A vessel is Not Under Command (NUC) when an exceptional circumstance leaves it unable to maneuver as COLREGS normally requires — typically a mechanical breakdown, steering gear failure, or similar genuine incapacity.
Required Signals
- By day — two black balls, displayed vertically
- By night — two all-round red lights, displayed vertically
These signals alert other vessels that the normal COLREGS give-way/stand-on rules don’t apply as expected — an NUC vessel cannot be relied upon to maneuver even where it would normally be obligated to.
What Qualifies as NUC vs What Doesn’t
NUC status requires a genuine exceptional circumstance making maneuvering impossible — a real steering failure or main engine breakdown, for instance. It’s distinct from a vessel that’s simply restricted in its ability to maneuver due to the nature of its work (like a dredger or vessel engaged in towing), which uses different signals (Restricted in Ability to Manoeuvre, RAM).
Why Other Vessels Must Respond Differently
Seeing NUC signals, other vessels must take early, positive action to keep clear — the normal expectation that the NUC vessel will give way or maintain course as usual simply doesn’t apply, since it genuinely cannot maneuver as required.
Exam Relevance
NUC signals, the distinction from RAM status, and appropriate response by other vessels are core COLREGS topics in deck officer competency exams.