Part C — Lights and shapes

COLREGs Rule 23: Power-driven vessels underway

Rule 23 says a power-driven vessel underway shows a masthead light forward, sidelights and a stern light. Vessels 50 m or longer add a second, higher masthead light aft. Small craft under 12 m may show an all-round white light and sidelights instead.

What Rule 23 says

  1. A masthead light forward; a second masthead light abaft and higher than the forward one (optional under 50 m); sidelights; a stern light.
  2. An air-cushion vessel (hovercraft) operating in non-displacement mode also shows an all-round flashing yellow light.
  3. A WIG craft taking off, landing or flying near the surface also shows a high-intensity all-round flashing red light.
  4. Under 12 m: an all-round white light and sidelights may replace the masthead and stern lights.
  5. Under 7 m with a maximum speed of 7 knots or less: an all-round white light, and sidelights if practicable.
  6. On a vessel under 12 m, the masthead or all-round white light may be off the centreline if that cannot be avoided, provided the sidelights are combined in one lantern on or near the centreline.

This is a plain-English summary, not the official wording. For the exact text, see the IMO convention. In UK waters the rules are given legal force by the Merchant Shipping (Distress Signals and Prevention of Collisions) Regulations.

What it means on a yacht

A sailing yacht under engine at night is a power-driven vessel and must show a steaming (masthead) light as well as its sidelights and stern light. Turn off the tricolour: it is for sailing only.

Lights and shapes under Rule 23

  • Power-driven, under 50 m Rule 23

    One masthead light, sidelights and a stern light. A second masthead light aft is optional. A sailing yacht motoring shows these too.

  • Power-driven, 50 m or more Rule 23

    Two masthead lights, the aft one higher. Their alignment shows which way the ship is heading.

  • Power-driven, under 12 m Rule 23(d)

    May combine the masthead and stern lights into one all-round white light, plus sidelights. Typical for small motor boats and RIBs.

  • Power-driven, under 7 m, max speed 7 knots Rule 23(d)(ii)

    Only needs one all-round white light. Should also show sidelights if practicable. Easily mistaken for a vessel at anchor.

  • Hovercraft (air-cushion vessel) Rule 23(b)

    Normal power-driven lights plus an all-round flashing yellow light when operating in non-displacement mode.

Drawn from the starboard side at night. See every aspect in the lights and shapes trainer.

Practise it

Test yourself on who gives way in the free COLREGs trainer, or print the rules of the road cheat sheets for the chart table.

Other rules in Part C

Frequently asked questions

What lights does a yacht show when motoring at night?

Sidelights, a stern light and a white masthead (steaming) light, as a power-driven vessel under Rule 23. A masthead tricolour must not be used when motoring.