COLREGS Rule 27 — Navigation Lights
Vessel Not Under Command — Navigation Lights (COLREGS Rule 27)
A vessel not under command (NUC) — unable to maneuver due to an exceptional circumstance such as a breakdown — shows two all-around red lights in a vertical line. When making way it adds sidelights and a sternlight. It does NOT show masthead lights. By day it shows two black balls in a vertical line.
Condition: Underway but unable to maneuver as required by the Rules due to an exceptional circumstance
Navigation lights required
| Light | Color | Arc | Position |
|---|---|---|---|
| All-around red light (upper) | Red | 360° | Upper of two vertical all-around lights |
| All-around red light (lower) | Red | 360° | Lower of two vertical all-around lights |
| Port sidelight (when making way only) | Red | 112.5° | Port side |
| Starboard sidelight (when making way only) | Green | 112.5° | Starboard side |
| Sternlight (when making way only) | White | 135° | Stern |
Port sidelight (when making way only): Added only when the vessel is making way through the water
Starboard sidelight (when making way only): Added only when the vessel is making way through the water
Sternlight (when making way only): Added only when the vessel is making way through the water
Dayshapes
| Shape | Position | When displayed |
|---|---|---|
| Two black balls in a vertical line | Where they can best be seen | When not under command, during daylight hours |
Exceptions and size rules
- →A NUC vessel does NOT show masthead lights at any time — not when making way, not when stopped.
- →A NUC vessel shows only two red lights plus sidelights/sternlight when making way — the overall appearance is simpler than RAM.
Inland Rules differences
- !US Inland Rules are identical to COLREGS for NUC lights — no practical difference.
Common exam mistakes
- ✗Adding a white masthead light to a NUC vessel — NUC vessels never show masthead lights.
- ✗Giving a NUC vessel three red lights — NUC has TWO red lights; three red lights in a vertical line indicate a vessel constrained by draft.
- ✗Confusing the NUC dayshape (two balls) with the RAM dayshape (ball/diamond/ball) or the aground dayshape (three balls).
- ✗Thinking NUC and RAM are interchangeable — NUC arises from an exceptional/unexpected circumstance (breakdown, casualty); RAM is a planned operational constraint.
USCG exam questions — Vessel Not Under Command
These questions are drawn from the same pool used in real USCG licensing exams. Correct answers and explanations are shown.
1. A vessel not under command (NUC) shall show:
- A.One all-round red light and a flashing white light
- B.Two all-round white lights in a vertical line
- C.An all-round red light over an all-round white light
- D.Two all-round red lights in a vertical line and, when making way, sidelights and sternlight✓
Why: Rule 27(a) prescribes two all-round red lights in a vertical line for a NUC vessel. When making way, she adds sidelights and a sternlight. Red over red — not white — is the critical identifier distinguishing NUC from fishing (red over white) or RAM (red-white-red).
2. A vessel restricted in her ability to maneuver (RAM) shall show which all-round lights?
- A.Red over red — two lights in a vertical line
- B.Green over white — two lights in a vertical line
- C.Red, white, red — three lights in a vertical line✓
- D.White, red, white — three lights in a vertical line
Why: Rule 27(b)(i) requires a RAM vessel to show three all-round lights in a vertical line: the uppermost and lowest lights red, and the middle light white. The mnemonic 'red-white-red' (or a ball-diamond-ball shape by day) signals restricted maneuverability.
3. You observe a vessel at night showing red-white-red all-round lights in a vertical line, two masthead lights, sidelights, and a sternlight. What is this vessel?
- A.A vessel not under command making way
- B.A vessel engaged in dredging or underwater operations making way
- C.A vessel restricted in ability to maneuver and making way✓
- D.A vessel constrained by her draft
Why: Red-white-red vertical lights identify a RAM vessel. The additional masthead lights, sidelights, and sternlight indicate she is underway and making way. A NUC vessel would show red over red, not red-white-red.
4. A vessel engaged in minesweeping operations shall show, in addition to the lights prescribed for a power-driven vessel underway:
- A.One all-round green light at the masthead and one at each end of the sweep
- B.Three all-round green lights or three black balls — one at the masthead and one at each end of the sweep✓
- C.Two all-round green lights in a vertical line
- D.A yellow flashing light
Why: Under COLREGS/Inland Rule 27(f) (33 CFR 83.27(f)), a vessel engaged in mine clearance operations exhibits, in addition to power-driven-underway lights, three all-round green lights or three balls — one near the foremast head and one at each end of the fore yard — signaling that it is dangerous to approach within 1000 metres. Only option B states the correct count (three green lights / three balls); one green light, two vertical green lights, and a yellow flashing light are not the mine-clearance signal.
5. By day, a vessel not under command shall display:
- A.A black cylinder
- B.Ball, diamond, ball in a vertical line
- C.A single black cone, apex up
- D.Two black balls in a vertical line✓
Why: Rule 27(a)(ii) requires a NUC vessel to show two black balls in a vertical line by day — the daytime equivalent of the two all-round red lights. Two balls means 'helpless'; three balls means 'at anchor.'
6. A vessel engaged in underwater operations that is unable to deviate from its course shall show the RAM lights PLUS, on the side with the obstruction, which additional lights?
- A.Two all-round red lights vertically and on the clear side two all-round green lights vertically✓
- B.A flashing red light on the obstruction side only
- C.An all-round white light on each side
- D.A yellow all-round light above the RAM lights
Why: Rule 27(d) prescribes that a vessel conducting underwater operations that is at anchor or underway but impeded shall show two all-round red lights (vertically) on the obstruction side and two all-round green lights (vertically) on the safe passing side, in addition to the RAM lights.
7. Which situation would most likely qualify a vessel as 'not under command' under Rule 3(f)?
- A.Complete loss of steering due to a rudder casualty in heavy weather✓
- B.Reduced speed due to fouled propeller requiring diver inspection
- C.Maneuvering restrictions caused by towing a barge astern
- D.Inability to alter course due to a narrow channel
Why: NUC status requires an 'exceptional circumstance' rendering the vessel unable to maneuver as required — a complete loss of steering from a rudder casualty is the classic example of such an exceptional circumstance, distinct from operational or navigational restrictions.
8. A vessel not under command (NUC) at anchor shall exhibit which lights?
- A.Two all-round red lights in a vertical line and anchor lights✓
- B.Two all-round red lights in a vertical line only
- C.One all-round red light and anchor lights
- D.Two all-round red lights and sidelights
Why: A vessel not under command shows two all-round red lights in a vertical line under Rule 27(a)(i), and when she is also at anchor she adds the anchor lights of Rule 30 because both conditions apply simultaneously — hence two all-round red lights plus anchor lights (option 0). (Note: the two reds are prescribed by Rule 27(a), the not-under-command rule; the at-anchor light addition for restricted-in-ability-to-maneuver vessels is the separate Rule 27(b)(iv).)
Frequently asked questions
- What lights does a vessel not under command show?
- A NUC vessel shows two all-around red lights in a vertical line. When making way it also shows sidelights and a sternlight. It never shows masthead lights. During daylight it shows two black balls in a vertical line.
- What is the difference between a NUC vessel and a RAM vessel in terms of lights?
- A NUC vessel shows two all-around red lights and never shows masthead lights. A RAM vessel shows three all-around lights in a vertical line — red, white, red — and when making way adds masthead lights, sidelights, and a sternlight. The NUC dayshape is two balls; the RAM dayshape is a ball over a diamond over a ball.
- Does a NUC vessel show masthead lights when it is making way?
- No. A NUC vessel does not show masthead lights under any circumstances. When making way it adds only sidelights and a sternlight to the two all-around red lights. This is a commonly tested distinction — do not add a masthead light to a NUC vessel.
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