Whistle Signals and Sound Signals: Rules of the Road Rules 32 to 37
Sound signals are where a lot of good students lose easy points. The blasts are simple, but the meaning shifts depending on whether you're in sight of the other vessel, in fog, or in Inland waters. This guide takes you through Rules 32 to 37 one at a time, then puts the Inland and International versions of Rule 34 side by side.
Rule 32: Short and prolonged blasts
Everything starts with two definitions:
- Short blast: about 1 second.
- Prolonged blast: 4 to 6 seconds.
A "whistle" is any sound signaling appliance that meets the Annex III specifications. In the tables below, a short blast is written as a dot and a prolonged blast as a dash, so "dash dot dot" means one prolonged and two short.
Rule 33: Required sound equipment
- 12 meters or more: a whistle.
- 20 meters or more: a whistle and a bell.
- 100 meters or more: a whistle, a bell, and a gong that can't be confused with the bell.
- Under 12 meters: not required to carry these, but must have some other means of making an efficient sound signal.
Rule 34: Maneuvering and warning signals (in sight of one another)
These signals only apply when vessels are in sight of one another. That phrase matters. In fog, you use Rule 35 instead.
International Rule 34(a): signals of action
A power-driven vessel underway, maneuvering as authorized or required by the rules, sounds:
| Signal | Meaning |
|---|---|
| 1 short | "I am altering my course to starboard" |
| 2 short | "I am altering my course to port" |
| 3 short | "I am operating astern propulsion" |
No reply is required. Under Rule 34(b), these may be supplemented with flashes from an all-round white light visible at least 5 miles, each flash about 1 second, flashes about 1 second apart, and at least 10 seconds between successive signals.
Inland Rule 34(a): signals of intent and agreement
When power-driven vessels are in sight of one another and meeting or crossing within half a mile:
| Signal | Meaning |
|---|---|
| 1 short | "I intend to leave you on my port side" |
| 2 short | "I intend to leave you on my starboard side" |
| 3 short | "I am operating astern propulsion" |
The other vessel, if she agrees, sounds the same signal back and takes the steps needed for a safe passing. If she doubts the maneuver is safe, she sounds the danger signal. Inland light signals may use an all-round white or yellow light visible at least 2 miles, synchronized with the whistle (Inland Rule 34(b)).
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Overtaking signals (Rule 34(c))
| International (narrow channel or fairway, Rule 9(e)) | Inland (in sight, power-driven vessels) | |
|---|---|---|
| Overtake on your starboard side | 2 prolonged, 1 short | 1 short |
| Overtake on your port side | 2 prolonged, 2 short | 2 short |
| Agreement | Prolonged, short, prolonged, short | Same signal as the overtaking vessel |
| In doubt | 5 or more short and rapid | 5 or more short and rapid |
Signals that are the same in both
- Danger or doubt signal (Rule 34(d)): at least five short and rapid blasts when you don't understand the other vessel's intentions or doubt she's taking enough action. It may be backed up with at least five short and rapid flashes.
- Bend signal (Rule 34(e)): one prolonged blast when nearing a bend or an area where other vessels may be hidden. Any vessel around the bend answers with one prolonged blast.
- Two whistles (Rule 34(f)): if whistles are more than 100 meters apart, only one is used for maneuvering and warning signals.
Inland-only additions
- Leaving a dock or berth (Inland Rule 34(g)): one prolonged blast.
- Radio agreement (Inland Rule 34(h)): vessels that agree on a passing by radiotelephone under the Vessel Bridge-to-Bridge Radiotelephone Act aren't obliged to sound whistle signals, but may. If agreement isn't reached, whistle signals are exchanged in a timely manner and prevail.
Rule 35: Sound signals in restricted visibility
These apply in or near an area of restricted visibility, day or night, whether or not you can see the other vessel's lights.
| Vessel | Signal | Interval |
|---|---|---|
| Power-driven, making way | 1 prolonged | Not more than 2 minutes |
| Power-driven, underway but stopped | 2 prolonged (about 2 seconds apart) | Not more than 2 minutes |
| NUC, RAM, sailing, fishing, towing or pushing (International adds constrained by her draft) | 1 prolonged, 2 short | Not more than 2 minutes |
| Last vessel of a tow, if manned | 1 prolonged, 3 short (right after the tug's signal when practicable) | Not more than 2 minutes |
| At anchor | Bell rung rapidly about 5 seconds (plus gong aft if 100 m or more); may add short, prolonged, short | Not more than 1 minute |
| Aground | Three distinct bell strokes, rapid ringing, three distinct strokes | Not more than 1 minute |
| Pilot vessel on duty | May add 4 short as an identity signal | With her other signals |
A fishing vessel at anchor and a RAM vessel working at anchor sound "dash dot dot" too, instead of the anchor bell. International Rule 35(d) says this in its own paragraph, while Inland Rule 35(c) folds it in with the words "whether underway or at anchor" and leaves 35(d) reserved. That's also why a vessel constrained by her draft appears only in International Rule 35(c): Inland has no Rule 28 status for her to signal. A pushing vessel rigidly connected to the vessel ahead (a composite unit) uses the power-driven signals (Rule 35(f)). The anchor bell is Rule 35(g), aground is 35(h), and the pilot identity signal is 35(k). Under Rule 35(i) and (j), vessels 12 to under 20 meters don't have to give the bell signals, and vessels under 12 meters don't have to give any of these, but each must make some other efficient sound signal at intervals of not more than 2 minutes. Inland Rule 35(l) adds that vessels under 20 meters, and barges, canal boats, scows, or other nondescript craft, don't need the anchor bell in a Coast Guard special anchorage area.
Rule 36: Signals to attract attention
Any vessel may use light or sound signals that can't be mistaken for any authorized signal, or point a searchlight toward the danger without embarrassing other vessels. The International version adds that any light used this way shouldn't be mistakable for an aid to navigation, and that high-intensity intermittent or revolving lights such as strobes should be avoided.
Rule 37: Distress signals
Rule 37 points to Annex IV. Distress signals include a continuous sounding of any fog signal apparatus, a gun fired at about one-minute intervals, SOS by any method, the spoken word "Mayday," red flares, orange smoke, and slowly raising and lowering outstretched arms. The Inland Annex IV (33 CFR Part 87) also lists a high-intensity white light flashing 50 to 70 times per minute.
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Frequently asked questions
What does one short blast mean on the Rules of the Road?
Under International rules, "I am altering my course to starboard." Under Inland rules, between power-driven vessels meeting or crossing within half a mile, "I intend to leave you on my port side" (Rule 34(a)).
What is the danger signal?
At least five short and rapid blasts (Rule 34(d)). It means you don't understand the other vessel's intentions or doubt she's taking enough action to avoid collision.
How long is a prolonged blast?
4 to 6 seconds. A short blast is about 1 second (Rule 32).
Do you use maneuvering signals in fog?
No. Rule 34 maneuvering signals are for vessels in sight of one another. In restricted visibility, use the Rule 35 signals.
Mini quiz: test yourself
- In fog, you hear one prolonged followed by two short blasts. Name two vessel types that might be sounding it.
- Under Inland rules, a power-driven vessel leaves her berth. What does she sound?
- Under International rules in a narrow channel, what does two prolonged and two short blasts mean?
- How often does a power-driven vessel making way in fog sound her signal?
- What does a vessel aground sound in restricted visibility?
Answers: 1) Any of: NUC, RAM, sailing, fishing, or towing or pushing, plus a vessel constrained by her draft in International waters (Rule 35(c)). 2) One prolonged blast (Inland Rule 34(g)). 3) "I intend to overtake you on your port side" (International Rule 34(c)). 4) One prolonged blast at intervals of not more than 2 minutes (Rule 35(a)). 5) Three separate strokes on the bell, the rapid ringing, then three separate strokes, with a gong if required (Rule 35(h)).