Fog Signals: What You Sound, and What You Are Hearing

What the rule requires

The rule switches on in or near an area of restricted visibility, whether by day or night 33 CFR §83.35. Both halves of that phrase are tested. Near means the signals begin as you stand toward the fog bank, not when you are already inside it, and day or night means daylight buys you nothing. The Section I rules apply in any condition of visibility COLREGs Rule 4; Rule 35 stacks a signalling duty on top of them.

Underway, on the whistle

Each of these goes at intervals of not more than 2 minutes .

One prolonged blast — a power-driven vessel making way through the water.
Two prolonged blasts, with about 2 seconds between them — a power-driven vessel underway but stopped and making no way.
One prolonged followed by two short — a vessel not under command; a vessel restricted in her ability to manoeuvre, whether underway or at anchor; a sailing vessel; a vessel engaged in fishing, whether underway or at anchor; and a vessel engaged in towing or pushing. International adds a vessel constrained by her draft to that list COLREGs Rule 35.
One prolonged followed by three short — a vessel towed, or where more than one is towed, the last vessel of the tow, if manned. When practicable it follows immediately after the towing vessel's signal.

A pushing vessel and a vessel pushed ahead that are rigidly connected in a composite unit count as one power-driven vessel and sound one or two prolonged blasts according to whether she is making way.

A tug with three manned barges in tow astern is in fog. Who sounds what?

The tug sounds one prolonged followed by two short at intervals of not more than 2 minutes, as a vessel engaged in towing. The third barge — the last vessel of the tow, and manned — sounds one prolonged followed by three short, immediately after the tug's signal when practicable. The first and second barges sound nothing under this paragraph; it names only the last vessel of the tow.

At anchor and aground

A vessel at anchor rings the bell rapidly for about 5 seconds at intervals of not more than 1 minute. At 100 metres or more in length the bell is sounded in the forepart, and immediately after it the gong is sounded rapidly for about 5 seconds in the after part. She may also sound one short, one prolonged and one short blast on the whistle to warn an approaching vessel of her position and of the possibility of collision.

A vessel aground gives that same bell signal, with the gong if her length requires it, and adds three separate and distinct strokes on the bell immediately before and after the rapid ringing. She may in addition sound an appropriate whistle signal.

Length exemptions, pilot vessels, special anchorages

These come from the Inland text at .

Less than 12 metres — not obliged to give any of the above signals, but if she does not, she must make some other efficient sound signal at intervals of not more than 2 minutes.
12 metres or more but less than 20 metres — not obliged to give the bell signals for anchoring and grounding; if she does not, some other efficient sound signal at intervals of not more than 2 minutes. Her underway whistle signals are untouched by this exemption.
Pilot vessel engaged on pilotage duty — may sound an identity signal of four short blasts in addition to her underway or anchor signal.
Special anchorage area designated by the Coast Guard — the anchor signal is not required from a vessel of less than 20 metres, or from a barge, canal boat, scow or other nondescript craft. The second limb carries no length qualifier.

Carriage of the gear behind those signals sits at 12, 20 and 100 metres: a whistle at 12 metres or more, a bell in addition at 20 metres or more, a gong in addition at 100 metres or more, its tone not to be confused with the bell's COLREGs Rule 33. Under 12 metres a vessel need not carry them but must have some other means of making an efficient sound signal. Bell or gong may be replaced by equipment of the same sound characteristics provided manual sounding of the prescribed signals remains possible.

International against Inland

Identical under both — one prolonged making way; two prolonged stopped; one prolonged and three short from the manned tow; the composite unit treated as power-driven; the 2-minute maximum interval , .
The one-prolonged-two-short group differs — International names a vessel constrained by her draft in it. Inland does not, and instead writes "whether underway or at anchor" into its entries for the vessel restricted in her ability to manoeuvre and the vessel engaged in fishing.
Anchored fishing and anchored RAM reach the same answer by different routes — International paragraph (d) sends them to the one-prolonged-two-short signal instead of the anchor signal; Inland reserves paragraph (d) and gets there through the words "whether underway or at anchor". Either way, that vessel sounds the whistle, not the bell.
Inland paragraph (d) is [Reserved] — a stem quoting Inland (d) as containing a requirement is quoting nothing.

The anchor and aground signals, the length exemptions, the pilot identity signal and the special anchorage exemption above are the Inland provisions. International Rule 35 carries its own anchor signal in its paragraph (g), which its paragraph (d) cross-refers to.

Telling it apart

Four pairs account for most of the wrong answers on this topic, and each turns on one criterion.

Making way against underway but stopped — the criterion is motion through the water, not whether the engines are turning. A vessel lying stopped in the water with her main engine running is on two prolonged blasts, and she does not go to one prolonged until she has way on.
The towing vessel against the vessel towed — the criterion is which end of the tow you are on. Two short after the prolonged is the tug or pushing vessel; three short is the manned vessel at the tail of the string.
At anchor against aground — the criterion is the pair of three-stroke brackets. Rapid ringing alone is anchored. Three strokes, rapid ringing, three strokes is aground.
A vessel restricted in her ability to manoeuvre at anchor against any other vessel at anchor — the criterion is her status, which overrides her position. She works the whistle at one prolonged and two short; an ordinary vessel at anchor works the bell.

Working a question

You are master of a 19-metre inspected passenger vessel. Visibility shuts down to under a cable at 1400. You take the way off her, lie stopped with the engine running, and wait.

Is the rule in force? Restricted visibility, and you are in it. Daylight is irrelevant, so signals start now.
Which category is the vessel in? Not under command, restricted in her ability to manoeuvre, sailing, fishing, towing or pushing — none apply. She is an ordinary power-driven vessel, so the answer lies in the first two paragraphs of the rule.
Making way, or not? No way on. That is two prolonged blasts with about 2 seconds between them, at intervals of not more than 2 minutes. Engine running does not change it.
Does an exemption reach her? At 19 metres she is above the under-12-metre exemption, and the 12-to-20-metre exemption reaches only the bell signals. The whistle signal is owed in full.
Now you anchor. The anchor signal is a rapid bell for about 5 seconds at intervals of not more than 1 minute. At 19 metres you are not obliged to give it — but if you do not, you must make some other efficient sound signal, and the interval for that substitute is not more than 2 minutes, not 1. If the anchorage is a special anchorage area designated by the Coast Guard, no anchor signal is required of you at all.

Step 5 is where the exemptions earn their keep on the exam: the exemption changes the interval as well as the instrument.

Where candidates lose the point

Reading the one-prolonged-two-short signal as belonging to one vessel type. A stem gives that signal and offers "a sailing vessel" as one option and "a vessel not under command" as another. Both are correct members of the group, so the question is written either as "could indicate" with a single member offered, or with a distractor from outside the group. Fix the whole membership: not under command, restricted in her ability to manoeuvre, sailing, fishing, towing or pushing, and constrained by her draft under International only.

Answering "one prolonged" for a vessel stopped in the water. The pull is the word underway in the stem. Underway is not making way, and the rule prices the difference at a second prolonged blast.

Putting the anchor bell on a 2-minute interval. Everything else in the rule is 2 minutes, so 1 minute looks like the odd value and gets discarded. It is the correct one for the bell, and an "all of the following EXCEPT" stem listing signals heard at 2-minute intervals is answered by the bell.

Treating "not more than" as a schedule. It is a maximum. A signal sounded more often than the stated interval satisfies the rule; one sounded less often does not.

Applying the special anchorage exemption by length alone. A 25-metre canal boat is over the 20-metre line, so candidates rule her in. The exemption lists barges, canal boats, scows and other nondescript craft as a separate class with no length attached.

Looking in Rule 35 for what to do when you hear a signal. The rule assigns signals to conditions; it does not set your speed, your helm or your engine order. A stem that asks what action you are required to take on hearing a fog signal ahead is answered from the conduct rules for restricted visibility, and no amount of blast-counting will separate its options. Signals meant to attract attention are governed separately, and must be incapable of being mistaken for any signal authorised elsewhere in the Rules or for an aid to navigation COLREGs Rule 36.

Check yourself

(Both) You are on a 12-metre power-driven vessel in fog, engine running, clutch out, drifting with no way on. What do you sound, and at what interval?

Two prolonged blasts in succession with about 2 seconds between them, at intervals of not more than 2 minutes. She is underway but stopped and making no way. At 12 metres she is required to carry a whistle, so the under-12-metre substitute signal is not available to her.

(Inland) In fog you hear one prolonged followed by two short at intervals of about 2 minutes. Which of these can it be: a vessel towed, a vessel aground, a vessel engaged in fishing, a 30-metre vessel at anchor?

A vessel engaged in fishing. The towed vessel sounds one prolonged and three short; the vessel aground sounds the bell with three distinct strokes before and after; the vessel at anchor sounds the rapid bell every minute. Under International the same signal could additionally be a vessel constrained by her draft.

(Both) Your 75-metre vessel is restricted in her ability to manoeuvre and is carrying out her work at anchor in fog. Bell or whistle?

Whistle: one prolonged followed by two short, at intervals of not more than 2 minutes. Inland reaches this through the words "whether underway or at anchor" in the paragraph; International reaches it through paragraph (d), which sends her to that signal instead of the anchor signal.

(Inland) You are at anchor in fog in a 15-metre vessel with no bell aboard. What are you required to do?

Make some other efficient sound signal at intervals of not more than 2 minutes. A vessel of 12 metres or more but less than 20 metres is not obliged to give the bell signal, and the substitute runs on the 2-minute interval rather than the 1-minute bell interval.

(Inland) Describe the full fog signal of a 120-metre vessel aground.

Three separate and distinct strokes on the bell; then the bell rung rapidly for about 5 seconds in the forepart; then, immediately after, the gong rung rapidly for about 5 seconds in the after part; then three separate and distinct strokes on the bell. Repeated at intervals of not more than 1 minute. She may in addition sound an appropriate whistle signal.

(Inland) A 25-metre canal boat lies at anchor in a special anchorage area designated by the Coast Guard, in fog. What must she sound?

Nothing under the anchor signal paragraph. Barges, canal boats, scows and other nondescript craft are exempt in a designated special anchorage area, and that limb of the exemption carries no length limit — her 25 metres does not put her back in.

(Inland) You hear one short, one prolonged and one short blast in fog. What is the vessel telling you, and was she obliged to say it?

She is at anchor and warning you of her position and of the possibility of collision. The signal is permissive, sounded in addition to the required rapid bell, so a stem asking what she must sound is answered by the bell.