COLREG 1972 · rule 21 of 38
EnglishEN

Rule 21. The kinds of light

Six definitions carry the whole of part C:

Part of the convention Lights and shapes

Six definitions carry the whole of part C:

  • Masthead light: a white light on the centreline showing 225 degrees, from right ahead to 22.5 degrees abaft the beam on either side.
  • Sidelights: green to starboard, red to port, each showing 112.5 degrees, from right ahead to 22.5 degrees abaft the beam on its own side. Under 20 m they may be combined in one lantern.
  • Sternlight: white, as near the stern as practicable, showing 135 degrees, 67.5 degrees on each side of right astern.
  • Towing light: yellow, with the same characteristics as a sternlight.
  • All-round light: shows the full 360 degrees.
  • Flashing light: flashes at regular intervals at a frequency of 120 flashes or more per minute.
At sea it looks like this

Add it up: the masthead 225 degrees plus the stern 135 makes 360. The sidelights cover the same forward arc as the masthead light. Hence the overtaking rule: 22.5 degrees abaft the beam is exactly where the sidelights stop and only the sternlight remains.

The rule says«“Masthead light” means a white light placed over the fore and aft centreline of the vessel showing an unbroken light over an arc of the horizon of 225 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on either side of the vessel».

Check yourself

From astern, what can you see of a vessel underway and what can you not?

You see the white sternlight (135 degrees) and any all-round lights. You cannot see the masthead lights (225 degrees forward) or the sidelights (112.5 degrees forward).

Rule 21.

You see a single white light and no colours. What can it be?

Either you are in the stern sector of a vessel underway (her sternlight), or it is a vessel at anchor or a small craft with an all-round white. Tell them apart by movement and by the second white light.

Rule 21, 23 (d), 30.

The light sectors: masthead, sidelights, sternlight, all-round?

Masthead 225 degrees, sidelights 112.5 each, sternlight 135, all-round 360.

Rule 21. 225 plus 135 makes 360.

What does a yellow light above a white sternlight mean?

A towing light: she is towing on a wire.

Rule 21 (d) and 24 (a) (iv).

What is a «towing light»?

A yellow light with the same characteristics as a sternlight (135 degrees), shown above the sternlight of a towing vessel.

Rule 21 (d).

How is a «flashing light» defined?

A light flashing at regular intervals at a frequency of 120 flashes or more per minute.

Rule 21 (f).

Where does the 22.5 degree angle in the rules come from?

It is the limit abaft the beam where the sidelights and masthead light stop. Beyond it only the sternlight shows, and that is the overtaking zone.

Rule 21 (a), (b) and 13 (b).

How many flashes per minute for a flashing light, and how long is a manoeuvring light flash?

A flashing light, 120 or more per minute. A manoeuvring flash about 1 second, about 1 second between flashes, not less than 10 seconds between signals.

Rule 21 (f) and 34 (b).

Part of the convention: Lights and shapes