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 | 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
Practice this rule#
One real exam question on this rule, cited to source. No sign-up.
Under the Inland Navigation Rules, three short blasts on the whistle by a power-driven vessel indicates which of the following?
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))
21 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.
Practise this rule against the real exam.
Real exam questions. No signup, no card.
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