Rules 20–31 | The lights and shapes every seafarer must recognise

At sea, navigation does not stop when daylight disappears. In darkness, restricted visibility and busy traffic conditions, a vessel communicates important information through navigation lights and day shapes. These visual signals help other vessels understand what type of vessel they are observing, whether it is underway, anchored, towing, fishing, restricted in its ability to manoeuvre or constrained by draught.

This is the purpose of Part C of the International Regulations for Preventing Collisions at Sea (COLREGs). Part C contains Rules 20–31, covering lights and shapes. The International Maritime Organization (IMO) identifies Part C as the section dealing with lights and shapes, supported by technical requirements in Annex I.

For a deck officer keeping a night watch, recognising a vessel's lights is not simply a matter of memorising colours. The colour, position, number, arrangement, relative bearing and visibility of lights, together with day shapes, can provide essential information for assessing the situation and applying the steering and sailing rules.

Rule 20 — Application

Rule 20 establishes when the requirements for lights apply.

The rules concerning lights apply from sunset to sunrise. During this period, the prescribed lights must be displayed and must comply with the relevant requirements.

The rule also provides for lights to be exhibited in conditions of restricted visibility during daylight when circumstances require them.

For a bridge team, this means that navigation lights should never be treated as merely a night-time formality. Their correct operation is an essential part of maintaining situational awareness and making other vessels visible.

A proper bridge watch therefore includes checking that navigation lights are operating correctly, particularly before and during hours of darkness.

Rule 21 — Definitions of Lights

Rule 21 establishes important definitions used throughout Part C.

Among the principal terms are:

Masthead light
Sidelights
Sternlight
Towing light
All-round light
Flashing light

Understanding these definitions is essential because the location and colour of a light help determine what aspect of a vessel is being observed.

For example, the sidelights consist of:

Green — starboard side

Red — port side

This simple colour distinction is one of the most fundamental pieces of knowledge for anyone standing a navigational watch.

A vessel observed at night is effectively communicating through a visual language. The bridge team must interpret that language correctly and combine it with information from radar, AIS, ECDIS, visual bearings and other available means.

Rule 22 — Visibility of Lights

It is not enough for a vessel to carry the correct lights. They must also have the required visibility range.

Rule 22 establishes minimum ranges at which prescribed lights should be visible, depending on the type and size of vessel.

The requirements are designed to ensure that vessels can detect one another at an appropriate distance.

This is particularly important during:

Night navigation
Heavy traffic
Restricted visibility
Coastal navigation
Port approaches
TSS operations
Overtaking situations

A weak, damaged, incorrectly positioned or obscured navigation light can significantly affect another vessel's ability to identify the situation correctly.

This is why navigation-light maintenance is an important part of safe shipboard operations.

Rule 23 — Power-Driven Vessels Underway

Rule 23 deals with the lights carried by power-driven vessels underway.

A typical merchant vessel at night may display:

Masthead light(s)
Port sidelight
Starboard sidelight
Sternlight

The arrangement provides information about the vessel's direction and aspect.

Imagine a deck officer observing another ship during a night watch. Seeing a green sidelight indicates that the observer is seeing the vessel's starboard side. Seeing a red sidelight indicates the vessel's port side.

Seeing both sidelights can indicate an approaching vessel presenting its forward aspect, while a sternlight indicates the vessel is being observed from behind.

Correct interpretation can therefore contribute directly to understanding the relative movement of another vessel.

Rule 24 — Towing and Pushing

Towing operations create a special navigation situation because the configuration may extend significantly beyond the towing vessel itself.

Rule 24 establishes the lights for vessels engaged in towing and pushing operations.

A towing vessel may display additional lights that distinguish it from an ordinary power-driven vessel.

The length of the tow and the arrangement of the towing operation are important considerations. A vessel approaching a towing operation must understand that the physical hazard may extend well beyond the hull of the towing vessel.

For deck officers, recognising towing lights is therefore particularly important when operating in busy waterways, coastal areas and restricted waters.

Rule 25 — Sailing Vessels and Vessels Under Oars

Rule 25 addresses lights for sailing vessels underway and vessels under oars.

Sailing vessels have specific lighting arrangements that distinguish them from power-driven vessels.

This distinction becomes important when a sailing vessel and a power-driven vessel encounter one another.

The visual identification provided by the lights allows the bridge team to determine what category of vessel is being observed before applying the appropriate COLREGs requirements.

For small vessels, the correct display of navigation lights is especially important because their size can make them difficult to detect among background lights, coastal illumination and other traffic.

Rule 26 — Fishing Vessels

Fishing vessels have specific light requirements because their operations can affect their ability to manoeuvre.

Rule 26 provides the requirements for vessels engaged in fishing.

The lights displayed can communicate the nature of the fishing operation and help other vessels understand that the fishing vessel may not be able to manoeuvre like an ordinary power-driven vessel.

This is especially significant in areas with concentrated fishing activity.

A vessel approaching a fishing vessel at night should not rely on a single visual clue. The bridge team should consider:

Navigation lights
Relative bearing
Radar information
AIS information where available
Vessel movement
Fishing gear and operational circumstances
Surrounding traffic

The objective is to build a complete picture of the situation before taking action.

Rule 27 — Vessels Not Under Command and Restricted in Their Ability to Manoeuvre

Some of the most important lights in Part C are those identifying vessels with special manoeuvring circumstances.

Rule 27 covers vessels:

Not Under Command (NUC)
Restricted in Their Ability to Manoeuvre (RAM)
Certain vessels engaged in specialised operations, including dredging or underwater operations
Mine-clearance operations

These vessels may display combinations of lights and shapes that communicate their special status.

For a navigating officer, recognising these signals can fundamentally change the interpretation of an encounter.

For example, a vessel engaged in operations that restrict its manoeuvrability may not be able to take avoiding action in the same way as an ordinary merchant vessel.

Therefore, the visual information supplied by Rule 27 connects directly with the responsibilities established elsewhere in COLREGs, particularly the rules concerning responsibilities between vessels.

Rule 28 — Vessels Constrained by Their Draught

Large merchant ships operating in shallow or restricted waters may be significantly limited by their draught.

Rule 28 provides a specific light and shape arrangement for vessels constrained by their draught.

The signal helps other vessels recognise that the vessel's ability to deviate from its course may be limited by available water depth.

This is particularly relevant around:

Narrow channels
Dredged approaches
River entrances
Port approaches
Shallow coastal waters

For deck officers, understanding this signal is part of appreciating the operational limitations of large vessels.

Rule 29 — Pilot Vessels

Pilot vessels perform an important role in maritime navigation by providing pilotage services.

Rule 29 specifies the lights for pilot vessels engaged on pilotage duty.

The distinctive light arrangement enables other vessels to recognise the pilot vessel at night.

Correct identification is particularly useful in approaches to ports and other areas where pilotage operations are common.

A bridge team should understand that recognising a pilot vessel is different from simply identifying a vessel's physical type. The lights communicate its operational status under the COLREGs.

Rule 30 — Anchored and Aground Vessels

A vessel that is not underway presents a different navigational situation.

Rule 30 covers vessels at anchor and aground.

An anchored vessel displays prescribed anchor lights and, where applicable, day shapes.

A vessel aground has additional requirements that distinguish it from an ordinary anchored vessel.

This distinction matters because a vessel observed at night with limited movement may initially appear simply to be another vessel in the area. Correct interpretation of its lights and shapes helps the approaching vessel understand its status.

Anchored vessels can also be particularly difficult to identify against a background of port and shore lights. Continuous lookout and careful radar correlation therefore remain important.

Rule 31 — Seaplanes

Rule 31 addresses seaplanes.

Where it is impracticable for a seaplane or WIG craft to exhibit the prescribed lights and shapes in the required characteristics or positions, it must display lights and shapes as closely similar as possible.

Although seaplane encounters are relatively uncommon for many merchant vessels, the rule demonstrates the broad scope of COLREGs.

The regulations are designed to provide a consistent framework for identifying different types of objects and vessels that may be encountered on navigable waters.

Why Part C Matters to Every Deck Officer

Part C may appear at first to be a collection of technical requirements about navigation lights. In practice, it forms an important part of situational awareness at sea.

Consider a night watch.

The officer of the watch sees:

Red + green lights ahead.

The next question is not simply, “What colour are those lights?”

The officer must determine:

What type of vessel is it?
What aspect am I seeing?
Is it underway?
What is its relative bearing?
Is the bearing changing?
What is its movement on radar?
Is there a risk of collision?
Which COLREGs rule applies?
What action is required?

This shows how Part C works together with Part B.

Part C helps identify the situation.
Part B governs the conduct of vessels.

A competent bridge team uses both.

Lights, Shapes and Modern Bridge Technology

Modern bridges contain sophisticated equipment such as radar, ARPA, AIS and ECDIS. These systems provide valuable information, but they do not eliminate the need to understand navigation lights and shapes.

The IMO's COLREG framework continues to recognise the importance of visual identification and proper lookout.

A deck officer should therefore be capable of combining:

Visual observation + hearing + radar + ARPA + AIS + ECDIS + professional judgement

rather than depending on one source alone.

AIS, for example, can provide useful identification and voyage information, but the visual appearance and actual movement of a vessel must still be assessed. Similarly, radar can detect targets that may not yet be visually obvious.

The strongest bridge watch is one where different sources of information are continuously compared.

Learning the Visual Language of the Sea

For maritime students and junior officers, COLREGs Part C is best learned through repeated visual practice.

Instead of memorising isolated rules, learners can study common night-time scenarios:

Power-driven vessel → identify masthead and sidelights

Towing vessel → recognise towing configuration

Fishing vessel → identify fishing signals

NUC/RAM vessel → recognise special status

Pilot vessel → identify pilotage lights

Anchored vessel → recognise anchor lights

Aground vessel → distinguish additional signals

This approach turns COLREGs from a theoretical subject into a practical bridge-watch skill.

Conclusion

COLREGs Part C — Rules 20–31 — provides the visual language used by vessels to communicate their status and circumstances, particularly at night and in conditions where lights and shapes are essential for identification.

From ordinary power-driven vessels to towing operations, fishing vessels, NUC/RAM vessels, pilot vessels, anchored vessels and vessels constrained by draught, each prescribed arrangement carries specific information.

For every seafarer, learning these signals is more than preparing for an examination. It is part of developing the ability to observe, identify, assess and respond during real navigation.

When the sun goes down, the sea does not become silent.
It begins to speak through lights and shapes.

The IMO identifies Part C as Rules 20–31, within the six-part COLREGs framework of 41 rules.

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COLREGs Part C: Lights and Shapes — Understanding the Visual Language of Ships at Sea