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Rule 23: Power-driven Vessels Underway

A power-driven vessel underway shows masthead, sidelights and sternlight at night; at 50 meters or more, two masthead lights.

Rule 23 of 41

Decide

An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.

Decision 1 of 3

It is night and you see only these lights. What vessel is it?

The rule in plain words

  1. Which lights?A masthead light forward, sidelights (green to starboard and red to port, see Rule 21) and a sternlight aft. These are the basic lights of every power-driven vessel underway.
  2. And the second masthead light?It goes abaft and higher than the forward one. It is required from 50 meters in length; under 50 meters you may carry it, but you don't have to.
  3. Air-cushion vessel or WIG craft?On top of the normal lights, an air-cushion vessel in non-displacement mode adds an all-round flashing yellow light; a WIG craft taking off, landing or flying near the surface adds a high-intensity all-round flashing red light.
  4. And small motorboats?Under 12 meters you may replace them all with an all-round white light and sidelights. Under 7 meters with a top speed of 7 knots, the white light is enough, with sidelights if practicable.
Starboard elevation of light fixtures: two masthead lights, the after one higher, and the sidelight abaft the forward masthead. Two masthead lights are required from 50 m and permitted below 50 m.
1 of 3

Before you read on: what would you do?

Question 1/1

An 80-meter power-driven vessel is underway at night. Which lights does she show?

See it in 3D

Drag the vessel to turn it: this is how her lights look at night.

An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.

Frequent mistakes

  • Wrong: "A 40-meter vessel may not carry a second masthead light."

    Right: She may carry it; she just isn't obliged to. It is required from 50 meters.

  • Wrong: "The second masthead light goes forward and lower."

    Right: It goes abaft the first one and higher than it.

  • Wrong: "A 10-meter motorboat always needs a separate masthead light and sternlight."

    Right: Under 12 meters she may show an all-round white light and sidelights instead.

Exam keys & interpretation

  1. Two masthead lights don't prove 50 meters or more: a smaller vessel may carry both voluntarily.
  2. "Flashing yellow" = air-cushion vessel in non-displacement mode; "high-intensity flashing red" = WIG craft near the surface.
  3. Watch the thresholds: "under 12 m" and "under 7 m and not more than 7 knots" are different exemptions.

How to read it

Read the masthead lights first: if there are two, the lower one marks the bow. Then the sidelight color tells you which side you see.

Under 12 meters, if the light can't go on the centerline it may be displaced, but the sidelights must then be combined in one lantern.

Key points

  • Power-driven underway = masthead forward + sidelights + sternlight.
  • 50 m or more → second masthead light, abaft and higher.
  • Under 12 m → all-round white + sidelights may be enough.
  • Under 7 m and max. 7 knots → all-round white; sidelights if practicable.

Test your knowledge

3 questions to check it. Pick an answer to see why.

Question 1/3

Where does the second masthead light go?

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Official text

IMO COLREG 1972 U.S. Coast Guard — International Navigation Rules (2024)

Full wording of the rule, reproduced from the official source below.

(a) A power-driven vessel underway shall exhibit: (i) a masthead light forward; (ii) 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; (iii) sidelights; and (iv) a sternlight. (b) An air-cushion vessel when operating in the non-displacement mode shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit an all-round flashing yellow light. (c) A WIG craft only when taking off, landing and in flight near the surface shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit a high intensity all-round flashing red light. (d) (i) 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; (ii) a power-driven vessel of less than 7 meters in length whose maximum speed does not exceed 7 knots may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and shall, if practicable, also exhibit sidelights; (iii) the masthead light or all-round white light on a power-driven vessel of less than 12 meters in length may be displaced from the fore and aft centerline of the vessel if centerline fitting is not practicable, provided that the sidelights are combined in one lantern which shall be carried on the fore and aft centerline of the vessel or located as nearly as practicable in the same fore and aft line as the masthead light or the all-round white light.
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Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed:

Frequently asked questions about COLREG Rule 23

Which lights does a power-driven vessel underway show?

A forward masthead light, sidelights and a sternlight, plus a second masthead light abaft of and higher than the forward one. A vessel of less than 50 metres in length is not obliged to exhibit that second light but may do so (Rule 23(a)).

How many masthead lights does a vessel of 50 metres or more show?

Two. Rule 23(a)(ii) requires a second masthead light abaft of and higher than the forward one, and only vessels of less than 50 metres in length are excused from it.

What lights may a power-driven vessel of less than 12 metres show?

Instead of masthead light, sidelights and sternlight she may exhibit an all-round white light and sidelights (Rule 23(d)(i)). A vessel of less than 7 metres whose maximum speed does not exceed 7 knots may show an all-round white light alone, with sidelights if practicable.