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Rule 27: Vessels Not Under Command or Restricted in Their Ability to Maneuver (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 27 says a vessel not under command shows two all-round red lights in a vertical line, or two balls by day. A vessel restricted in her ability to maneuver shows red, white, red all-round lights, or ball, diamond, ball. Both add sidelights and a sternlight when making way, and RAM adds masthead lights too.

The rule text

two all-round red lights in a vertical line where they can best be seen;

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 27

The highest and lowest of these lights shall be red and the middle light shall be white;

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 27

The highest and lowest of these shapes shall be balls and the middle one a diamond;

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 27

What it means on the water

  • NUC: two all-round reds, two balls; add sidelights and sternlight when making way (Rule 27(a)).
  • RAM: red-white-red, ball-diamond-ball; add masthead light or lights, sidelights, and sternlight when making way; add Rule 30 anchor signals at anchor (Rule 27(b)).
  • Dredging or underwater operations with an obstruction: two reds or two balls on the obstructed side, two greens or two diamonds on the side where another vessel may pass (Rule 27(d)).
  • Diving vessel too small for the full set: red-white-red plus a rigid replica of Code flag "A" not less than 1 meter high (Rule 27(e)).
  • Mine clearance: three all-round greens or three balls, dangerous to approach within 1000 meters (Rule 27(f)). Under 12 meters, except diving, these signals aren't required (27(g)).

International vs Inland

Mostly the same. Inland Rule 27(d)(iv) adds lighting for floating or trestle-supported dredge pipelines, starting with a row of yellow lights flashing 50 to 70 times per minute. The towing paragraph 27(c) is worded differently but pairs the RAM signals with the Rule 24 towing signals in both.

Common exam traps

  • Mixing NUC (two reds) with aground (two reds plus anchor lights, Rule 30).
  • Thinking these are distress signals. Rule 27(h) says they're not.
  • Reading two greens on a dredge as danger. Greens mark the side where another vessel may pass.

Try 3 questions on Rule 27

  1. Ball, diamond, ball in a vertical line means:

    1. Not under command
    2. Restricted in ability to maneuver
    3. Aground
    4. Constrained by draft
    Show answer

    B. Restricted in ability to maneuver Rule 27(b)(ii).

  2. A dredge shows two all-round green lights on her port side. That side is:

    1. Where the obstruction is
    2. Where another vessel may pass
    3. Her anchor side
    4. Off limits
    Show answer

    B. Where another vessel may pass Rule 27(d)(ii).

  3. A NUC vessel making way at night shows:

    1. Two all-round reds only
    2. Two all-round reds, sidelights, and sternlight
    3. Red-white-red and masthead lights
    4. Three all-round reds
    Show answer

    B. Two all-round reds, sidelights, and sternlight Rule 27(a)(iii).

Frequently asked questions

What does red over red mean?

Two all-round red lights in a vertical line mark a vessel not under command (Rule 27(a)). With anchor lights, two reds mean aground (Rule 30(d)).

Are Rule 27 signals distress signals?

No. Rule 27(h) says they are not; distress signals are in Annex IV.

What flag does a small diving vessel show?

A rigid replica of International Code flag "A" not less than 1 meter in height (Rule 27(e)).

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