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34

Manoeuvring and Warning Signals

For power-driven vessels underway and in sight manoeuvring as authorised: one short blast means altering to starboard, two to port, and three operating astern propulsion. Three blasts do not establish sternway. Timing example: short blasts are about 1 s and prolonged blasts 4–6 s; the illustrated 1 s gaps are not prescribed intervals.

Rule 34 prescribes manoeuvring and warning signals including signals between vessels in sight of one another and a prolonged-blast warning at a blind bend or obstruction.

(a) When vessels are in sight, a power-driven vessel underway indicates a manoeuvre authorized or required by these Rules with:

  • 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
b
These whistle signals may be supplemented by equivalent light signals.
c
In narrow channels and fairways, special signals are used for overtaking: two prolonged plus one short blast to overtake on the other vessel's starboard side, and two prolonged plus two short blasts to overtake on her port side. The vessel being overtaken answers with one prolonged, one short, one prolonged and one short if she agrees.
d
If a vessel does not understand the other vessel's intentions, or doubts whether sufficient action is being taken, she shall sound at least five short and rapid blasts.
e
A vessel nearing a bend or area of a channel where another vessel may be hidden by an obstruction shall sound one prolonged blast; any approaching vessel that hears it shall answer with a prolonged blast.
For vessels in sight of one another. Narrow-channel overtaking proposal: two prolonged blasts then one short to pass on the other vessel’s starboard side, or two shorts for its port side. Agreement: prolonged-short-prolonged-short. Timing example: short blasts are about 1 s and prolonged blasts 4–6 s; the illustrated 1 s gaps are not prescribed intervals.

IMO COLREG 1972Official text

Full wording of the rule, reproduced from the official source below.

(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.
Source:

Signal Workflow

Decide the context first: manoeuvring in sight, restricted visibility, or distress.

The same whistle can mean very different things in a different context.

Check both the pattern and the interval.

For fog signals, the time spacing is part of the rule, not just the blast sequence.

When vessels in sight are approaching and you do not understand the other vessel’s intentions or actions, or doubt its collision-avoiding action, immediately sound at least five short, rapid blasts (Rule 34(d)).

Exam Focus

Three short blasts are astern propulsion, not a fog signal.

Rule 35(a)–(f) signals have a maximum interval of 2 minutes; the anchor and aground signals in (g)–(h) have a maximum interval of 1 minute.

Paragraphs (i)–(j) provide small-vessel alternatives.

Key Takeaways

1

1 short = starboard, 2 short = port, 3 short = astern propulsion

2

5 or more rapid blasts are the doubt or danger signal

3

Channel overtaking uses the prolonged-plus-short combinations, not the basic one- and two-blast signals

4

A single prolonged blast warns of a blind bend or obstruction in a channel

Common Mistakes

Using the basic manoeuvring blasts when the channel overtaking signals are required

Confusing three short blasts with a fog signal

Forgetting that Rule 34 depends on vessels being in sight, except for the bend signal

Test Your Knowledge

Practise this topic and review the explanation for each answer.

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Frequently asked questions about COLREG Rule 34

Which sound signal means “I am altering my course to port”?

Two short blasts. One short blast means I am altering my course to starboard and three short blasts mean I am operating astern propulsion (Rule 34(a)).

What is the warning signal in COLREG Rule 34?

At least five short and rapid blasts. It is sounded when a vessel does not understand the other vessel's intentions, or doubts whether sufficient action is being taken to avoid collision (Rule 34(d)).

Which signal warns of a blind bend in a channel?

One prolonged blast, sounded by a vessel nearing a bend or an area of a channel where another vessel may be hidden by an obstruction. Any approaching vessel that hears it shall answer with a prolonged blast (Rule 34(e)).

What are the overtaking sound signals in a narrow channel?

Two prolonged plus one short blast to overtake on the other vessel's starboard side, and two prolonged plus two short blasts to overtake on her port side. The vessel being overtaken agrees with one prolonged, one short, one prolonged and one short (Rule 34(c)).