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Rule 23: Power-driven Vessels Underway (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 23 says a power-driven vessel underway shows a masthead light forward, a second masthead light abaft and higher (optional under 50 meters), sidelights, and a sternlight. Under 12 meters she may show an all-round white light and sidelights instead. Air-cushion vessels and WIG craft add flashing lights.

The rule text

a second masthead light abaft of and higher than the forward one; except that a vessel of less than 50 meters in length shall not be obliged to exhibit such light but may do so;

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 23

A power-driven vessel of less than 12 meters in length may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and sidelights;

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 23

What it means on the water

  • Basic set (Rule 23(a)): masthead light forward, second masthead light abaft and higher, sidelights, sternlight.
  • Under 50 meters, the second masthead light is optional, so one masthead light is common.
  • Air-cushion vessel in non-displacement mode: add an all-round flashing yellow light (Rule 23(b)).
  • WIG craft taking off, landing, or in flight near the surface: add a high intensity all-round flashing red light (Rule 23(c)).
  • Under 12 meters: an all-round white light and sidelights may replace the Rule 23(a) lights (International 23(d)(i), Inland 23(d)).

International vs Inland

International Rule 23(d)(ii) lets a power-driven vessel under 7 meters whose maximum speed does not exceed 7 knots show an all-round white light, with sidelights if practicable; Inland has no such provision. Inland Rule 23(e) lets a vessel on the Great Lakes carry an all-round white light in place of the second masthead light and sternlight.

Common exam traps

  • Requiring two masthead lights on a vessel under 50 meters. The second one is optional.
  • Giving the air-cushion vessel a flashing red light. It's flashing yellow; flashing red is the WIG craft.
  • Applying the 7 meter / 7 knot option to Inland waters.

Try 3 questions on Rule 23

  1. A 40-meter power-driven vessel underway must show how many masthead lights at minimum?

    1. None
    2. One
    3. Two
    4. Three
    Show answer

    B. One Rule 23(a)(ii): under 50 meters the second masthead light is not obligatory.

  2. An air-cushion vessel in non-displacement mode adds which light?

    1. All-round flashing red
    2. All-round flashing yellow
    3. Special flashing light
    4. Second sternlight
    Show answer

    B. All-round flashing yellow Rule 23(b): an all-round flashing yellow light.

  3. On the Great Lakes, what may replace the second masthead light and sternlight?

    1. A towing light
    2. An all-round white light
    3. Two all-round red lights
    4. Nothing
    Show answer

    B. An all-round white light Inland Rule 23(e).

Frequently asked questions

What lights does a small powerboat show?

Under 12 meters, an all-round white light and sidelights may be shown instead of the full Rule 23(a) set.

Is the second masthead light required?

Only for vessels 50 meters or more. Below that it's optional (Rule 23(a)(ii)).

What does a flashing yellow light on a power-driven vessel mean?

Under Rule 23(b), an air-cushion vessel operating in the non-displacement mode.

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