Boat Navigation Lights Explained: Red, Green, White & What to Show

Why the Arcs Are the Numbers They Are

The odd-looking figures in the COLREGs — 112.5°, 225°, 135° — look arbitrary until you add them up. Each sidelight covers 112.5°, from dead ahead round to 22.5° behind the beam. Two of them make 225°, which is exactly the arc of the masthead light above them. The sternlight's 135° is precisely what is left: 225 + 135 = 360. The lights tile a circle, with no gaps and no overlaps, so that from any bearing on earth you see one — and only one — combination.

That is the whole design. It is why a colour tells you an aspect, why a single white light means you are behind someone rather than in front of them, and why a light that shines the wrong way is worse than one that does not shine at all. It also explains the shape of every rule below.

One naming trap while we are here: the masthead light is not, on a sailing yacht, at the top of the mast. It is the forward-facing white light mounted partway up the front of the mast, and sailors usually call it the steaming light precisely to avoid the confusion. The light at the actual top of your mast is either the tricolour or the anchor light, and neither of them is a masthead light in the sense the rules mean.

How Far They Have to Be Seen

Range is set by boat length, and this is the reason a chandlery-cheap lamp can be technically illegal on your boat and fine on a dinghy. The figures below are minimum required visibility in clear conditions.

Two practical consequences. First, a typical 12 m cruising yacht needs a 2 nm anchor light — which is more than a solar garden lamp in the rigging will do, however tempting. Second, these are minimums in clear air: in haze, rain or against a lit shoreline, assume everyone sees you later than the table says, including the ship whose bridge is thirty metres above the water and looking over you.

What to Show: Under Power

A power-driven vessel under way shows a masthead light forward, sidelights and a sternlight. Big ships — 50 m and over — add a second masthead light, further aft and higher than the first, which is the pair that lets you read their heading from miles away.

• Under 12 m: may show an all-round white light plus sidelights, instead of a separate masthead light and sternlight.
• Under 7 m and under 7 knots maximum speed: may show an all-round white light alone, plus sidelights if practicable.
• Under oars: may show sailing-vessel lights, or simply keep a torch or lantern ready to show in time to prevent a collision.

What to Show: Under Sail

A sailing vessel under way shows sidelights and a sternlight, and no masthead light. The absence is the point — the missing white light forward is precisely what tells the world you are sailing and therefore likely to be the stand-on vessel against a power-driven boat.

Under 20 m you have an option: combine the sidelights and sternlight into a single tricolour lantern at the top of the mast. It is a genuinely good offshore choice — one bulb, one circuit, far less current, and mounted where a distant ship actually looks and where waves do not bury it. Two conditions come with it. It is shown instead of the deck-level lights, never as well; and it is only for sailing. There is a second option, rarely used, of two all-round lights at the masthead, red over green — the “red over green, sailing machine” of the mnemonic — and those may not be combined with a tricolour either.

What to Show: At Anchor

At anchor you show an all-round white light where it can best be seen, and you switch everything else off. The sidelights, sternlight and steaming light all belong to a vessel that is under way; leaving any of them on turns your boat into a moving target in someone else's mental picture, which is exactly the wrong thing to be when you are lying still in the dark across their route.

• Under 50 m: one all-round white light. On a yacht that means hoisted at or near the masthead — high, so it clears the boom and the sprayhood, and visible from every direction.
• 50 m and over: two — one forward, and a second lower one near the stern.
• 100 m and over: must also use available working lights to illuminate the decks.
• Under 7 m: no anchor light required unless anchored in or near a narrow channel, fairway, anchorage, or where other vessels normally navigate — which describes most real anchorages, so the exemption is narrower than it sounds.
• Aground: anchor lights plus two all-round red lights in a vertical line.

For what else changes after dark at anchor — the watch, the gear, the bearings you take before you go below — see our night anchoring safety guide and the wider overnight anchoring routine.

The Five Mistakes

Almost every lighting error afloat is one of these, and all five are made by boats whose lights are working perfectly.

1. Motorsailing under a tricolour. The engine is in gear, so you are a power-driven vessel: steaming light on, deck-level sidelights and sternlight on, tricolour off. Keeping the tricolour up tells an approaching ship you are sailing, which claims a priority you do not have — and you are the one who will lose that argument.
2. Tricolour and deck lights on together. The tricolour replaces the lower lights; showing both puts two sets of red and green at different heights and makes your size and aspect unreadable at exactly the range where they matter.
3. Steaming light left on at anchor. A common one after a late arrival: you drop the hook, hoist the anchor light, and forget the switch. Now you are showing a white light forward and an all-round white, which reads as a vessel under way.
4. Deck, spreader or cockpit lights drowning the anchor light. Bright white light at deck level destroys both your own night vision and the contrast that lets someone pick your masthead light out of a lit shoreline. Anchor light on, everything else off.
5. All-round white with no sidelights, on a boat that does not qualify. The white-light-only option belongs to power-driven vessels under 7 m doing under 7 knots. Under 12 m still needs sidelights alongside the all-round light, and without them nobody can tell which way you are pointing.

International vs Inland

The COLREGs are the international rules and apply at sea everywhere. Many countries add an inland version for rivers, lakes and enclosed waters — in the United States the Inland Navigation Rules differ from the international set in a handful of places, including an extra flashing yellow light for some towing situations and slightly different masthead arrangements. The lights in this guide are the international ones, which cover coastal and offshore sailing; if you boat on inland waters, check your national inland rules before you assume the two match.

The Anchor Light Is for Them. The Alarm Is for You.

It is worth being precise about what the anchor light does, because it is easy to feel protected by it and be wrong. The all-round white light at your masthead is a statement to other vessels: I am here, I am anchored, I am not going anywhere, plan around me. It works in exactly one direction — outward. It cannot see. It does not know where your anchor is. It will keep burning, faithfully and legally, all the way across the anchorage while you drag, right up to the moment it stops being visible because your mast is lying on a rock.

The two are complementary halves of the same night. The light protects other boats from you; a GPS anchor alarm protects you from your own anchor. It watches your position with the phone locked, and wakes you the moment you leave the radius you set — while the problem is still a winch job. If you are deciding what to trust, our comparison of anchor watch versus an alarm app covers the difference, and the anchor scope calculator works out the radius from your rode and depth. The full method is in our complete guide to anchoring a boat.

The Bottom Line

Three colours, four arcs, one circle. Green is starboard, red is port, white astern and white above; the arcs tile the compass so that what you see is always enough to work out what someone is and where they are going. Show the lights that describe what you are actually doing right now — sailing, motoring, or lying still — and change them the moment that changes, because the engine going into gear changes your lights and your obligations at the same instant. Then, when you have dropped the hook and hoisted the one light that says you are going nowhere, set the alarm that will tell you if that stops being true.