On her whistle, two short blasts from a power-driven vessel means:

I am altering my course to port. Rule 34(a): one short blast = altering to starboard, two short blasts = altering to port, three = operating astern propulsion.

COLREGs trainerPart D: Sound and Light Signals

Rule 34

Manoeuvring and Warning Signals

One short = stbd, two = port, three = astern; five+ = doubt.

Paragraph (a): Whistle signals

When vessels are in sight of one another, a power-driven vessel underway, when manoeuvring as authorized or required by these Rules, shall indicate that manoeuvre on her whistle: • 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".

Sound signals

Played at true length: a short blast lasts about 1 second and a prolonged blast 4 to 6 seconds (Rule 32). Here a prolonged blast plays for 5 seconds.

Whistle
Whistle
Whistle

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Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules

Read the whole of Rule 34 on one page

Paragraph (a): Whistle signals

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."

Paragraph (b): Light signals

These whistle signals may be supplemented by light signals, repeated as appropriate while the manoeuvre is carried out: (i) one flash = altering to starboard; two flashes = altering to port; three flashes = operating astern propulsion; (ii) each flash about one second, the interval between flashes about one second, and between successive signals not less than ten seconds; (iii) the light, if fitted, shall be an all-round white light visible at a minimum range of 5 miles, complying with Annex I.

Paragraph (c): Overtaking in a narrow channel

When in sight of one another in a narrow channel or fairway: (i) a vessel intending to overtake shall, under Rule 9(e)(i), signal her intention: two prolonged + one short blast ("I intend to overtake you on your starboard side"); two prolonged + two short blasts ("…on your port side"); (ii) the vessel about to be overtaken shall indicate agreement: one prolonged, one short, one prolonged and one short blast, in that order.

Two prolonged + one short: "I intend to overtake you on your starboard side."
Two prolonged + two short: "I intend to overtake you on your port side."
Prolonged–short–prolonged–short, the overtaken vessel’s agreement.

Paragraph (d): Doubt signal

When vessels in sight of one another are approaching and either fails to understand the other’s intentions or doubts whether sufficient action is being taken, the vessel in doubt shall immediately give at least five short and rapid blasts on the whistle. This may be supplemented by at least five short and rapid flashes.

At least five short and rapid blasts: doubt. The vessel sounding it does not understand the other’s intentions, or doubts that she is taking enough action to avoid collision.

Paragraph (e): Bends

A vessel nearing a bend or an area where other vessels may be obscured by an intervening obstruction shall sound one prolonged blast, answered with a prolonged blast by any approaching vessel within hearing around the bend or behind the obstruction.

One prolonged blast, sounded by a vessel nearing a bend or an obstruction that hides other vessels.
One prolonged blast in answer, given by any approaching vessel within hearing around the bend or behind the obstruction.

Paragraph (f): Whistles far apart

If whistles are fitted more than 100 m apart, only one whistle shall be used for giving manoeuvring and warning signals.

Test yourself on Rule 34

On her whistle, two short blasts from a power-driven vessel means:: I am altering my course to port. Rule 34(a): one short blast = altering to starboard, two short blasts = altering to port, three = operating astern propulsion.

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A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.