7.4 Sound and Light Signals: Manoeuvring, Warning & Fog (Rules 32-37)
Key Takeaways
Short blasts last approximately 1 second; prolonged blasts last 4 to 6 seconds; sound appliances scale by vessel length with whistles required from 12m, bells from 20m, and gongs aft from 100m.
Manoeuvring whistle signals (1 short = altering to starboard, 2 short = altering to port, 3 short = astern propulsion) are given by power-driven vessels underway when vessels are in sight of one another.
The doubt/danger signal of 5 or more rapid short blasts must be sounded immediately when a vessel fails to understand an approaching ship's intentions or doubts whether sufficient action is being taken.
In fog, power-driven vessels making way sound 1 prolonged blast every 2 minutes, while vessels stopped sound 2 prolonged blasts separated by 2 seconds.
NUC, RAM, CBD, sailing, fishing, towing and non-composite pushing vessels use 1 prolonged followed by 2 short blasts within 2 minutes. A rigid pushing composite unit uses the power-driven signal.
7.4 Sound and Light Signals: Manoeuvring, Warning & Fog (Rules 32-37)
Note
When visual contact is obscured by fog or during urgent close-quarters encounters, sound signals provide vital collision avoidance communication. Part D of the Collision Regulations (Rules 32 through 37) prescribes whistle signals for manoeuvring, overtaking, warning, blind bends, fog, and recognized distress signals.
Rules 32 & 33: Definitions and Sound Appliances
Sound signals are governed by standardized durations defined in Rule 32:
- Short Blast: A blast of about 1 second duration.
- Prolonged Blast: A blast of from 4 to 6 seconds duration.
Statutory Appliance Requirements (Rule 33)
Sound signalling equipment mandates scale with vessel length:
- Vessels ≥ 100 metres: Must carry a whistle, a bell forward, and a gong aft.
- Vessels 20 metres to < 100 metres: Must carry a whistle and a bell.
- Vessels 12 metres to < 20 metres: Must carry a whistle (a bell is optional, but an efficient sound signal is required).
- Vessels < 12 metres: Must provide some other means of making an efficient sound signal (e.g., mouth horn or canister horn).
Rule 34: Manoeuvring and Warning Signals (In Sight)
Under Rule 34(a), power-driven vessels underway signal manoeuvres authorized or required by the Rules when vessels are in sight of one another:
1. Action Signals When Altering Course or Operating Astern
- One Short Blast (•): "I am altering my course to starboard."
- Two Short Blasts (••): "I am altering my course to port."
- Three Short Blasts (•••): "I am operating astern propulsion."
- Light Supplement: May be supplemented by 1-second light flashes (1 = starboard, 2 = port, 3 = astern).
2. Overtaking Signals in a Narrow Channel (Rule 34(c))
In a narrow channel where overtaking requires the overtaken vessel to permit safe passage:
- Two Prolonged followed by One Short (—— •): "I intend to overtake on your starboard side."
- Two Prolonged followed by Two Short (—— ••): "I intend to overtake on your port side."
- Agreement Signal (— • — •): Sounded by the overtaken vessel if agreeing: prolonged-short-prolonged-short (Morse 'C').
- If in doubt, the overtaken vessel sounds the danger signal.
3. Danger / Doubt Signal (Rule 34(d))
When approaching vessels in sight fail to understand intentions or doubt whether sufficient action is being taken to avoid collision:
- Five or More Short Rapid Blasts (•••••) on the whistle, optionally supplemented by five rapid light flashes.
4. Blind Bend Signal (Rule 34(e))
Nearing an obscured bend or channel obstruction:
- Sounds One Prolonged Blast (—), answered immediately with one prolonged blast by any approaching vessel.
Rule 35: Sound Signals in Restricted Visibility (Fog Signals)
In or near restricted visibility, use the signal for the vessel's status. Whistle signals below recur within 2 minutes; anchor/aground bell signals recur within 1 minute:
1. Power-Driven Vessels Underway
- Making Way: One prolonged blast (—) every 2 minutes.
- Stopped and Making No Way: Two prolonged blasts (——) separated by ~2 seconds, every 2 minutes.
2. Vessels with Restricted Manoeuvrability
A universal signal: One prolonged followed by two short blasts (— • •) every 2 minutes ("Long, short, short"). Applies to:
- Not Under Command (NUC)
- Restricted in Ability to Manoeuvre (RAM)
- Constrained by Draught (CBD)
- Sailing vessels underway
- Fishing vessels
- Vessels towing or pushing, except a rigid pushing composite unit, which uses one prolonged blast when making way or two when stopped and making no way
3. Towed Vessels
A towed vessel—or the last vessel of a tow if several are towed—if manned, sounds one prolonged followed by three short blasts (— • • •) at intervals not exceeding 2 minutes. When practicable, it follows immediately after the tug's signal.
4. Anchored Vessels in Fog
A vessel engaged in fishing at anchor, or a RAM vessel carrying out her work at anchor, uses one prolonged and two short blasts under Rule 35(d), instead of the following ordinary anchor signal.
- Vessels < 100 metres: Rapid ringing of the bell forward for 5 seconds every 1 minute. May additionally sound one short, one prolonged, one short blast (• — •, Morse 'R') on the whistle to warn approaching craft.
- Vessels ≥ 100 metres: Bell forward for 5 seconds, followed immediately by gong aft for 5 seconds, every 1 minute.
- Vessels < 20m: May sound an efficient signal every 2 minutes in lieu of bell/gong.
5. Vessels Aground in Fog
Vessels ≥20m: three distinct bell strokes, rapid ringing for about 5 seconds, then three distinct strokes; add the gong aft if ≥100m. Vessels <20m may instead make an efficient sound signal at intervals not exceeding 2 minutes.
6. Pilot Vessel on Duty
May sound an identity signal of four short blasts (••••) in addition to regular fog signals.
Rules 36 & 37: Attention & Distress Signals
Signals to Attract Attention (Rule 36)
Any vessel may make light or sound signals that cannot be mistaken for authorized signals (such as directing a searchlight beam onto a danger, without blinding other skippers).
Recognized Distress Signals (Rule 37 & Annex IV)
Signals indicating distress and need of immediate assistance:
| Distress Signal | Category | Details |
|---|---|---|
| Gun / Explosive Signal | Sound / Pyrotechnic | Fired at intervals of ~1 minute |
| Continuous Sounding | Sound | Continuous sounding of any fog horn |
| Rockets / Shells | Pyrotechnic | Red stars fired one at a time |
| Morse SOS | Light / Sound | • • • — — — • • • by lamp or whistle |
| MAYDAY | Radio Voice | Spoken on VHF Ch 16 (156.8 MHz) or 2182 kHz |
| Code Flags NC | Visual Flags | International Code flags November over Charlie |
| Square Flag & Ball | Visual | Square flag with ball above or below |
| Flames on Vessel | Visual Fire | Burning tar or oil drum on deck |
| Rocket Parachute Flare | Pyrotechnic | Parachute flare displaying red light |
| Hand Flare | Pyrotechnic | Red hand flare held downwind |
| Orange Smoke | Pyrotechnic | Buoyant or hand canister orange smoke |
| Arm Motion | Visual Body | Slowly raising and lowering outstretched arms |
| DSC Distress Alert | Digital Radio | VHF Ch 70 / MF 2187.5 kHz digital alert |
| Satellite Distress Alert | Satellite Radio | Ship-to-shore alert through an Inmarsat or other recognised mobile satellite service ship earth station |
| Satellite EPIRB | Beacon | 406 MHz emergency radio beacon signal |
| SART Transponder | Radar Beacon | 12-dot blip code on 9 GHz marine radar |
While navigating in dense fog with visibility under 200 metres, you hear a fog signal consisting of one prolonged blast followed by two short blasts sounding every two minutes. Which of the following vessels could be sounding this signal?
A power-driven container ship stopped and making no way through the water
A vessel aground in a shallow channel
A high-speed craft operating in non-displacement mode
A sailing yacht underway under sail alone
In clear daylight in a coastal channel, an approaching vessel on a reciprocal course suddenly sounds one short blast on her whistle. What does this signal legally mean under Rule 34(a)?
"I am altering my course to starboard"
"I am maintaining my course and speed as the stand-on vessel"
"I intend to overtake you on your starboard side"
"I am operating astern propulsion"
You are overtaking a slower power-driven vessel in a narrow tidal channel where you need her cooperation to pass safely. To signal your intention to overtake on her port side, what sound signal must you sound under Rule 34(c)?
Two short blasts
One prolonged blast followed by one short blast
Two prolonged blasts followed by two short blasts
Five rapid short blasts
Sections you finish are checked off in the contents.