Navigation lights: what lights a boat carries and how to read them at night

Published on 26 September 2026 · By Ricard Prehn, professional recreational skipper (PPER), Spanish Yacht Captain (capitán de yate), Yacht Captain up to 200 GT, GMDSS General Operator and sailing instructor for over 27 years

What are navigation lights?

They are the lights that COLREGs (rules 20 to 31) require every vessel to show from sunset to sunrise, and by day in restricted visibility. From them you can tell, without seeing the hull, what kind of vessel is ahead, what she is doing and where she is heading.

They are part C of COLREGs, the rules that decide who has right of way at sea. While they are on, you must not show other lights that could be mistaken for them or that spoil your lookout, and it is worth checking them before you leave: a blown bulb at night is a boat nobody else can read.

What lights does a boat carry?

A power-driven vessel carries four: the white masthead light shining forward (225°); the sidelights, green to starboard and red to port (112.5° each); and the white sternlight shining aft (135°). A sailing vessel under sail carries the same except the masthead light.

• Masthead light: white, on the fore-and-aft centreline, visible from right ahead to 22.5° abaft the beam on either side (225°).
• Sidelights: green to starboard and red to port, each from right ahead to 22.5° abaft the beam on its side (112.5°).
• Sternlight: white, as near the stern as possible, 67.5° either side of dead astern (135°).
• All-round light: visible through 360°. It is the anchor light and the one used for special signals.

Power-driven vessels of 50 m or more also carry a second masthead light, aft of and higher than the forward one (vessels under 50 m may carry it too). Those two whites are very useful at night: if you see them one above the other, the ship is heading towards you; as they open out, the lower one (the forward one) points where she is going.

Why are sidelights 112.5° and the sternlight 135°?

Because each sidelight shines from right ahead to 22.5° abaft the beam on her side (90° + 22.5° = 112.5°), and the sternlight covers what is left astern: 67.5° either side of the stern (135°). Together the three cover 360°: from any point you see at least one.

That 22.5° abaft the beam is the same limit COLREGs use to decide whether you are overtaking: if you come up inside her sternlight arc, where you only see her white sternlight, you are the one who keeps clear.

How far away can they be seen?

COLREGs (rule 22) set the minimum range of each light according to the vessel's length, in nautical miles:

Look at the first row: the sidelights of a small motorboat only have to be visible at 1 mile. By the time you can make out the colour you are already close, so at night you look early and act early.

How can you tell where a boat is heading at night?

From the sidelights you can see. If you see her red, she is showing you her port side and crossing ahead of you from right to left; if you see her green, she is showing her starboard side and crossing from left to right; if you see red and green together, she is coming straight at you; and if you only see a white light, you are looking at her stern.

And what the lights do not tell you, the bearing does: if an approaching vessel stays in the same direction from you (for example, fixed against a stanchion), you are on a collision course, however reassuring her lights look.

Who gives way at night?

Between two crossing power-driven vessels, the one that sees the other's red light: she has the other on her starboard side and must keep clear (rule 15). If you see both sidelights, it is a head-on situation and both turn to starboard. If you only see the sternlight and you are closing, you are overtaking and you give way.

A trick that works: the other vessel's red light is a red traffic light for you. But careful, that holds between two power-driven vessels: if the one showing red is a sailing boat, a vessel engaged in fishing or one not under command, you keep clear anyway, and if you are the sailing boat and you are overtaking, so do you. Every case, with the encounter simulator, in who has right of way under COLREGs.

What lights does a sailing boat show?

Under sail alone, sidelights and sternlight, with no masthead light. If she is under 20 m she may combine them in a tricolour lantern at the masthead. And as soon as her engine is running she becomes a power-driven vessel: she switches on the masthead light even with sails up.

The tricolour saves battery and is easier to see above the waves, but it only applies under sail: with the engine running you switch it off and show sidelights, sternlight and masthead light. Optionally, a sailing vessel may also show red over green all-round lights at the masthead, though never together with the tricolour. And by day, when sailing with the engine running, she must hoist a cone, point down.

What lights does a small motorboat or dinghy show?

A power-driven vessel under 12 m may replace the masthead light and sternlight with a single all-round white light, plus sidelights. A sailing vessel under 7 m and a rowing boat may carry just a white torch, ready to show in time to prevent a collision.

That is why at night a small motorboat often looks like a white light with a red or green one below: don't mistake it for a big ship showing you her side from far off. If you go out fishing before sunrise, you are the one who has to have those lights on.

What do red, green and white lights in a vertical line mean?

They are all-round lights warning that the vessel is limited: two reds, not under command; red-white-red, restricted in her ability to manoeuvre; three reds, constrained by her draught; green over white, trawling; red over white, fishing with other gear. A single all-round white light on a vessel under 50 m means she is at anchor.

• Not under command: two reds in a vertical line (by day, two black balls). If making way, she also shows sidelights and sternlight.
• Restricted in ability to manoeuvre: red, white, red (by day, ball, diamond, ball).
• Constrained by her draught: three reds in a vertical line, as well as her power-driven lights (by day, a cylinder).
• Trawling: green over white. Fishing with other gear: red over white. By day, two cones joined at their points.
• Towing: two masthead lights in a vertical line (three if the tow exceeds 200 m) and a yellow towing light above the sternlight.
• At anchor: one all-round white if under 50 m; two if longer, the aft one lower (by day, one black ball). Aground: anchor lights plus two reds in a vertical line.

Be careful not to mix them up with buoy lights. A white light flashing in quick groups is not a vessel: it is a cardinal mark (see buoyage).

How are lights tested in the exam?

Lights and shapes are among the most reliable COLREGs questions in the Spanish PER and PNB exams: either they describe some lights and ask which vessel it is, or they ask for the arc of a light. This one came up in Barcelona on 15 March 2025 (model 2), in Spanish, translated:

"What is the name of the white light placed as near the stern as possible, with an arc of visibility of 135° split between both sides?"

• a) Towing light.
• b) After masthead light.
• c) Sternlight. (correct)
• d) Sternpost light.

The trap is b: the second masthead light is aft, but it covers 225° and shines forward. Option a is tricky too: the towing light has the same 135° arc, but it is yellow. Another that comes up: "At night, we see two red lights in a vertical line (and no other light). What type of vessel is it?" A vessel not under command and not making way (Barcelona, March 2024).

In the online classroom you practise with real official exams from previous sittings, marked instantly. If you are planning your study, see how to pass the PER in a month.

Source: COLREGs (RIPA), rules 20 to 31 (BOE-A-1977-15605), and Topic 6 of Altair's PER syllabus.