Navigation lights exist so that, on a pitch-black sea, one glance tells you three things about another vessel: roughly what she is, which way she’s heading relative to you, and (combined with the COLREGS rules) who has to give way. Every configuration below is a fixed, internationally agreed picture: once you can read the four or five basic patterns, night sailing stops being guesswork.
The building blocks
Every vessel’s lights are made from the same small set of parts, each visible only over a specific arc:
- Sidelights: red to port, green to starboard, each visible from dead ahead to 22.5° abaft the beam (112.5° total). If you can see a sidelight, you’re somewhere in that 112.5° wedge on that side of her.
- Masthead light: white, forward-facing, visible across a 225° arc (112.5° either side of the bow). Carried by power-driven vessels underway; a sailing vessel under sail alone never shows one.
- Sternlight: white, visible over the 135° arc directly astern. Combined with the masthead light, the three lights together cover the full 360° around a power vessel.
- All-round lights: visible through the full 360°, used for anchor lights and status signals (not under command, restricted in ability to manoeuvre, fishing, and so on).
Reading the basic pictures
Sailing vessel under sail: red and green sidelights plus a white sternlight. No masthead light. Small sailing vessels may combine all three into a single tricolour light at the masthead instead.
Power-driven vessel underway: sidelights, sternlight, and one masthead light (two if she’s 50m or longer, mounted with the after one higher, which also helps you judge her heading at a distance).
At anchor: a single all-round white light, nothing else (two lights, fore and aft, on vessels 50m+). No sidelights, no sternlight: she isn’t going anywhere.
Towing, fishing, not under command, restricted in ability to manoeuvre: each has its own combination of coloured all-round lights (see the quiz diagrams below for four of the most common). These exist specifically to warn you that the normal rules of the road may not apply cleanly, because that vessel can’t manoeuvre freely.
Why this matters at 2am in a shipping lane
The whole point of this system is that a single glance (red only, red+green together, white masthead only, red-over-red) should be enough to make a safe decision before you’re close enough for it to matter. Combine what you see here with the COLREGS give-way rules and you have the two pieces you need to stand a safe night watch.
Test yourself on the eight scenarios below: each one mirrors something you’ll actually see from the cockpit.