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23

Power-driven Vessels Underway

Power-driven vessel under 50 m underway — masthead, sidelights, sternlight.

Rule 23 specifies lights for power-driven vessels underway.

a
A power-driven vessel underway shall exhibit:
  • 1A masthead light forward
  • 2A second masthead light abaft of and higher than the forward one — except that a vessel of less than 50 metres in length shall not be obliged to exhibit such light but may do so
  • 3Sidelights — red (port) and green (starboard)
  • 4A sternlight
b
An air-cushion vessel when operating in the non-displacement mode shall, in addition, exhibit an all-round flashing yellow light.
c
A WIG (Wing-In-Ground) craft only when taking off, landing and in flight near the surface shall, in addition to the lights prescribed in paragraph (a), exhibit a high-intensity all-round flashing red light.
d
  • 1A power-driven vessel of less than 12 metres in length may, instead of the lights in paragraph (a), exhibit an all-round white light and sidelights.
  • 2A power-driven vessel of less than 7 metres in length whose maximum speed does not exceed 7 knots may, instead of the lights in paragraph (a), exhibit an all-round white light and shall, if practicable, also exhibit sidelights.
  • 3The masthead light or all-round white light on a power-driven vessel of less than 12 metres may be displaced from the fore-and-aft centreline if centreline fitting is not practicable, provided sidelights are combined in one lantern carried on the centreline (or as nearly as possible in the same fore-and-aft line as the masthead/all-round white light).
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.
Schematic plan view of the light fixtures. A power-driven vessel under 12 m may use an all-round white light and sidelights instead of the standard arrangement.

IMO COLREG 1972Official text

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.
Source:

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

Recognition Sequence

Classify the vessel state first: underway, making way, stopped, at anchor, aground, towing, fishing, pilotage or special condition.

Read special lights vertically from top to bottom before using sidelights and sternlight to confirm aspect.

Use the complete display and any vessel length stated in the scenario.

Optional second masthead lights, two anchor lights or deck illumination do not by themselves prove a minimum vessel length.

Exam Focus

Avoid identifying a vessel from one colour alone.

Many mistakes come from spotting a red light and guessing before checking the full pattern.

If the question mentions 'making way', 'underway but stopped', 'at anchor' or 'aground', that wording usually determines which extra lights or shapes appear.

Key Takeaways

1

Vessels 50m+ must show two masthead lights

2

≥12m PDVs show masthead light(s) + sidelights + sternlight; smaller vessels have simplified options under (d)

3

Second masthead light must be higher than the forward one

4

Hovercraft show additional flashing yellow light

5

WIG craft taking off/landing/in low flight: additional high-intensity flashing red

6

<12m PDV may show 1 all-round white + sidelights instead of masthead + stern

7

<7m PDV with max speed ≤7 kn may show just an all-round white, with sidelights if practicable

Common Mistakes

Assuming a vessel under 50 metres is forbidden to show a second masthead light; Rule 23 expressly permits it

Confusing the vertical positioning of forward and after masthead lights

Forgetting that <12m vessels have a simplified light option (single all-round white + sidelights)

Forgetting the <7m + ≤7kn even-simpler option (single all-round white only, sidelights if practicable)

Test Your Knowledge

Practise this topic and review the explanation for each answer.

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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.