1The rule in plain words
In one sentence…
In sight or near a bend, COLREG tells you which whistle signal to sound for manoeuvring, overtaking, expressing doubt and warning.
What do one, two or three short blasts mean?
In sight of another vessel, if power-driven and underway: one short blast means altering course to starboard, two to port, and three means astern propulsion. You may supplement with one, two or three flashes of the same meaning.
How do you announce an overtaking manoeuvre in a narrow channel or fairway?
Two prolonged blasts and one short means you intend to overtake on starboard; two prolonged and two short means on port. The vessel about to be overtaken agrees with the prolonged-short-prolonged-short sequence, in that order.
What if you do not understand the other vessel's action or doubt it is taking enough action?
When vessels in sight approach, immediately show your doubt with at least five short and rapid blasts. You may add at least five short and rapid flashes.
What do you sound near a bend where an obstruction hides other vessels?
Sound one prolonged blast. Any approaching vessel that may be within hearing around the bend or behind the obstruction must answer with a prolonged blast.
2Listen to it
Play each signal and listen: this is how it sounds at sea.
3Key points
- 1.
Each flash ≈ 1 second · interval between flashes ≈ 1 second.
- 2.
Between successive light signals → never less than 10 seconds.
- 3.
If fitted: all-round white light · minimum range 5 miles · Annex I.
- 4.
Whistles more than 100 metres apart → use one for manoeuvring and warning signals.
4Frequent mistakes
Wrong: "Any colour of light works if the signal is visible."
Right: If fitted, it is all-round white, visible at a minimum range of 5 miles, and Annex I compliant.
Wrong: "The gap between successive light signals may be five seconds."
Right: The interval between successive signals must be at least 10 seconds.
Wrong: "Use every whistle when they are more than 100 metres apart."
Right: Use one whistle only for manoeuvring and warning signals.
5Exam keys & interpretation
Exam keys
First identify the setting: in sight, narrow channel, doubt or bend; each paragraph changes the signal.
Count two separate variables: duration (short/prolonged) and number of blasts; do not mix them.
"At least" sets a minimum, not an exact figure: look for that word in the option.
How to read it
Read the Rule as a context code: the place and situation determine which signal you should sound.
The signals help the other vessel understand your action or doubt before the situation worsens.
6Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
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.7Test your knowledge
What do the short blasts mean when vessels are in sight of one another?
Keep studying…
Last reviewed: