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35

Sound Signals in Restricted Visibility

Restricted visibility: a power-driven vessel making way gives one prolonged blast; underway but stopped and making no way, two prolonged blasts about two seconds apart. Repeat at intervals no longer than two minutes.

1The rule in plain words

In one sentence…

In restricted visibility, the signal depends on whether the vessel is making way, stopped or anchored, or aground.

What does a power-driven vessel sound?

Making way through the water: one prolonged blast at intervals of not more than 2 minutes. Underway but stopped and making no way: two prolonged blasts about 2 seconds apart at the same interval.

Who uses one prolonged and two short blasts?

The listed vessels—not under command, maneuver-restricted, draft-constrained, sailing, fishing, towing or pushing—use one prolonged plus two short; the last manned towed vessel uses one prolonged plus three short, when practicable right after the towing vessel; both at intervals of not more than 2 minutes.

What do anchored and aground vessels sound?

At anchor: rapidly ring the bell about 5 seconds every not more than 1 minute; at 100 metres or more, add the gong about 5 seconds aft. May add short-prolonged-short. Aground: add three separate strokes before and after and may add an appropriate whistle.

What about vessels under 20 metres and pilot vessels?

12 metres or more but less than 20: no bell signals; under 12: no above-mentioned signals. If omitted, make some other efficient sound signal at intervals of not more than 2 minutes. A pilot vessel on pilotage duty may add four short blasts.

2Listen to it

Play each signal and listen: this is how it sounds at sea.

3Key points

  1. 1.

    Power-driven: making way = 1 prolonged · stopped = 2, about 2 seconds apart.

  2. 2.

    Signal (c): 1 prolonged + 2 short · not under command, restricted, draft-constrained, sailing, fishing, towing or pushing.

  3. 3.

    Fishing at anchor or restricted in ability to maneuver while working at anchor → signal (c), not (g).

  4. 4.

    Anchor/aground: bell; 100 metres or more adds gong; aground adds three strokes before and after.

4Frequent mistakes

  • Wrong: "In restricted visibility every vessel gives one prolonged blast."

    Right: The signal depends on the situation: stopped power-driven vessels give two; several listed vessels give three.

  • Wrong: "An anchored vessel uses only the bell."

    Right: It may add short-prolonged-short; at 100 metres or more, it also sounds the gong aft.

  • Wrong: "A vessel under 12 metres makes no sound signal."

    Right: If it omits the listed signals, it must make another efficient sound signal at most every 2 minutes.

5Exam keys & interpretation

Exam keys

  • "Making way" versus "stopped" separates one prolonged blast from two, with about 2 seconds between the latter.

  • "Last vessel of the tow" and "if manned" limit one prolonged followed by three short blasts.

  • "100 metres or more" includes exactly 100: bell forward, then gong immediately aft.

How to read it

  • A rigidly connected composite pushing unit is treated as a power-driven vessel: use the making-way or stopped signal.

  • Length exemptions do not remove sound: if the listed signals are omitted, make another efficient sound signal.

6Official text

IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)

Full wording of the rule, reproduced from the official source below.

In or near an area of restricted visibility, whether by day or night, the signals prescribed in this Rule shall be used as follows: (a) A power-driven vessel making way through the water shall sound at intervals of not more than 2 minutes one prolonged blast. (b) A power-driven vessel underway but stopped and making no way through the water shall sound at intervals of not more than 2 minutes two prolonged blasts in succession with an interval of about 2 seconds between them. (c) A vessel not under command, a vessel restricted in her ability to maneuver, a vessel constrained by her draft, a sailing vessel, a vessel engaged in fishing and a vessel engaged in towing or pushing another vessel shall, instead of the signals prescribed in paragraphs (a) or (b) of this Rule, sound at intervals of not more than 2 minutes three blasts in succession, namely one prolonged followed by two short blasts. (d) A vessel engaged in fishing, when at anchor, and a vessel restricted in her ability to maneuver when carrying out her work at anchor, shall instead of the signals prescribed in paragraph (g) of this Rule sound the signal prescribed in paragraph (c) of this Rule. (e) A vessel towed or if more than one vessel is towed the last vessel of the tow, if manned, shall at intervals of not more than 2 minutes sound four blasts in succession, namely one prolonged followed by three short blasts. When practicable, this signal shall be made immediately after the signal made by the towing vessel. (f) When a pushing vessel and a vessel being pushed ahead are rigidly connected in a composite unit they shall be regarded as a power-driven vessel and shall give the signals prescribed in paragraphs (a) or (b) of this Rule. (g) A vessel at anchor shall at intervals of not more than one minute ring the bell rapidly for about 5 seconds. In a vessel of 100 meters or more in length the bell shall be sounded in the forepart of the vessel and immediately after the ringing of the bell the gong shall be sounded rapidly for about 5 seconds in the after part of the vessel. A vessel at anchor may in addition sound three blasts in succession, namely one short, one prolonged and one short blast, to give warning of her position and of the possibility of collision to an approaching vessel. (h) A vessel aground shall give the bell signal and if required the gong signal prescribed in paragraph (g) of this Rule and shall, in addition, give three separate and distinct strokes on the bell immediately before and after the rapid ringing of the bell. A vessel aground may in addition sound an appropriate whistle signal. (i) A vessel of 12 meters or more but less than 20 meters in length shall not be obliged to give the bell signals prescribed in paragraphs (g) and (h) of this Rule. However, if she does not, she shall make some other efficient sound signal at intervals of not more than 2 minutes. (j) A vessel of less than 12 meters in length shall not be obliged to give the above-mentioned signals but, if she does not, shall make some other efficient sound signal at intervals of not more than 2 minutes. (k) A pilot vessel when engaged on pilotage duty may in addition to the signals prescribed in paragraphs (a), (b) or (g) of this Rule sound an identity signal consisting of four short blasts.
Source:

7Test your knowledge

Question 1/4

A power-driven vessel making way through the water sounds what?

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Frequently asked questions about COLREG Rule 35

In restricted visibility, what does a power-driven vessel making way sound?

One prolonged blast at intervals of not more than 2 minutes (Rule 35(a)). If she is underway but stopped and not making way she sounds two prolonged blasts in succession at the same interval.

What is the sound signal for a vessel aground?

The anchor bell signal, plus three distinct separate bell strokes immediately before and after the rapid ringing, and the gong if required (Rule 35(h)). She may add an appropriate whistle signal; the short-prolonged-short pattern is specifically described for an anchored vessel, not prescribed for an aground vessel. Rules 35(i)–(j) provide small-vessel alternatives.

Which vessels sound one prolonged followed by two short blasts in fog?

NUC, RAM, constrained-by-draught, sailing, fishing and towing/pushing vessels use this signal instead of Rule 35(a) or (b), at intervals of not more than 2 minutes (Rule 35(c)). Fishing at anchor and RAM vessels carrying out their work at anchor also use it instead of the ordinary anchor signal (Rule 35(d)). A rigid composite pushing unit uses the power-driven-vessel signal (Rule 35(f)).

What sound signal does a vessel at anchor give in restricted visibility?

Normally, ring the bell rapidly for about 5 seconds at intervals of no more than one minute (Rule 35(g)). At 100 m or more, sound the bell forward, immediately followed by rapid gong sounding for about 5 seconds aft; an optional short-prolonged-short whistle warns of position and collision risk. Fishing or RAM working at anchor use 35(d); small-vessel alternatives are in 35(i)–(j).