Part C — Lights and shapes
COLREGs Rule 21: Definitions of lights
Rule 21 defines the navigation lights by colour and arc: a white masthead light over 225°, a green starboard and red port sidelight over 112.5° each, a white stern light over 135°, a yellow towing light over 135°, and all-round lights over 360°.
What Rule 21 says
- Masthead light: white, over the fore-and-aft centreline, an unbroken arc of 225° from right ahead to 22.5° abaft the beam on each side.
- Sidelights: green on the starboard side and red on the port side, each over 112.5° from right ahead to 22.5° abaft the beam. On a vessel under 20 m they may be combined in one lantern.
- Stern light: white, at the stern, over 135°, 67.5° from right aft on each side.
- Towing light: yellow, with the same arc as the stern light.
- All-round light: an unbroken arc of 360°.
- Flashing light: flashing at 120 or more flashes per minute.
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
The arcs fit together: masthead 225° plus stern light 135° make 360°, so you can always see either white masthead light or stern light. If you see only a white light, you are looking at a vessel's stern and are probably overtaking it.
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
- Rule 20: Application of Part C
- Rule 22: Visibility of lights
- Rule 23: Power-driven vessels underway
- Rule 24: Towing and pushing
- Rule 25: Sailing vessels underway and vessels under oars
- Rule 26: Fishing vessels
- Rule 27: Vessels not under command or restricted in their ability to manoeuvre
- Rule 28: Vessels constrained by their draught
- Rule 29: Pilot vessels
- Rule 30: Anchored vessels and vessels aground
- Rule 31: Seaplanes