Not Under Command
Abbreviation: NUC
Vessel which through exceptional circumstance cannot manoeuvre as required by the rules; shows two black balls / two red lights vertically.
In practice
NUC vessel reports status via VHF and lights. Other vessels keep clear; salvors and tugs may be inbound.
Regulatory detail & full definition
Vessel Not Under Command — defined in COLREG Rule 3(f) as a vessel which through some exceptional circumstance is unable to manoeuvre as required by the Rules and is therefore unable to keep out of the way of another vessel. Examples include propulsion failure, steering failure, or major engine breakdown. NUC vessels show by day two black balls in a vertical line and by night two all-round red lights in a vertical line; they have priority over all other vessel categories under Rule 18.
Authoritative source: COLREG 1972 ↗
Related
Source: COLREG 1972 Rule 3(f)
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