In short
At night a vessel is a pattern of lights, identified by colour, number and relative position — never by brightness.
Green is starboard, red is port, a white masthead light gives heading and size. All-round coloured lights say what she is doing.
Rule 21 fixes the arcs: sidelight 112.5 degrees, sternlight 135, masthead 225, all-round 360.
Work through the picture in order: how many lights, what colour, how arranged, what aspect.
The Rule 22 ranges hint at size: a masthead light showing at six miles belongs to a vessel of 50 metres or more.
Mnemonics save seconds but say nothing about where she is going. The aspect still has to be read.
What the picture is made of: the arcs
Rule 21 sets out how much of the horizon each light covers — the key to everything else:
Masthead light — white, on the centreline, over 225 degrees: from right ahead to 22.5 degrees abaft the beam each side.
Sidelights — green to starboard, red to port, 112.5 degrees each, from right ahead to 22.5 degrees abaft the beam. Under 20 metres they may share one lantern.
Sternlight — white, 135 degrees, from right astern to 67.5 degrees each side.
Towing light — yellow, with the characteristics of a sternlight.
All-round light — 360 degrees, visible from any direction. All-round lights mark a vessel's occupation: fishing, towing, restricted, at anchor.
Flashing light — 120 flashes a minute or more.
Add the masthead's 225 to the sternlight's 135 and you have the full circle, so a white light shows from every direction. The question is always the same: masthead or stern? The sidelights answer it — red or green in view and you are forward of her beam, a lone white and you are astern.
How far it shows: Rule 22
Ranges depend on length, so they hint at what you are seeing:
50 metres and over: masthead 6 miles, sidelights and sternlight 3 miles;
12 to 50 metres: masthead 5 miles (3 if under 20 metres), sidelights and sternlight 2 miles;
under 12 metres: masthead 2 miles, sidelights 1 mile, sternlight 2 miles.
The conclusion is uncomfortable: a ship shows at six miles while your yacht shows at one or two. You have more time to act than she does — plan on that.
A working order: four questions
Do not try to read a vessel in one glance — go in steps.
1How many lights? One, two, three; a column or a row.
2What colour? White, red, green, yellow. Count flashing lights separately.
3How arranged? What is above what. A vertical column almost always means occupation; white lights spread horizontally give length and heading.
4What aspect? What shows from here and what does not. The same tug looks nothing alike from ahead, abeam and astern, and that is where people go wrong.
The last step matters most: lights do not draw the vessel, they show the sector of her that faces you.
Under power: Rule 23
A power-driven vessel underway carries a masthead light forward, sidelights and a sternlight. At 50 metres and over she must carry a second masthead light abaft of and higher than the first; under 50 metres she may.
Those two whites are the most useful detail at night. Their spread gives the aspect: the further apart, the more of her side you see. In line, with both sidelights showing, she is heading straight at you.
Small craft live in paragraph (d): under 12 metres a vessel may show an all-round white and sidelights instead of the full set, and under 7 metres at no more than 7 knots, an all-round white and, if practicable, sidelights. Hence the rule of thumb — a lone all-round white is a small motorboat or a vessel at anchor.
Under sail: Rule 25
A sailing vessel underway shows sidelights and a sternlight, nothing else. The missing masthead light is the identifying feature: no white above, therefore sail.
Under 20 metres she may combine all three in a tricolour lantern at the masthead, and she may in addition carry two all-round lights there, red over green — but not together with the tricolour. Under 7 metres, and under oars, a torch shown in time to prevent collision will do.
And the detail most often forgotten: a yacht under sail with the engine running is a power-driven vessel. At night she carries a masthead light; by day, a cone point down.
Towing: Rule 24
A towing vessel carries two masthead lights in a vertical line — three if the tow is longer than 200 metres — plus sidelights, a sternlight and a yellow towing light above the sternlight. Over 200 metres she shows a diamond by day.
The vessel or barge being towed carries sidelights and a sternlight, no masthead light. That is the trap: between the two groups of lights lies darkness, and in the darkness lies the towline. Never pass between a tug and what she is pulling.
Pushing ahead and towing alongside differ: two masthead lights in a vertical line, no yellow towing light.
Fishing: Rule 26
A vessel engaged in fishing shows a vertical pair of all-round lights:
green over white — trawling;
red over white — fishing other than trawling.
Sidelights and a sternlight appear only when she is making way through the water. Where gear extends more than 150 metres, a white all-round light or a cone point up marks its direction. By day the signal is two cones, apexes together.
Green over white and red over white are hard to tell apart on a lively sea, but for a yacht the conclusion is the same: keep clear, and keep well clear, because the gear can be anywhere.
Special cases: Rules 27 and 28
Two all-round reds in a vertical line — not under command. Making way, she adds sidelights and a sternlight; stopped, the two reds are all there is.
Red, white, red in a vertical line — restricted in her ability to manoeuvre: dredging, diving, laying marks. A dredger adds which side to take — two all-round reds mark the obstruction, two greens the side you may pass.
Three all-round greens — mine clearance: keep a thousand metres off.
Three all-round reds (Rule 28) — constrained by her draught; a cylinder by day. She follows the deep water and cannot turn out of it.
White over red (Rule 29) — a pilot vessel on duty.
At anchor and aground: Rule 30
A vessel at anchor shows an all-round white forward and a second, lower, at or near the stern. Under 50 metres a single all-round white will do. Over 100 metres she must also light her decks — a useful clue that you are looking at an anchorage.
A vessel aground shows her anchor lights plus two all-round reds in a vertical line, and three black balls by day. Aground is easily confused with not under command, two reds in both cases; the difference is the anchor lights beside them, and that she is going nowhere.
Mnemonics, and where they stop
The rhymes save seconds, and everyone uses them:
"Red over red, the captain is dead" — two reds, not under command.
"Red over white, fishing tonight" — fishing other than trawling.
"Green over white, trawling tonight" — a trawler.
"White over red, pilot ahead" — a pilot vessel.
"Red, white, red — restricted ahead" — restricted in her ability to manoeuvre.
Three reds in a column — deep draught below: constrained by her draught.
The honest caveat: a mnemonic identifies the vessel and says nothing about her heading, whether she is making way, or who gives way. That is settled by the sidelights, the aspect and the steering rules. Vertical pairs are also easy to misread in a swell, against shore lights, or at a range where two lights merge into one.
Common questions
What does a white light on the water at night mean?
It depends which one. A lone all-round white is a vessel at anchor or a small motorboat. A lone white with no sidelights is a stern, so you are overtaking. Two whites in a vertical line means towing or pushing. Two whites spread horizontally is a vessel of 50 metres or more, and the spread tells you which side she shows.
How do I tell a sailing vessel from a power-driven one?
By the masthead light. A sailing vessel underway has none: sidelights and sternlight only. A white above the sidelights means a power-driven vessel, sails up or not. A tricolour at the masthead is still a sailing vessel — the same three lights in one place.
What lights does a fishing vessel show?
A vertical pair of all-round lights: green over white for a trawler, red over white for any other kind of fishing. Sidelights and a sternlight appear only when she is making way through the water. If the gear extends more than 150 metres from the vessel, an extra all-round white light marks the direction it runs in.
What lights does a tug with a barge show?
Two masthead lights in a vertical line (three if the tow is longer than 200 metres), sidelights, a sternlight and a yellow towing light above it. The barge shows sidelights and a sternlight, no masthead light. The dark gap between them is the towline.
How do I know a vessel is aground?
Anchor lights together with two all-round reds in a vertical line; three black balls by day. She is going nowhere, so the manoeuvring is yours — and she is telling you the depth here is not what the chart says.
Where to go next
The catalogue gathers the configurations of lights and shapes in one place: "Learning → Lights and shapes". Each card has an aspect switch — bow, port, starboard, stern — the most useful thing on it, because the aspect is what catches people out. Alongside are the day shapes and the rule that prescribes the lights.
Once the catalogue stops surprising you, go to the lights trainer: situations on a black scene, no hull and no hints, each answer followed by an explanation with the rule number. Mistakes are grouped by topic, so a weak spot shows up quickly. Short guides to these screens are "The catalogue of lights and shapes" and "The lights trainer".
Identifying the vessel is half the job; the other half is who gives way. Read "Who gives way at sea: the COLREGs in plain English", then build your own case in "Who gives way", which has a night-view switch. The full text of the rules lives under "Learning → COLREGs".