1The rule in plain words
In one sentence…
A power-driven vessel underway shows masthead, sidelights and sternlight at night; at 50 meters or more, two masthead lights.
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.
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.
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.
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.
2See it in 3D
Turn the vessel or pick bow, stern, port or starboard: 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.
3Key points
- 1.
Power-driven underway = masthead forward + sidelights + sternlight.
- 2.
50 m or more → second masthead light, abaft and higher.
- 3.
Under 12 m → all-round white + sidelights may be enough.
- 4.
Under 7 m and max. 7 knots → all-round white; sidelights if practicable.
4Frequent 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.
5Exam keys & interpretation
Exam keys
Two masthead lights don't prove 50 meters or more: a smaller vessel may carry both voluntarily.
"Flashing yellow" = air-cushion vessel in non-displacement mode; "high-intensity flashing red" = WIG craft near the surface.
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.
6Official text
IMO COLREG 1972U.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.7Test your knowledge
An 80-meter power-driven vessel is underway at night. Which lights does she show?
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