Sound Signals in Restricted Visibility (Rule 35)

Introduction

When visibility is reduced by fog, mist, falling snow, heavy rainstorms, or any other condition, the visual system of navigation lights becomes insufficient. Vessels cannot see each other's lights until dangerously close, making collision avoidance extremely difficult. Sound signals in restricted visibility serve a critical function: alerting other vessels to your presence, status, and movement before visual contact is possible.

Rule 35 establishes a completely separate sound signal system that applies when vessels are NOT in sight of one another due to restricted visibility. These signals bear no relation to the maneuvering signals in Rule 34-they serve an entirely different purpose.

The "Fog" Rules: A Separate Language

Restricted visibility signals are often called "fog signals," though they apply in any restricted visibility, not just fog. They must be sounded:

• At intervals of not more than 2 minutes for whistle signals
• At intervals of not more than 1 minute for bell signals (anchored vessels)

These are mandatory signals-not optional. When visibility is restricted, vessels MUST sound the appropriate signal even if no other traffic is known to be nearby.

Critical Distinction: These signals do NOT indicate intent or action. They indicate vessel type and status only. A vessel hearing fog signals knows what type of vessel is nearby and whether it's moving or stopped, but not what maneuver it might take.

Whistle Signals in Restricted Visibility (Rule 35)

The whistle signals in fog are based on two factors:

1. Vessel type (power-driven, sailing, fishing, towing, etc.)
2. Whether the vessel is making way (moving through the water) or stopped

Power-Driven Vessel Making Way (Rule 35a)

Signal: One prolonged blast (-)

Interval: Not more than 2 minutes

Meaning: "I am a power-driven vessel and I am making way (moving through the water)"

Mnemonic: One prolonged = "I am moving"

This is the most common fog signal, heard frequently in busy harbors during foggy conditions.

Power-Driven Vessel Underway but Stopped (Rule 35b)

Signal: Two prolonged blasts with 2-second interval between them (- -)

Interval: Not more than 2 minutes

Meaning: "I am a power-driven vessel, I am underway (afloat, not anchored), but I am stopped and not making way through the water"

Mnemonic: Two prolonged = "I am stopped"

Why This Matters: A vessel stopped in fog presents a stationary hazard. Unlike an anchored vessel (which has chosen a safe location), a stopped underway vessel may be anywhere-in a channel, fairway, or unexpected location. Extreme caution is required when this signal is heard.

Sailing Vessel, Fishing Vessel, RAM, NUC, or Towing (Rule 35c, d, e)

All "hampered" vessels-vessels with special status that restricts their maneuverability-sound the same fog signal:

Signal: One prolonged blast followed by two short blasts (- • •)

Interval: Not more than 2 minutes

Meaning: "I am a vessel with restricted maneuverability" (could be sailing, fishing, towing, RAM, or NUC)

Mnemonic: - • • = "I am NOT a normal vessel"

This signal groups together all the special-status vessels from Rule 18 hierarchy (except power-driven vessels). It warns approaching traffic: "I have limited ability to maneuver-YOU must take action to avoid me."

Towed Vessel (Manned) (Rule 35e)

If a vessel is being towed and has crew aboard (manned tow), that towed vessel shall sound:

Signal: One prolonged followed by three short blasts (- • • •)

Interval: Not more than 2 minutes, immediately after the towing vessel's signal

Meaning: "I am a manned vessel being towed"

Purpose: Alerts other vessels that there are two vessels in proximity (the tug and its tow), and helps establish the size/length of the total towing configuration.

Pilot Vessel (Rule 35f)

A pilot vessel engaged in pilotage duty may sound, in addition to the signals for its status (underway, stopped, etc.):

Signal: Four short blasts (• • • •)

Meaning: "I am a pilot vessel on duty"

This additional signal helps other traffic identify the pilot boat, which is often maneuvering near channel entrances and fairways in challenging conditions.

The Critical Sound: Distinguishing Signals

When you hear fog signals, rapid interpretation is essential:

One prolonged (-):

• Power-driven vessel making way
• Or: Vessel approaching a blind bend (Rule 34e signal, which also applies in fog)

Two prolonged (- -):

• Power-driven vessel stopped (dangerous-may be anywhere)

One prolonged + two short (- • •):

• Sailing vessel, OR
• Fishing vessel, OR
• Vessel towing, OR
• Vessel restricted (RAM), OR
• Vessel not under command (NUC)
• Translation: "Give me wide berth-I have limited maneuverability"

How to distinguish which type: You can't always tell from sound alone. That's why the signal groups them-all require the same response from you: wide berth and extreme caution.

Bell and Gong Signals: Stationary Hazards (Rule 35g, h, i)

Vessels at anchor or aground use bell and gong signals instead of whistle signals. This distinguishes them from underway vessels and alerts approaching traffic to stationary hazards.

Vessel at Anchor (Rule 35g)

Signal: Rapid ringing of bell forward for 5 seconds

Interval: Not more than 1 minute

For vessels 100 meters or more: After the bell, rapidly sound gong aft for 5 seconds

Purpose: The distinctive bell sound immediately identifies a stationary anchored vessel. The bell can be heard at significant distance in fog, providing critical warning.

Approaching fog bank: If you hear a bell ringing, STOP IMMEDIATELY. There is an anchored vessel ahead. Do NOT proceed until you have determined its exact position and can pass safely clear.

Vessel Aground (Rule 35h)

Signal: Three distinct bell strikes, followed by rapid bell ringing for 5 seconds, followed by three distinct bell strikes

Interval: Not more than 1 minute

Pattern: Ding-ding-ding ... rapid ring ... ding-ding-ding

For vessels 100 meters or more: May also sound gong aft after the bell sequence

Purpose: The three-ring, rapid-ring, three-ring pattern is unmistakable and warns of both a stationary hazard AND shallow water (the vessel is touching bottom).

Optional Whistle Signal for Anchored/Aground Vessels (Rule 35g, h)

An anchored or aground vessel may ALSO sound the following whistle signal to alert approaching traffic:

Signal: One short, one prolonged, one short (• - •)

Purpose: This optional signal helps vessels equipped with radar or approaching from directions where the bell may not be audible. It's the International Code Signal for "I require assistance," though in this context it simply means "I am anchored/aground here."

Exemptions for Small Vessels (Rule 35i, j, l)

The rules recognize that very small vessels may not have the electrical power or equipment to sound prolonged blasts.

Vessels under 12 meters are not required to sound the above signals, BUT they must:

• Make some other efficient sound signal at intervals of not more than 2 minutes

"Some other efficient sound signal" can include:

• Handheld air horn
• Mouth whistle
• Banging on the hull with a wrench or hammer
• Any loud, audible sound that warns nearby vessels

Vessels under 20 meters at anchor (not in narrow channel, fairway, or anchorage):

• Are not required to sound bell signals
• BUT should sound some signal if approaching traffic is detected

Practical Reality: Even when exempted, prudent seamanship dictates making sound signals in restricted visibility. The exemption does not eliminate the hazard-it only acknowledges equipment limitations.

Practical Application and Safety

Scenario 1: Operating in Fog

You're operating a 25-foot powerboat at 8 knots when fog suddenly rolls in, reducing visibility to less than 100 yards.

Required actions:

1. Slow to minimum steerage speed (Rule 19 - Restricted Visibility)
2. Post additional lookout
3. Immediately begin sounding one prolonged blast every 2 minutes (- ... -)
4. Activate radar (if equipped) and monitor closely
5. Listen intently for other fog signals

Scenario 2: Hearing a Bell in Fog

You're navigating slowly in fog when you hear a bell ringing ahead.

Required actions:

1. Stop immediately or slow to bare steerage
2. Attempt to determine exact direction and distance of bell
3. Use radar (if equipped) to locate the anchored vessel
4. Plot a course to pass well clear
5. Continue sounding your own fog signals
6. Do NOT proceed until you have confirmed safe clearance

Scenario 3: Hearing Two Prolonged Blasts

You hear two prolonged blasts (- -) on your port bow.

Interpretation: Power-driven vessel stopped in the water, not making way

Critical assessment: This is a hazard-a stopped vessel may be disabled, conducting emergency operations, or stopped due to traffic. It's not in a designated anchorage.

Required actions:

1. Alter course to starboard to increase separation
2. Slow down significantly
3. Attempt to establish VHF contact to determine vessel's status
4. Sound danger signal (• • • • •) if collision risk exists
5. Do NOT pass close aboard-the vessel may suddenly get underway

Summary

Fog signals in restricted visibility serve to communicate vessel type and status (making way vs. stopped) to prevent collision when visual signals are ineffective. Power vessels making way sound one prolonged blast every 2 minutes. Power vessels stopped sound two prolonged blasts. Special status vessels (sailing, fishing, towing, RAM, NUC) sound one prolonged plus two short.

Anchored vessels ring a bell for 5 seconds every minute. Aground vessels ring three bells, rapid ring, three bells. Large vessels (≥100m) add a gong aft after the bell.

When fog signals are heard, immediate action is required: slow down, determine the type and location of the other vessel, and maneuver to pass safely clear. The bell signal requires immediate stopping-there's an anchored vessel ahead.

Key Takeaways

• Fog signals must be sounded in any restricted visibility, not just fog
• One prolonged (-) = power vessel making way
• Two prolonged (- -) = power vessel stopped (DANGER-unexpected location)
• One prolonged + two short (- • •) = sailing, fishing, towing, RAM, or NUC (hampered vessels)
• One prolonged + three short (- • • •) = manned tow
• Bell ringing = anchored vessel (STOP immediately when heard)
• Three bells, rapid ring, three bells = aground vessel
• Fog signals sounded every 2 minutes (whistle) or 1 minute (bell)
• Small vessels (<12m) must make "efficient sound signal" even if not full prolonged blast
• Hearing fog signals requires immediate action: slow, locate, and avoid