COLREGs Part D: Sound and Light Signals — The Language of Warning at Sea
Understanding Rules 32–37 for Safe Communication Between Vessels
When visibility becomes poor, ships cannot depend entirely on visual observation. Fog, heavy rain, darkness and other conditions can make it difficult to identify another vessel or determine its intentions. In these situations, sound and light signals become an essential part of communication at sea.
Part D of COLREGs covers Rules 32–37, dealing with sound and light signals. The six rules cover the definitions of whistle signals, sound-signalling equipment, manoeuvring and warning signals, restricted-visibility signals, attention signals and distress signals.
For a seafarer, learning Part D is not simply about memorising patterns of short and prolonged blasts. The real objective is to understand what a signal means, when it should be used, how it should be interpreted and how it fits into the overall navigational situation.
Rule 32 — Definitions
Before understanding the individual signals, COLREGs first establish the terminology used for sound signals.
Rule 32 defines:
- Whistle
- Short blast
- Prolonged blast
A short blast is a blast of about one second.
A prolonged blast lasts approximately four to six seconds.
These durations are important because the meaning of a signal depends not only on the number of blasts but also on whether the blasts are short or prolonged.
For example, a sequence containing two short blasts communicates something different from a sequence containing one prolonged blast.
This makes the ship's whistle a form of standardised maritime communication.
Rather than relying on spoken radio communication alone, vessels can communicate certain manoeuvring intentions through internationally recognised sound signals.
Rule 33 — Sound Equipment
Rule 33 establishes requirements for sound-signalling appliances according to vessel length.
Under the rule, vessels 12 metres or more in length are required to carry a whistle and a bell. Vessels 100 metres or more in length are required to carry a gong in addition to the whistle and bell.
These appliances serve different purposes.
Whistle
The whistle is particularly important for manoeuvring and warning signals and for signals used in restricted visibility.
Bell
The bell is used for prescribed signals, particularly by vessels in situations such as anchoring and during restricted visibility.
Gong
For larger vessels, the gong provides an additional sound signal as required by the rules.
These requirements demonstrate that sound-signalling equipment is not simply emergency equipment. It forms part of the vessel's normal navigational safety arrangements.
The equipment must therefore be maintained and ready for immediate use.
Rule 34 — Manoeuvring and Warning Signals
Rule 34 is one of the most important parts of Part D for everyday ship navigation.
It deals with manoeuvring and warning signals made by vessels in sight of one another.
The signals provide information about a vessel's manoeuvre and can help reduce uncertainty during close encounters.
Some of the most commonly learned signals include:
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.”
These signals allow vessels to communicate their intended manoeuvre through sound.
The rule also provides for corresponding light signals in certain circumstances.
However, a sound signal should never be interpreted in isolation.
A bridge team should simultaneously consider:
- Visual observation
- Relative bearing
- Radar information
- ARPA tracking
- AIS information where appropriate
- Vessel speed
- Course
- Distance
- Existing COLREGs situation
For example, hearing one short blast should prompt the officer to understand the intended manoeuvre, but the actual movement of the vessel should also be monitored.
Communication and verification go together.
The Danger of Misinterpreting a Signal
Imagine two vessels approaching each other at night.
The officer hears a whistle signal.
Instead of immediately assuming what the other vessel will do, the bridge team should identify the signal, observe the vessel's movement and continuously assess the situation.
A whistle tells you about an intended manoeuvre; it does not replace the requirement for proper lookout and collision-risk assessment.
This is particularly important in busy shipping areas where several vessels may be manoeuvring simultaneously.
A professional bridge team therefore uses sound signals as one part of the navigational picture, not as a substitute for situational awareness.
Rule 35 — Sound Signals in Restricted Visibility
Restricted visibility presents one of the most challenging situations for navigational watchkeeping.
Fog, heavy rain, snow or similar conditions can significantly reduce the ability to see another vessel.
Rule 35 establishes the sound signals to be used by vessels in or near areas of restricted visibility.
In such conditions, a vessel may be detected by sound before it becomes visible.
For a power-driven vessel making way, the prescribed signal consists of one prolonged blast at intervals of not more than two minutes.
A power-driven vessel underway but stopped and making no way through the water uses two prolonged blasts in succession, with the interval between them being about two seconds, at intervals of not more than two minutes.
Other vessel categories have their own prescribed signals.
These include vessels such as:
- Vessels not under command
- Vessels restricted in their ability to manoeuvre
- Sailing vessels
- Vessels engaged in fishing
- Vessels engaged in towing or pushing
The signals help other vessels understand that a vessel is nearby even when visual contact has not been established.
Restricted Visibility Is Not the Time to Depend on One System
A fog signal may indicate the presence of another vessel, but it does not necessarily tell the bridge team everything they need to know.
The officer may need to combine:
Sound + Radar + ARPA + AIS + Echo information + Visual observation when available
Radar becomes particularly valuable because another vessel may be detected electronically before it becomes visible.
At the same time, the bridge team must understand the limitations of every system.
Rain clutter, sea clutter, poor radar settings, weak targets, AIS inaccuracies and other factors can affect the information available.
This is why COLREGs Part D works together with Rule 5 — Proper Look-out and Rule 6 — Safe Speed from Part B.
Rule 36 — Signals to Attract Attention
Sometimes a vessel may need to attract the attention of another vessel without using one of the specific manoeuvring signals.
Rule 36 addresses signals used to attract the attention of another vessel.
The rule allows a vessel, when necessary, to make light or sound signals that cannot be mistaken for signals authorised elsewhere in the COLREGs.
This is an important distinction.
A signal intended simply to attract attention must not create confusion with an established manoeuvring or warning signal.
In practical navigation, clarity is essential.
If a signal can be misunderstood, it can potentially create uncertainty rather than improve safety.
Therefore, the bridge team should always understand why a signal is being used and whether another vessel could reasonably interpret it correctly.
Rule 37 — Distress Signals
Rule 37 deals with distress signals.
These signals indicate that a vessel or person is in distress and requires assistance. The COLREGs reference the distress signals contained in Annex IV.
Distress signals include a range of internationally recognised methods.
Examples include:
- MAYDAY transmitted by radiotelephony
- SOS in Morse code
- Red rockets or red flares
- Orange smoke
- Continuous sounding with a fog-signalling apparatus
- Appropriate distress flags or visual signals
- Approved distress alerts transmitted through radiocommunication systems
- Emergency position-indicating radio beacons and other approved systems
The purpose is always the same:
To clearly communicate that assistance is required.
The IMO notes that these signals are intended to indicate distress and the need for assistance, and their use for other purposes is prohibited because of the possibility of confusion.
Sound Signals and Modern GMDSS
Modern maritime distress communication extends far beyond traditional whistles, bells and flares.
Today's ships operate within an integrated communications environment that includes systems associated with the Global Maritime Distress and Safety System (GMDSS).
Digital Selective Calling, satellite communications, EPIRBs and other approved systems can transmit distress alerts.
This does not make traditional COLREGs signals irrelevant.
Instead, it shows how maritime safety combines:
Traditional signalling + radio communication + digital systems + visual observation
A seafarer should therefore understand both the traditional COLREGs signals and the modern communication systems used onboard.
Why Part D Is Important on the Bridge
Part D connects directly with the practical responsibilities of a deck officer.
Consider a vessel navigating through dense fog.
The officer cannot see clearly ahead.
A prolonged whistle is heard.
The bridge team must immediately become alert to the possibility of another vessel.
Radar is checked.
ARPA targets are assessed.
AIS information is reviewed where appropriate.
The vessel's own speed and course are considered.
The bridge team determines whether a close-quarters situation or risk of collision may exist.
This demonstrates an important principle:
A sound signal is not the end of the assessment — it is the beginning of greater awareness.
The safest bridge teams develop the habit of responding to sound signals with systematic observation and assessment.
Learning COLREGs Part D as a Seafarer
For maritime students preparing for examinations, Part D can initially seem like a collection of numbers and signal patterns.
A better approach is to connect every signal with a situation.
Learn the signal together with the scenario
One short blast
→ Course alteration to starboard.
Two short blasts
→ Course alteration to port.
Three short blasts
→ Operating astern propulsion.
Prolonged blasts in restricted visibility
→ Identify the vessel's situation and understand that visibility is limited.
Special signals
→ Identify the vessel category and operational circumstance.
Distress signals
→ Recognise that immediate assistance may be required.
This method makes the rules easier to remember because each signal has a practical context.
The Bridge Team Must Listen as Well as Look
Modern navigation often focuses heavily on screens.
Radar displays, ECDIS, AIS targets and electronic alerts provide enormous amounts of information.
But COLREGs Rule 5 specifically requires a proper lookout by sight and hearing, as well as by all appropriate available means.
That makes listening a fundamental navigational skill.
A sound heard in fog may provide the first indication that another vessel is nearby.
The direction, timing and repetition of the signal can provide important clues that must then be correlated with other information.
For this reason, a good lookout is not simply someone watching the sea.
It is someone who is actively observing, listening, interpreting and reporting.
Part D in the Larger COLREGs Framework
COLREGs contains 41 rules divided into six parts:
- Part A — General: Rules 1–3
- Part B — Steering and Sailing: Rules 4–19
- Part C — Lights and Shapes: Rules 20–31
- Part D — Sound and Light Signals: Rules 32–37
- Part E — Exemptions: Rule 38
- Part F — Verification of Compliance: Rules 39–41
Part D therefore does not operate independently.
It complements Part B's steering and sailing rules and Part C's visual identification requirements.
A deck officer may first identify another vessel through lights, receive additional information through sound, confirm the target using radar and AIS, and then apply the appropriate steering and sailing rules.
That is how the different parts of COLREGs come together in real navigation.
Conclusion
COLREGs Part D — Rules 32–37 — establishes the sound and light signals that help vessels communicate, warn one another and respond to emergency situations.
From the basic definitions of short and prolonged blasts to manoeuvring signals, restricted-visibility signals, attention signals and distress signals, these rules provide a standardised language for maritime operations.
For seafarers, the most important lesson is not simply to memorise signal patterns.
It is to understand their meaning, context and relationship with the overall navigational situation.
When visibility disappears, sound becomes a vital source of information.
When a vessel needs to communicate its manoeuvre, the whistle provides a recognised signal.
And when a vessel is in distress, internationally recognised signals can communicate one of the most important messages at sea:
Assistance is required.
Understanding Part D helps seafarers listen more carefully, communicate more clearly and contribute to safer navigation.
Comments
Post a Comment