisions of Annex I to these Rules (33 CFR part 84).

(c) When in sight of one another:

(i) A power-driven vessel intending to overtake another power-driven vessel shall indicate her intention by the following signals on her whistle:

(1) One short blast to mean “I intend to overtake you on your starboard side”;

(2) Two short blasts to mean “I intend to overtake you on your port side”; and

(ii) The power-driven vessel about to be overtaken shall, if in agreement, sound a similar sound signal. If in doubt she shall sound the signal prescribed in paragraph (d) of this Rule.

(d) When vessels in sight of one another are approaching each other and, from any cause, either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle. Such signal may be supplemented by a light signal of at least five short and rapid flashes.

(e) A vessel nearing a bend or an area of a channel or fairway where other vessels may be obscured by an intervening obstruction shall sound one prolonged blast. This signal shall be answered with a prolonged blast by any approaching vessel that may be within hearing around the bend or behind the intervening obstruction.

(f) If whistles are fitted on a vessel at a distance apart of more than 100 meters, one whistle only shall be used for giving maneuvering and warning signals.

(g) When a power-driven vessel is leaving a dock or berth, she shall sound one prolonged blast.

(h) A vessel that reaches agreement with another vessel in a head-on, crossing, or overtaking situation, as for example, by using the radiotelephone as prescribed by the Vessel Bridge-to-Bridge Radiotelephone Act (85 Stat. 164; 33 U.S.C. 1201 et seq.), is not obliged to sound the whistle signals prescribed by this Rule, but may do so. If agreement is not reached, then whistle signals shall be exchanged in a timely manner and shall prevail.

Sections it refers to

The plain-English explanation on this page was written with AI from the regulation text above and checked automatically before publishing: every quote appears in the text, every number comes from it, and a second model confirmed each statement adds nothing the text doesn't say. Regulation text current as of August 12, 2024. The regulation text governs. RegKnot is not affiliated with the U.S. Coast Guard.