What is the difference between a vessel not under command and a vessel restricted in her ability to manoeuvre?

A vessel not under command is one which through some exceptional circumstance is unable to manoeuvre as required by these Rules (Rule 3(f)). A vessel restricted in her ability to manoeuvre is one which from the nature of her work is restricted in her ability to manoeuvre (Rule 3(g)): NUC is exceptional, RAM is work-related.

What does “underway” mean in the COLREGs?

Rule 3(i) defines underway as not at anchor, not made fast to the shore and not aground. Underway does not mean making way: a vessel adrift with her engines stopped is still underway.

General Definitions

Q: Is a sailing vessel using her engine still a sailing vessel?

No — she is a power-driven vessel. Rule 3(c) defines a sailing vessel as any vessel under sail provided that propelling machinery, if fitted, is not being used, and Rule 3(b) defines a power-driven vessel as any vessel propelled by machinery.

Top row: power-driven, sailing, fishing, and not under command. Bottom row: restricted in ability to manoeuvre while working, constrained by draught, seaplane, and WIG craft in near-surface flight. These categories depend on operating circumstances, not hull appearance alone.

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 and is therefore unable to keep out of the way of another vessel.

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.

Length & breadth, vessels in sight, restricted visibility — visual definitions.

IMO COLREG 1972Official text

Full wording of the rule, reproduced from the official source below.

For the purpose of these Rules, except where the context otherwise requires: (a) 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. (b) The term power-driven vessel means any vessel propelled by machinery. (c) The term sailing vessel means any vessel under sail provided that propelling machinery, if fitted, is not being used. (d) The term vessel engaged in fishing means any vessel fishing with nets, lines, trawls or other fishing apparatus which restrict maneuverability, but does not include a vessel fishing with trolling lines or other fishing apparatus which do not restrict maneuverability. (e) The word seaplane includes any aircraft designed to maneuver on the water. (f) The term vessel not under command means a vessel which through some exceptional circumstance is unable to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel. (g) The term vessel restricted in her ability to maneuver means a vessel which from the nature of her work is restricted in her ability to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel. The term vessels restricted in their ability to maneuver shall include but not be limited to: (i) a vessel engaged in laying, servicing or picking up a navigation mark, submarine cable or pipeline; (ii) a vessel engaged in dredging, surveying or underwater operations; (iii) a vessel engaged in replenishment or transferring persons, provisions or cargo while underway; (iv) a vessel engaged in the launching or recovery of aircraft; (v) a vessel engaged in mine clearance operations; (vi) a vessel engaged in a towing operation such as severely restricts the towing vessel and her tow in their ability to deviate from their course. (h) The term vessel constrained by her draft means a power-driven vessel which, because of her draft 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. (i) The word underway means that a vessel is not at anchor, or made fast to the shore, or aground. (j) The words length and breadth of a vessel mean her length overall and greatest breadth. (k) Vessels shall be deemed to be in sight of one another only when one can be observed visually from the other. (l) 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. (m) 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.

Source:

U.S. Coast Guard — International Navigation Rules (2024)

NUCNot Under Command — by exceptional circumstance

RAMRestricted Ability to Manoeuvre — by the nature of her work

When

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

(g) The term vessel restricted in her ability to maneuver means a vessel which from the nature of her work is restricted in her ability to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.

What it says

What it demands

NUC: two all-round red lights and two black balls vertically. When making way, add sidelights and sternlight, without masthead lights. Rule 27(g) provides the under-12 m exemption except diving vessels.

RAM other than mine clearance: red-white-red all-round lights and ball-diamond-ball. When making way, add masthead light(s), sidelights and sternlight. Rule 27(b)(iv) adds anchor signals; dredging/underwater operations have the distinct provisions of 27(d), and mine clearance uses 27(f).

Typical failure

Rule 18 does not rank NUC above RAM. Its opening exceptions are Rules 9, 10 and 13; fishing vessels underway keep clear of NUC/RAM so far as possible. Equipment failure only creates NUC when Rule 3(f) is actually met; responsibility under Rule 2 remains.

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: (f) The term vessel not under command means a vessel which through some exceptional circumstance is unable to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.
What it says: (f) The term vessel not under command means a vessel which through some exceptional circumstance is unable to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.
What it demands: NUC: two all-round red lights and two black balls vertically. When making way, add sidelights and sternlight, without masthead lights. Rule 27(g) provides the under-12 m exemption except diving vessels.
Typical failure: Rule 18 does not rank NUC above RAM. Its opening exceptions are Rules 9, 10 and 13; fishing vessels underway keep clear of NUC/RAM so far as possible. Equipment failure only creates NUC when Rule 3(f) is actually met; responsibility under Rule 2 remains.

RAMRestricted Ability to Manoeuvre — by the nature of her work

When: (g) The term vessel restricted in her ability to maneuver means a vessel which from the nature of her work is restricted in her ability to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.
What it says: (g) The term vessel restricted in her ability to maneuver means a vessel which from the nature of her work is restricted in her ability to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.
What it demands: RAM other than mine clearance: red-white-red all-round lights and ball-diamond-ball. When making way, add masthead light(s), sidelights and sternlight. Rule 27(b)(iv) adds anchor signals; dredging/underwater operations have the distinct provisions of 27(d), and mine clearance uses 27(f).
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

Rules 4–10 apply in any visibility.

Rules 11–18 apply to vessels in sight of one another; Rule 19 applies to vessels not in sight when navigating in or near an area of 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.

For vessels in sight, every overtaking vessel keeps clear until finally past and clear.

Rule 13 takes precedence over Part B Sections I and II; it does not override the other Parts of COLREG.

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 also being propelled by machinery 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

Practise this topic and review the explanation for each answer.

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: September 9, 2026