A Vessel is "Not Under Command" (NUC) when it is unable to maneuver as required. The vessel cannot physically comply with the Rules for ordinary vessels, and so they are granted special privileges.
A vessel not under command shall exhibit:
- two all-round red lights in a vertical line where they can best be seen;
- two balls or similar shapes in a vertical line where they can best be seen;
- when making way through the water, in addition to the lights prescribed in this paragraph, sidelights and a sternlight.
Examples: a vessel with a disabled rudder.