IALACOLREG
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).

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.

Then confirm the answer with the day shape, vessel length and any extra signal such as towing lights, deck illumination or a cylinder.

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

Small vessels displaying two masthead lights unnecessarily

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

Test your knowledge and prove your mastery.

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