Rule 35 - Fog signals are not optional noise

Rule 35

Fog signals are not optional noise

Official rule text as published in the U.S. Coast Guard Navigation Rules

Source: USCG NavCen / eCFR Navigation Rules · Last updated September 7, 2026

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 [Inland: whether underway or at anchor], [International: 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 (a) or (b), sound at intervals of not more than 2 minutes three blasts in succession, namely one prolonged followed by two short blasts.

International Only
(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 (g), sound the signal prescribed in (c).

(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 (a) or (b).

(g) A vessel at anchor shall at intervals of not more than 1 minute ring the bell rapidly for about 5 seconds. In a vessel 100 meters or more in length the bell shall be sounded in the forepart of the vessel and, immediately after, 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 — 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 (g) 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 (g) and (h). 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 (a), (b) or (g), sound an identity signal consisting of four short blasts.

Inland
(l) The following vessels shall not be required to sound signals as prescribed in (g) when anchored in a special anchorage area designated by the Coast Guard: (i) a vessel of less than 20 meters in length; and (ii) a barge, canal boat, scow, or other nondescript craft.

Color note: black text applies generally; blue is International Waters; green is Inland Waters.

MyCOLREGS explanation

Summary

In or near fog, vessels signal their type and state at least every two minutes so others can build a picture without seeing anything. A power-driven vessel making way sounds one prolonged blast; stopped and not making way, two prolonged blasts. Vessels not under command, restricted in their ability to maneuver, sailing, fishing, or towing/pushing sound one prolonged followed by two short blasts instead, and a vessel being towed sounds one prolonged followed by three short blasts. At anchor it is bell strokes rather than whistle blasts; aground, bell strokes plus three distinct strokes before and after. None of this is optional in restricted visibility — it is how vessels that cannot see each other stay apart.

Real World Example

In fog, the whistle pattern is often the only information another crew has about what kind of vessel you are and whether you're moving - mistake a stopped vessel's two prolonged blasts for one making way, and you can commit to passing a boat that isn't going anywhere, without ever laying eyes on her until it's too late to correct.

Practice Questions

Q1. You are underway and making way through the water in fog. What sound signal must you give, and how often?

A. One prolonged blast at intervals of not more than 2 minutes.

B. Two prolonged blasts at intervals of not more than 2 minutes.

C. One prolonged followed by two short blasts, every minute.

D. Five short blasts every 2 minutes.

Q2. A sailing vessel underway in restricted visibility signals with one prolonged blast followed by two short blasts every 2 minutes. Is this correct, and why?

A. Yes, but only if she is also flying a day shape.

B. Yes — Rule 35(c) prescribes that exact pattern for sailing, fishing, towing/pushing, not-under-command, and restricted-ability vessels.

C. No, sailing vessels must sound two prolonged blasts like a stopped power-driven vessel.

D. No, sailing vessels are exempt from all sound signals in fog.

Practice Question Answers

Q1.

A. One prolonged blast at intervals of not more than 2 minutes. Rule 35(a) requires a power-driven vessel making way through the water to sound one prolonged blast at intervals of not more than 2 minutes. B doubles the blast count incorrectly. C describes the pattern for a different vessel status (the one Rule 35(c) covers), not a plain power-driven vessel making way. D reaches for the danger/doubt signal from Rule 34, which is unrelated to fog signaling.

Q2.

B. Yes — Rule 35(c) prescribes that exact pattern for sailing, fishing, towing/pushing, not-under-command, and restricted-ability vessels. Rule 35(c) groups sailing vessels with several other vessel types that all use one prolonged plus two short blasts, distinct from the plain prolonged-blast signals for power-driven vessels making way or stopped. A adds a day-shape condition that isn't part of the sound-signal requirement. C describes the two-prolonged-blast pattern used by a stopped power-driven vessel instead. D wrongly exempts sailing vessels from fog signals altogether.

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