General Definitions

Vessel types: power-driven, sailing, fishing, NUC, RAM, CBD, seaplane, WIG.

Rule 3 provides essential definitions used throughout the COLREGs. Key definitions include:

The word "vessel" includes every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water.

The term "power-driven vessel" means any vessel propelled by machinery.

The term "sailing vessel" means any vessel under sail provided that propelling machinery, if fitted, is not being used.

The term "vessel engaged in fishing" means any vessel fishing with nets, lines, trawls or other fishing apparatus which restrict manoeuvrability, but does not include a vessel fishing with trolling lines or other fishing apparatus which do not restrict manoeuvrability.

The word "seaplane" includes any aircraft designed to manoeuvre on the water.

The term "vessel not under command" means a vessel which through some exceptional circumstance is unable to manoeuvre as required by these Rules and is therefore unable to keep out of the way of another vessel.

The term "vessel restricted in her ability to manoeuvre" means a vessel which from the nature of her work is restricted in her ability to manoeuvre as required by these Rules.

The term "vessel constrained by her draught" means a power-driven vessel which because of her draught in relation to the available depth and width of navigable water is severely restricted in her ability to deviate from the course she is following.

The word "underway" means that a vessel is not at anchor, or made fast to the shore, or aground.

The words "length" and "breadth" of a vessel mean her length overall and greatest breadth.

Vessels shall be deemed to be "in sight of one another" only when one can be observed visually from the other.

The term "restricted visibility" means any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or any other similar causes.

The term "Wing-In-Ground (WIG) craft" means a multimodal craft which, in its main operational mode, flies in close proximity to the surface by utilizing surface-effect action.

NUCNot Under Command — by exceptional circumstance

RAMRestricted Ability to Manoeuvre — by the nature of her work

When

An unplanned event prevents the vessel from manoeuvring (engine failure, broken rudder, dragging anchor in heavy weather).

The vessel's work itself prevents her from manoeuvring as required (cable laying, dredging, mine clearance, replenishment, launching aircraft).

What it says

Through some exceptional circumstance, the vessel is unable to manoeuvre as required by these Rules.

From the nature of her work the vessel is restricted in her ability to manoeuvre as required by these Rules.

What it demands

Two all-round red lights vertical at night, two black balls by day. She cannot keep out of the way of other vessels.

Three all-round vertical lights at night — red / white / red. By day, ball / diamond / ball.

Typical failure

Calling a working vessel NUC because she's restricted — RAM is the correct status when the work itself causes the limitation.

Treating any slow or busy ship as RAM. The restriction must come from the work, not from cargo, draught, or weather.

NUCNot Under Command — by exceptional circumstance

When: An unplanned event prevents the vessel from manoeuvring (engine failure, broken rudder, dragging anchor in heavy weather).
What it says: Through some exceptional circumstance, the vessel is unable to manoeuvre as required by these Rules.
What it demands: Two all-round red lights vertical at night, two black balls by day. She cannot keep out of the way of other vessels.
Typical failure: Calling a working vessel NUC because she's restricted — RAM is the correct status when the work itself causes the limitation.

RAMRestricted Ability to Manoeuvre — by the nature of her work

When: The vessel's work itself prevents her from manoeuvring as required (cable laying, dredging, mine clearance, replenishment, launching aircraft).
What it says: From the nature of her work the vessel is restricted in her ability to manoeuvre as required by these Rules.
What it demands: Three all-round vertical lights at night — red / white / red. By day, ball / diamond / ball.
Typical failure: Treating any slow or busy ship as RAM. The restriction must come from the work, not from cargo, draught, or weather.

STCW Bridge Watch Lens

Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.

Build the traffic picture with sight, hearing, radar/ARPA and chart context.

Do not let AIS or one isolated bearing replace systematic observation.

After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.

Exam Focus

Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.

If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.

Key Takeaways

A vessel under sail using its engine is classified as power-driven

Fishing vessels must have apparatus that restricts manoeuvrability

NUC status requires exceptional circumstances preventing manoeuvring

RAM status comes from the nature of the work being performed

"Underway" means not anchored, moored, or aground — a vessel adrift IS underway

"In sight of one another" requires VISUAL observation — radar contact alone does not count

Common Mistakes

Classifying a sailing vessel with engine running as a sailing vessel

Confusing NUC with RAM — NUC is exceptional, RAM is work-related

Confusing CBD with NUC/RAM — CBD is specifically about draught vs depth/width of navigable water

Thinking "underway" means making way — a vessel adrift with engines stopped is still underway

Test Your Knowledge

Test your knowledge and prove your mastery.

ApplicationCOLREG applies to all vessels on the high seas and in connected waters navigable by seagoing vessels, subject to closely conforming local special rules and IMO-adopted traffic separation schemes.

Last reviewed: May 20, 2026

STCW Bridge Watch Lens

Exam Focus

Key Takeaways

Common Mistakes

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