COLREGs · Rule 34 · 33 CFR §83.34

Maneuvering and Warning Signals

A blast under COLREGs reports what you are already doing. Under Inland Rule 34 the same blast states what you intend to do — and the other vessel has to answer it.

After this page you can

Name the five signal patterns and what each one asserts
Tell an Inland-only signal from one that works in both waters
State the in-sight, half-mile condition that switches the Inland signals on

The signals at a glance#

Signal and meaning under the Inland Rules
Signal	Means — Inland
1 short blast	I intend to leave you on my port sidediffers
2 short blasts	I intend to leave you on my starboard sidediffers
3 short blasts	I am operating astern propulsion
1 prolonged blast	Nearing a bend or an obstruction that hides other vessels — answered by any vessel within hearing
5 or more short and rapid blasts	Danger, or doubt that enough is being done to avoid collision

What does Rule 34 actually say?#

Source

Verbatim 33 CFR §83.34, public domain. The international COLREGs text is published by the IMO and cannot be reproduced here; this U.S. wording tracks it closely.

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:
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Shall indicate that maneuver by the following signals on her whistle:
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One short blast to mean “I intend to leave you on my port side”;
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Two short blasts to mean “I intend to leave you on my starboard side”; and
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Three short blasts to mean “I am operating astern propulsion”.
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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.
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A vessel may supplement the whistle signals prescribed in paragraph (a) of this Rule by light signals:
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These signals shall have the following significance:
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One flash to mean “I intend to leave you on my port side”;
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Two flashes to mean “I intend to leave you on my starboard side”;
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Three flashes to mean “I am operating astern propulsion”;
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The duration of each flash shall be about 1 second; and
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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).
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When in sight of one another:
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A power-driven vessel intending to overtake another power-driven vessel shall indicate her intention by the following signals on her whistle:
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One short blast to mean “I intend to overtake you on your starboard side”;
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Two short blasts to mean “I intend to overtake you on your port side”; and
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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.
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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.
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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.
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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.
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When a power-driven vessel is leaving a dock or berth, she shall sound one prolonged blast.
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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. [USCG-2012-0102, 79 FR 37912, July 2, 2014, as amended by USCG-2015-0433, 80 FR 44280, July 27, 2015; USCG-2017-1002, 83 FR 3274, Jan. 24, 2018]
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Practice this rule#

One real exam question on this rule, cited to source. No sign-up.

Rule 34 — Maneuvering and Warning Signals

Under the Inland Rules, a power-driven vessel departing a berth shall sound ______.

How does Inland Rule 34 differ from COLREGs?#

When the signals apply

International

Whenever vessels are in sight of one another and one manoeuvres as the Rules authorise or require

Inland

Only between power-driven vessels in sight of one another, meeting or crossing within half a mile (§83.34(a))

The answer

International

No agreement is needed — the signal announces what the vessel is doing

Inland

The vessel that hears it answers with the same signal if it agrees, or five or more short and rapid blasts if it doubts; both take precautionary action until a safe passing agreement is made

Overtaking in a narrow channel

International

Two prolonged plus one short to pass on her starboard side, two prolonged plus two short for her port side; she answers prolonged–short–prolonged–short

Inland

One short to overtake on her starboard side, two short for her port side; she answers with the same signal to agree (§83.34(c))

Leaving a dock or berth

International

No departure signal

Inland

One prolonged blast (§83.34(g))

Agreement reached by radiotelephone

International

Not addressed — the whistle signals stand

Inland

A vessel that reaches agreement under the Vessel Bridge-to-Bridge Radiotelephone Act need not sound them; where no agreement is reached the whistle signals must be exchanged in a timely manner and they prevail (§83.34(h))

22 of 38 rules differ somewhere; the five the exam leans on are Rule 9, Rule 10, Rule 24, Rule 28 and Rule 34, side by side on Inland vs International Rules.

What does the exam actually test here?#

Blast length
1 second vs 4–6 seconds
Every Rule 34 signal is built from two lengths defined in 33 CFR §83.32: a short blast is about 1 second, and a prolonged blast is from 4 to 6 seconds. A prolonged blast is never just a drawn-out short blast, and distractors that call it 8 or 10 seconds are wrong.
Light signals
Supplement, never replace
33 CFR §83.34(b) allows the manoeuvring signals to be repeated by light — one, two, or three flashes of about a second each, from an all-round white or yellow light visible at least 2 miles, synchronised with the whistle and compliant with Annex I (33 CFR part 84). §83.34(d) allows at least five short and rapid flashes with the danger signal. In every case the light accompanies the whistle; it is never an alternative to it.
Which rule set is the question in?
The signal tells you
A question that rewards one prolonged blast on getting under way, or that requires the two vessels to agree before passing, is an Inland question — COLREGs has neither. A question where the blast reports a course change already begun is an International one.

Rule 34 — common exam questions#

Is one short blast an action or an intention?

Both, depending on the water. Under COLREGs it reports a course change already under way; under Inland Rule 34 it states an intention the other vessel must answer before either of you acts on it.

What does five short blasts mean?

Danger or doubt — you do not understand another vessel's intentions, or you doubt that enough is being done to avoid collision. It is the same signal in both rule sets, and 33 CFR §83.34(d) lets you supplement it with at least five short and rapid flashes.

How long is a short blast compared with a prolonged blast?

About 1 second against 4 to 6 seconds, defined in 33 CFR §83.32. Those two lengths are the whole alphabet of Rule 34.

Do you still have to sound whistle signals if you agreed to pass on the VHF radio?

Not under the Inland Rules: 33 CFR §83.34(h) releases a vessel that has reached agreement under the Vessel Bridge-to-Bridge Radiotelephone Act, though she may sound them anyway. If no agreement is reached, the whistle signals must be exchanged in a timely manner and they prevail.

Authoritative source#

33 CFR §83.34 — this section with its neighbours. 33 CFR Part 83 is public domain; checked 2026-07-23.
COLREGs Rule 34 is published by the IMO and is authoritative internationally.

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