COLREGS Rule 27 — Vessels Not Under Command or Restricted in Ability to Maneuver

Rule Text

What it means on the water

• →NUC: 2 all-around RED lights vertical + 2 balls by day. Sidelights and sternlight when making way.
• →RAM: 3 all-around lights vertical (RED/WHITE/RED) + ball/diamond/ball by day. Full set of underway lights when making way.
• →RAM dredging/underwater ops: on obstructed side = 2 red; on clear side = 2 green (night). Day: 2 balls on obstructed, 2 diamonds on clear.
• →NUC vessel making way shows no masthead light — only red/red + sidelights + sternlight.
• →RAM vessel making way shows masthead light(s) + sidelights + sternlight in addition to the red/white/red.

Common exam mistakes

• ✗Giving NUC three red lights — NUC gets TWO red lights. RAM gets three (red/white/red).
• ✗Thinking NUC must show masthead lights when making way — NUC does NOT show masthead lights.
• ✗Confusing the dayshapes: NUC = two balls, RAM = ball/diamond/ball.
• ✗Missing that RAM dredging vessels show different colors on obstructed vs. clear sides.

USCG exam questions — Rule 27

These questions are drawn from the same pool used in real USCG licensing exams. Correct answers and explanations are shown.

1. 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. 2. By day, what shapes does a vessel restricted in its ability to maneuver display?

   • A.A black cylinder
   • B.Two black balls in a vertical line
   • C.Ball, diamond, ball — three shapes in a vertical line✓
   • D.Two cones with apices together

   Why: Rule 27(b)(ii) requires three shapes in a vertical line by day for a RAM vessel: ball on top, diamond in the middle, and ball at the bottom. This mirrors the night pattern of red-white-red lights.

3. 3. 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.

4. 4. 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.'

5. 5. Which of the following is specifically listed as an example of a vessel 'restricted in her ability to maneuver' (RAM) under Rule 3(g)?

   • A.A vessel engaged in laying, servicing, or picking up a navigation mark✓
   • B.A vessel that has run aground and cannot move
   • C.A vessel with a steering failure that cannot change course
   • D.A vessel anchored in a narrow channel

   Why: Rule 3(g) lists vessels engaged in laying, servicing, or picking up a navigation mark, submarine cable, or pipeline as examples of RAM vessels — these operations limit maneuverability and are thus explicitly recognized.

6. 6. A vessel conducting replenishment-at-sea (RAS) operations is best classified as:

   • A.Restricted in her ability to maneuver✓
   • B.Not under command
   • C.Constrained by her draft
   • D.A power-driven vessel underway with no special status

   Why: COLREGS Rule 3(g)(iii) expressly classifies a vessel "engaged in replenishment or transferring persons, provisions or cargo while underway" as restricted in her ability to maneuver, so a vessel conducting replenishment-at-sea (RAS) is RAM. The nature of the work — maintaining precise course and speed alongside the supplying/receiving ship — severely restricts her ability to deviate, distinguishing her from a not-under-command vessel (Rule 3(f)), which is disabled by exceptional circumstance rather than by the nature of her operations.

7. 7. 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).)

8. 8. A vessel not under command that is making way through the water at night shall exhibit which lights?

   • A.Two all-round red lights in a vertical line, sidelights, and a stern light✓
   • B.Two all-round red lights in a vertical line only
   • C.Two all-round red lights, masthead lights, and sidelights
   • D.Three all-round red lights in a vertical line

   Why: Rule 27(b) requires a NUC vessel making way to exhibit two all-round red lights in a vertical line plus sidelights and a stern light, while a NUC vessel not making way shows only the two red lights without sidelights or stern light.

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 does not show masthead lights. During the day, it shows two black balls in a vertical line.

What is the difference between NUC and RAM lights?

NUC shows two all-around red lights (no masthead). RAM shows three all-around lights in a vertical line — red, white, red — and when making way adds masthead lights, sidelights, and sternlight. NUC dayshape is two balls; RAM dayshape is a ball over a diamond over a ball.

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