The three white arcs that add up to a circle
A power-driven vessel underway shows a white masthead (steaming) light covering 225° ahead, a white stern light covering 135° astern, and red and green sidelights of 112.5° each. 225 + 135 = 360. The sidelights sit inside the masthead sector: from dead ahead to 22.5° abaft the beam on each side.
If you see only a white light, it may be a stern light (you are overtaking) or a masthead of a vessel whose sidelights are not yet open. If you see red and green together, the other vessel is end-on, coming toward you.
Power-driven vs sailing
A power-driven vessel under 50 m coming toward you typically shows one white masthead plus red and green sidelights. Over 50 m she adds a second masthead further aft.
A sailing vessel underway does not show a masthead steaming light. She shows sidelights and a stern light. Optionally she may show all-round red over green at the masthead (the “red over green, sailing machine” pair) instead of, or in addition to, combined sidelights — never together with a steaming light, because that would look like a power vessel.
Special states: NUC and RAM
Not under command (NUC) at night: two all-round reds in a vertical line. If she still has way on, you also see sidelights and stern light. By day: two black balls.
Restricted in ability to manoeuvre (RAM): red–white–red all-round, vertically. That is not the same as NUC (two reds) or constrained by draught (three reds). If RAM is making way, sidelights, stern, and masthead lights are added as for a power vessel.
Memory lines that survive the exam
| Pattern | Meaning |
|---|---|
| Red + green together | End-on, meeting |
| Only green, or only red | Crossing; you see one side |
| Only white | Stern or distant masthead — think overtaking |
| Two reds vertical | NUC |
| Red–white–red | RAM |
| Three reds | Constrained by draught |
| Green over white | Trawling |
| Red over white | Fishing, not trawling |
COLREGs also set the day shapes and sound signals; those sit in the COLREGs quick revision guide. Lights are the night language of the same rules.
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This article is for study and revision. It is not legal advice. Always check the official COLREGs, IALA, CEVNI, and local authority texts. SailPrep is an independent study aid and is not affiliated with LBS, the RYA, or any certification body.