Rule 35 — Sound Signals in Restricted Visibility

TL;DR — In or near restricted visibility, a power-driven vessel making way sounds one prolonged blast every 2 minutes; a vessel stopped and making no way sounds two prolonged blasts every 2 minutes; vessels such as NUC, RAM, sailing, fishing, and towing vessels sound one prolonged followed by two short blasts every 2 minutes. 33 CFR §83.35

---

What the Rule Says

Restricted visibility is defined as any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms, or any other similar causes. 33 CFR §83.03 The signals prescribed by Rule 35 apply in or near an area of restricted visibility, whether by day or night. 33 CFR §83.35 Note carefully: you do not have to be inside the fog bank — being near it is sufficient to trigger the obligation.

Power-Driven Vessels Underway

A power-driven vessel making way through the water shall sound, at intervals of not more than 2 minutes, one prolonged blast. 33 CFR §83.35

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. 33 CFR §83.35

The distinction between these two signals is critical: making way versus stopped-and-making-no-way. A vessel is underway whenever she is not at anchor, made fast to the shore, or aground — but underway does not mean making way. 33 CFR §83.03 A vessel can be underway (drifting, engines stopped) yet making no way through the water, and the two-blast signal applies in that case.

Vessels Sounding One Prolonged + Two Short Blasts

The following vessel types shall sound, at intervals of not more than 2 minutes, one prolonged blast followed by two short blasts: 33 CFR §83.35

• A vessel not under command (NUC)
• A vessel restricted in her ability to maneuver (RAM), whether underway or at anchor
• A sailing vessel
• A vessel engaged in fishing, whether underway or at anchor
• A vessel engaged in towing or pushing another vessel

This single signal pattern covers a wide range of vessel types. The exam frequently tests whether candidates can identify which vessels share this signal.

Manned Vessel Being Towed

A vessel being 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: one prolonged followed by three short blasts. When practicable, this signal shall be made immediately after the signal made by the towing vessel. 33 CFR §83.35

Composite Unit (Pushing Vessel Rigidly Connected)

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 for a power-driven vessel making way or stopped. 33 CFR §83.35 This is a common exam trap — a rigid composite unit is treated as a single power-driven vessel, not as a towing situation.

Vessel at Anchor

A vessel at anchor shall, at intervals of not more than 1 minute, ring the bell rapidly for about 5 seconds. 33 CFR §83.35 Note the interval: 1 minute, not 2 minutes.

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. 33 CFR §83.35 This requires a gong, which is mandatory equipment only on vessels 100 meters or more in length. 33 CFR §83.33

A vessel at anchor may in addition sound three blasts in succession — one short, one prolonged, and one short blast — to warn an approaching vessel of her position and the possibility of collision. This supplemental signal is optional, not mandatory. 33 CFR §83.35

Vessel Aground

A vessel aground shall give the bell signal (and if required, the gong signal) prescribed for a vessel at anchor, 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. 33 CFR §83.35

The sequence for a vessel aground is therefore: three distinct bell strokes — rapid bell ringing for about 5 seconds — three distinct bell strokes. The whistle signal is optional.

Small Vessel Exemptions

A vessel of 12 meters or more but less than 20 meters in length is not obliged to give the bell signals prescribed for vessels at anchor or aground. However, if she does not, she shall make some other efficient sound signal at intervals of not more than 2 minutes. 33 CFR §83.35

A vessel of less than 12 meters in length is not obliged to give any of the above-mentioned signals, but if she does not, she shall make some other efficient sound signal at intervals of not more than 2 minutes. 33 CFR §83.35

Pilot Vessel Identity Signal

A pilot vessel when engaged on pilotage duty may, in addition to the signals prescribed for a power-driven vessel making way, stopped, or at anchor, sound an identity signal consisting of four short blasts. 33 CFR §83.35

Special Anchorage Area Exemption

The following vessels are not required to sound the anchor bell signal when anchored in a special anchorage area designated by the Coast Guard: a vessel of less than 20 meters in length, and a barge, canal boat, scow, or other nondescript craft. 33 CFR §83.35

---

Why It Matters on the Exam

Rule 35 is one of the highest-yield topics on both the OUPV and Master 100 GT written examinations. Expect multiple questions that require you to identify the correct fog signal for a specific vessel type, or to match a signal pattern to the vessel producing it.

The exam tests several recurring themes:

Signal pattern identification. You must know all four distinct patterns: one prolonged blast (power-driven, making way); two prolonged blasts (power-driven, stopped, no way); one prolonged + two short (NUC, RAM, sailing, fishing, towing/pushing); one prolonged + three short (manned vessel being towed). 33 CFR §83.35

Interval precision. Most fog signals are required at intervals of not more than 2 minutes. The anchor bell signal is the exception — not more than 1 minute. Exam writers exploit this distinction. 33 CFR §83.35

Equipment requirements. A vessel of 12 meters or more must carry a whistle. A vessel of 20 meters or more must carry a bell in addition to a whistle. A vessel of 100 meters or more must carry a gong in addition to the whistle and bell. 33 CFR §83.33 These thresholds directly determine which fog signals a vessel is capable of — and obligated — to produce.

Rule 19 interaction. Rule 35 governs what signals to make; Rule 19 governs how to navigate. When a vessel hears a fog signal apparently forward of her beam, she must reduce speed to the minimum at which she can be kept on course, and if necessary take all way off. 33 CFR §83.19 Exam questions sometimes combine both rules in a scenario.

Day or night applicability. Rule 35 signals apply whether by day or night. Restricted visibility is not a nighttime-only condition. 33 CFR §83.35

---

Common Pitfalls

Confusing "underway" with "making way." A vessel is underway if she is not at anchor, made fast to shore, or aground. 33 CFR §83.03 She may be underway but making no way — drifting with engines stopped. That vessel sounds two prolonged blasts, not one. 33 CFR §83.35

Forgetting that RAM at anchor sounds one prolonged + two short. The rule explicitly states "whether underway or at anchor" for RAM vessels. A RAM vessel at anchor does not sound the anchor bell — she sounds one prolonged + two short blasts. 33 CFR §83.35 Same applies to a vessel engaged in fishing at anchor.

Misidentifying the composite unit signal. A pushing vessel rigidly connected to the vessel being pushed is treated as a single power-driven vessel. She does not sound the towing signal (one prolonged + two short). She sounds one prolonged blast if making way, or two prolonged blasts if stopped. 33 CFR §83.35

Confusing the anchor optional signal with the aground signal. A vessel at anchor may optionally sound one short + one prolonged + one short. A vessel aground sounds three distinct bell strokes before and after the rapid bell ringing. These are different signals with different meanings. 33 CFR §83.35

Applying the 2-minute interval to the anchor bell. The anchor bell is rung at intervals of not more than 1 minute. Every other fog signal in Rule 35 uses a 2-minute interval. 33 CFR §83.35

Assuming small vessels are exempt from all fog signals. A vessel under 12 meters is not obligated to use the prescribed signals, but she must make some other efficient sound signal at intervals of not more than 2 minutes if she does not. 33 CFR §83.35 There is no complete exemption from making a sound signal.

---

Quick Check

Q1 — A power-driven vessel is underway in fog, engines stopped, making no way through the water. What fog signal is required and at what interval?

Two prolonged blasts in succession, with an interval of about 2 seconds between them, at intervals of not more than 2 minutes. 33 CFR §83.35

Q2 — You hear one prolonged blast followed by two short blasts in restricted visibility. Which of the following could be producing that signal: (A) power-driven vessel making way, (B) sailing vessel, (C) manned vessel being towed, (D) vessel at anchor?

(B) A sailing vessel. The one prolonged + two short signal is used by NUC, RAM, sailing vessels, vessels engaged in fishing, and vessels towing or pushing. A power-driven vessel making way sounds one prolonged blast. A manned vessel being towed sounds one prolonged + three short blasts. A vessel at anchor rings the bell. 33 CFR §83.35

Q3 — A 150-meter vessel is at anchor in restricted visibility. Describe the complete required sound signal.

The bell shall be rung rapidly for about 5 seconds in the forepart of the vessel, at intervals of not more than 1 minute. Immediately after the bell, the gong shall be sounded rapidly for about 5 seconds in the after part of the vessel. The gong is required because the vessel is 100 meters or more in length. 33 CFR §83.35 33 CFR §83.33

Q4 — A pushing vessel and the vessel being pushed ahead are rigidly connected as a composite unit. What fog signal do they sound?

They are regarded as a power-driven vessel. If making way, one prolonged blast at intervals of not more than 2 minutes. If stopped and making no way, two prolonged blasts at intervals of not more than 2 minutes. 33 CFR §83.35

Q5 — A vessel of 15 meters in length is at anchor in restricted visibility. Is she required to ring a bell?

No. A vessel of 12 meters or more but less than 20 meters in length is not obliged to give the bell signals prescribed for vessels at anchor. However, if she does not ring the bell, she must make some other efficient sound signal at intervals of not more than 2 minutes. 33 CFR §83.35

Q6 — A pilot vessel is underway and making way in restricted visibility while engaged on pilotage duty. What signals may she sound?

She shall sound one prolonged blast at intervals of not more than 2 minutes (as a power-driven vessel making way), and may in addition sound an identity signal of four short blasts. 33 CFR §83.35

Q7 — What action must a vessel take upon hearing a fog signal apparently forward of her beam, unless it has been determined that no risk of collision exists?

She shall reduce her speed to the minimum at which she can be kept on course. She shall if necessary take all her way off and, in any event, navigate with extreme caution until danger of collision is over. 33 CFR §83.19