A vessel at anchor in fog rings her bell how often?

At intervals of not more than 1 minute

Rule 35: Sound Signals in Restricted Visibility (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 35 sets fog signals. A power-driven vessel making way sounds one prolonged blast at least every 2 minutes, or two prolonged if stopped. Not under command, restricted, sailing, fishing, and towing vessels sound one prolonged and two short. Anchored vessels ring a bell for about 5 seconds at least every minute.

The rule text

A power-driven vessel making way through the water shall sound at intervals of not more than 2 minutes one prolonged blast.
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 35

A vessel at anchor shall at intervals of not more than one minute ring the bell rapidly for about 5 seconds.
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 35

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
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 35

What it means on the water

Applies in or near restricted visibility, day or night.
Power-driven making way: one prolonged (35(a)). Underway but stopped: two prolonged about 2 seconds apart (35(b)).
NUC, RAM, sailing, fishing, towing or pushing: one prolonged plus two short (35(c)).
Manned last vessel of a tow: one prolonged plus three short, right after the tug's signal when practicable (35(e)).
At anchor: bell about 5 seconds each minute, with a gong aft at 100 meters or more, optional short-prolonged-short (35(g)). Aground: three distinct bell strokes before and after (35(h)). Pilot vessel may add four short (35(k)).

International vs Inland

International 35(c) includes a vessel constrained by her draft; Inland doesn't. International 35(d) covers fishing and RAM vessels at anchor; Inland 35(c) covers them with the words "whether underway or at anchor" and 35(d) is reserved. Inland 35(l) exempts vessels under 20 meters, and barges, canal boats, scows, or other nondescript craft, from the anchor signal in a Coast Guard special anchorage area.

Common exam traps

Using Rule 34 maneuvering signals in fog.
Giving a sailing vessel one prolonged blast. She sounds one prolonged and two short.
Mixing the 2-minute interval (whistle) with the 1-minute interval (anchor bell).

Try 3 questions on Rule 35

A power-driven vessel underway but stopped in fog sounds:
1. One prolonged every 2 minutes
2. Two prolonged every 2 minutes
3. One prolonged, two short
4. Bell every minute
Show answer
B. Two prolonged every 2 minutes Rule 35(b).
A sailing vessel in fog sounds:
1. One prolonged
2. One prolonged, two short
3. Two prolonged
4. Four short
Show answer
B. One prolonged, two short Rule 35(c).
A vessel at anchor in fog rings her bell how often?
1. At intervals of not more than 1 minute
2. Every 2 minutes
3. Every 30 seconds
4. Only when a vessel approaches
Show answer
A. At intervals of not more than 1 minute Rule 35(g).

Frequently asked questions

What is the fog signal for a vessel at anchor?

Ring the bell rapidly for about 5 seconds at intervals of not more than one minute; 100 meters or more adds a gong aft (Rule 35(g)).

What does one prolonged and three short mean in fog?

A manned towed vessel, or the last vessel of a tow if manned (Rule 35(e)).

Do small boats have to sound fog signals?

Under 12 meters, not the Rule 35 signals, but some other efficient sound signal at intervals of not more than 2 minutes (Rule 35(j)).