Lights and shapes of vessels — What she shows
Reading notes · Lights and shapes of vessels
When they are lit, the four kinds of light and the arcs they cover, her length read off her mastheads, and the five day shapes.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
What she shows · topic 1
The same sentence, twice
Everything in this module is one idea, and here it is before anything else: what she shows by day and what she shows by night say the same thing. They are not two systems to learn. They are one sentence written twice, once for the light and once for the dark.
On the left she is carrying two black balls. On the right, the same vessel at night, carrying two all-round red lights. Both mean exactly one thing: I am not under command — something has failed and I cannot get out of your way.
Learn to read one and you have read the other. That is why they are taught together and never in two lists.
When they are burning
Before any of it: when are they lit? Rule 20 answers in one sentence and most people only hear the first half of it.
From sunset to sunrise, in all weathers. Not «when it gets dark» and not «when you think you need them» — the sun goes down and they go on, clear night or filthy one. That much everybody knows.
And by day, in restricted visibility. That is the half that gets missed, and it is the one that changes what you expect to see. Fog at noon and every vessel around you has her navigation lights burning — in daylight, with her day shapes up as well.
Which is why this is also the door into the last part of this module. In fog she is lit and she is sounding her whistle, and the two go together: the same conditions switch on both. A ship blowing a long blast at midday is not making a mistake — she is obeying Rule 35, and her lights are on for the same reason.
There is a second half to Rule 20 about not showing other lights that could be mistaken for these. It belongs to navigation in reduced visibility and it is not in this module, which is about reading the lights themselves.
The horizon, divided once
A vessel's lights are not told apart by colour or by height. They are told apart by where you have to be standing to see them. Rule 21 does nothing but divide the horizon, and every light in the book is one of these four slices.
The two sidelights take 112.5° each, from right ahead to 22.5° abaft their own beam. Put them together and they are 225° — which is exactly the masthead light, sitting on top of both. In a vessel of less than 20 m the two of them may live in a single lantern on her centreline — Rule 21(b). One lamp, and still the same two arcs and the same two colours.
What is left behind is 135°, and that is the sternlight. 225 and 135 make 360: the horizon is covered once, with nothing doubled and nothing missed.
An all-round light is the fifth kind, and it is the easy one: 360°, seen from anywhere.
From ahead you see two. From astern, one.
Stand right ahead of her and you are inside both sidelight arcs at once, because both of them start at right ahead. Red on your right, green on your left, and her mastheads above. That sight means one thing: she is coming at you.
Step round to her beam and one sidelight goes out — her own hull hides it. Now you see a single colour, and that colour tells you which side of her you are on.
Go one degree past 112.5° and both sidelights and both mastheads cut off together, and the sternlight comes on. From anywhere abaft that line you see one white light and nothing else.
That is the whole of overtaking, and it is in the arcs before it is in Rule 13.
What she shows · topic 2
Under way does not mean moving
Two words that sound alike and are not. Under way means only this: she is not at anchor, not made fast to the shore, and not aground. She may be dead in the water and still be under way. Making way is the second thing — she is moving through the water.
Between the first panel and the second, everything changes: anchor lights go out, her own lights come on. Between the second and the third, only the sidelights and the sternlight are added — and that is the whole of it. Those two are the only lights in the rules that mean moving .
Which is why you do not have to know what she is to know whether she is moving. Read the sidelights first and the identity afterwards.
One warning, and the next screen will charge you for it: a power-driven vessel keeps her sidelights whether she is making way or not. On her, the lights answer under way and stop there.
What she shows · topic 3
One masthead light, or two
Rule 23 hands you her length for nothing, and most people never collect it. A power-driven vessel under way carries one masthead light if she is under 50 m and two if she is 50 m or more, the after one higher than the forward one and further aft.
Two answers, not three. The rule has no way of saying 30 m or 200 m — it only ever says which side of fifty she is on. At anchor, Rule 30 gives you the same count: one all-round white under 50 m, two at 50 m and over. But not the same picture — the two of Rule 30 are stacked the other way up, the forward one higher, where the two of Rule 23 have the after one higher. Same count, mirrored.
And under 12 m the count stops working altogether. Rule 23(d) lets a small power-driven vessel put her masthead light and her sternlight together as one all-round white, with her sidelights below it. So she has no masthead light to count — not because she is hiding it, but because the rule let her fold two lights into one. One all-round white with sidelights under it is a motorboat, and it looks nothing like a ship.
And the third panel is the one to remember. A sailing vessel carries no masthead light at all, whatever her length — so there is nothing to count and nothing to conclude. The same happens to any vessel once you are abaft her beam: the mastheads cut off at 112.5° and all that is left is her sternlight.
When the count is not there, the honest answer is that her lights do not say. Guessing is how a ship becomes a fishing boat.
What she shows · topic 4
How far each light has to be seen
Every light in the book has a range: the distance at which it must still be visible on a clear night. Rule 22 sets it, and it depends on two things only — what kind of light it is and how long she is.
Read the table down a column and you see the first: a big ship has to be seen from further off than a small one. Read it across a row and you see the second, which matters more: the masthead light always reaches furthest, and the sidelights least — on a big ship, twice as far.
That is not a detail about lamps. It is what happens to a vessel as she goes away from you.
And it is a minimum, not a promise: a light may be seen further in clear air and less in haze. What the rule guarantees is that inside the range it is there.
She does not vanish. She comes apart.
Here is the table from the last screen, happening. One vessel of 120 m, one bearing, and you backing away from her.
At two miles you have all of it. At three and a half the sidelight goes — 3 miles is all Rule 22 asks of it. Past four and a half everything low has gone with it, and what is left is one white light six miles out.
So a single distant white light is not a small boat. It is just as likely to be a ship whose colour you have not reached yet — and that mistake, made at night, is the one that ends in a report.
Read what is missing as hard as you read what is there.
What she shows · topic 5
Four shapes, and the arrangements they make
By day there are no lights, so the rules say the same things in black shapes. The convention defines four: a ball, a cone, a cylinder and a diamond — and a diamond is itself two cones, base to base. Put two cones point to point instead and you have the fishing vessel’s mark: the same cone, arranged another way.
Read the row and you will recognise every arrangement, because you have already met all of them at night. Two balls is the vessel with the two all-round reds. Ball, diamond, ball is red, white, red, with the diamond standing exactly where the white light goes.
The odd one out is the single cone, point down: a sailing vessel that is also using her engine. At night she shows the lights of a power-driven vessel and there is nothing to see; by day the cone is the only thing that says it. It is the one place where the day says more than the night.
And a fishing vessel gets the same two cones whichever way she is fishing — which is why the next screen asks you to go from her lights to her shape, and never the other way.
What she shows · topic 6
At anchor: one ball, and one white light
The last one, and the one you will use most. A vessel at anchor shows one all-round white light where it can best be seen — and, if she is 50 m or more, a second one aft and at least 4.5 m lower. By day, one black ball forward.
Notice what is not there: no sidelights, no sternlight, no masthead. An anchor light comes alone, and that is the whole of the reading. Her type does not come into it — a tanker of 300 m and a fishing boat of 15 show you the same thing.
And there is one trap, which the next screen will charge you for. A sternlight is also a single white light. From abaft the beam of a vessel under way you see one white light and nothing else — the same count as a vessel of under 50 m at anchor.
You might think you could tell them apart by where the light sits: an anchor light up forward, a sternlight low and right aft. You cannot. Rule 30(b) lets a vessel of under 50 m carry her anchor light where it can best be seen — which is nowhere in particular, and includes low and right aft. The rule leaves it free, so the position tells you nothing.
When two readings are the same picture, the answer is that you cannot tell. Saying so is not a failure — it is the reason you keep watching, and it is the only honest thing to write in a log.
What you can read now
That is the vocabulary. Not one rule about who gives way, and not one vessel identified by name: what you have is the alphabet, and it is smaller than it looks.
You can now say, of any contact at night: which way she is pointing, from the arcs; whether she is under way and whether she is moving, from her sidelights; which side of 50 m she is, from her mastheads; and roughly how far off, from what has stopped reaching you. And you can say the same things in daylight, from five black shapes.
What you cannot do yet is name her. Two all-round reds, red over white, three reds in a line — you have seen them all in these fifteen screens and you have not been asked to say what any of them means. That is deliberate: reading comes before naming, and most people try it the other way round and never get the reading.
Part 2 takes one vessel at a time, by day and by night together.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.