COLREGs · Rule 33 · 33 CFR §83.33
Equipment for Sound Signals
Rule 33 sets three vessel-length thresholds — whistle at 12 m, bell added at 20 m, gong added at 100 m — but Annex III, which the same sentence incorporates by reference, sets three different ones at 20 m, 75 m and 200 m. Sound-signal carriage therefore turns on six length bands, not three (verified 2026-08-15 against 33 CFR §83.33 and §86.01).
After this page you can
- Name the equipment a vessel of any given length must carry, across all six bands — not just the three Rule 33 states outright.
- Say how far a whistle is designed to be heard, and why the answer changes at 20 m, 75 m and 200 m rather than at Rule 33's own breakpoints.
- Tell Rule 33 apart from Rules 34 and 35 — what you carry, versus what you sound and when.
The signals at a glance#
| Vessel length | Means — Inland |
|---|---|
| Under 12 m | No whistle, bell or gong required — but she must have some other means of making an efficient sound signal. |
| 12 m to under 20 m | Whistle. Fundamental 250–700 Hz; designed range ≈0.5 nautical mile. |
| 20 m to under 75 m | Whistle + bell (bell mouth at least 300 mm). Fundamental 250–700 Hz; designed range ≈1 nautical mile. |
| 75 m to under 100 m | Whistle + bell. Fundamental drops to 130–350 Hz; designed range ≈1.5 nautical miles. |
| 100 m to under 200 m | Whistle + bell + gong, the gong's tone not confusable with the bell. Fundamental 130–350 Hz; designed range ≈1.5 nautical miles. |
| 200 m or more | Whistle + bell + gong. Fundamental drops again to 70–200 Hz; designed range ≈2 nautical miles. |
What does Rule 33 actually say?#
Source
Practice this rule#
One real exam question on this rule, cited to source. No sign-up.
Rules 11 through 18 of the Inland Navigation Rules apply only when which condition is met?
Does Inland Rule 33 change anything?#
Inland Rule 33 is 33 CFR §83.33, reproduced above, and carries no exam-relevant divergence from COLREGs Rule 33. One answer serves both the International and the Inland version of a Rules of the Road question on this rule — which is worth knowing precisely because several neighbouring rules do not work that way.
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?#
12 m or more
Whistle
A vessel at least 12 meters long must carry a whistle that can make the prescribed sound signals.
20 m or more
Whistle + bell
At 20 meters, the bell is added to the whistle requirement. This is the most common exam breakpoint.
100 m or more
Whistle + bell + gong
At 100 meters, the vessel also needs a gong whose tone cannot be confused with the bell.
Annex III's own breakpoints
20 m · 75 m · 200 m
Rule 33 says the whistle, bell and gong "shall comply with the specifications in Annex III" — and Annex III (33 CFR §86.01) then divides vessels at 20, 75 and 200 meters, not at 12, 20 and 100. Those bands set the whistle's fundamental frequency (70–200 Hz at 200 m or more, 130–350 Hz from 75 m, 250–700 Hz below 75 m) and its designed audibility range. A 150-metre vessel is a single-bell vessel under Rule 33 and a 1.5-mile, 130–350 Hz whistle under Annex III; neither scale predicts the other.
Range is indicative
Not a legal minimum
§86.01(c) states the audibility figures in Table 86.01(c) are "shown for information purposes only" — measured on the whistle's forward axis, in still air, with 90% probability against an assumed 68/63 dB background. What the rule actually mandates is the sound pressure level in dB at 1 metre; the miles are the consequence. In strong wind or high ambient noise the real range is shorter, which is the reasoning behind Rule 35's short signal intervals.
Rule 33 — common exam questions#
What is sound Rule 33?
Rule 33 is the equipment-for-sound-signals rule. It tells you which vessels must carry a whistle, bell, and gong so they can make the signals required elsewhere in the Navigation Rules.
How far away can a ship's whistle be heard under Rule 33?
Rule 33 does not say — it defers to Annex III, which does. Table 86.01(c) of 33 CFR §86.01 gives the designed range as about 2 nautical miles for a vessel of 200 meters or more, 1.5 miles from 75 up to 200 meters, 1 mile from 20 up to 75 meters, and 0.5 mile below 20 meters. The regulation labels those distances as information only: the binding requirement is a sound pressure level in decibels measured 1 metre from the whistle, and real-world range falls off sharply in wind or high background noise.
Which sound signal means "I am altering my course to port"?
Two short blasts — but that is Rule 34, not Rule 33, and the wording is International only. Under the International Rules a power-driven vessel in sight of another sounds one short blast for "I am altering my course to starboard" and two short for "I am altering my course to port". The Inland Rules (33 CFR §83.34(a)) use intent instead of helm: one short blast means "I intend to leave you on my port side", two short "I intend to leave you on my starboard side", three short "I am operating astern propulsion". Rule 33 only decides whether you carry a whistle capable of making any of them.
What sound signal equipment do the fog rules require?
The bell and gong Rule 33 makes you carry are, in practice, the fog-and-anchor equipment. Under 33 CFR §83.35(g) a vessel at anchor rings the bell rapidly for about 5 seconds at intervals of not more than a minute, and a vessel of 100 meters or more sounds the bell forward and then the gong aft — which is why Rule 33 puts the bell at 20 m and the gong at 100 m. The bands still do not line up: §83.35(i) excuses a vessel of 12 up to 20 meters from the bell signals entirely, provided she makes some other efficient sound signal at intervals of not more than 2 minutes, even though Rule 33 has required her to carry a whistle since 12 meters.
Does a boat under 12 meters need a whistle under Rule 33?
No. A vessel under 12 meters is not obliged to carry the whistle, bell, or gong listed for larger vessels, but it must still have some efficient means of making a sound signal.
Is Rule 33 the same as the maneuvering and warning signals rule?
No. Rule 33 covers the equipment carried aboard. Rule 34 covers maneuvering and warning signals, and Rule 35 covers sound signals in restricted visibility.
Authoritative source#
- 33 CFR §83.33 — this section with its neighbours. 33 CFR Part 83 is public domain; checked 2026-08-15.
- COLREGs Rule 33 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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