Module 04 · Standard port heights
Reading notes · Module 04 · Tides
The almanac page and the tidal curve, walked forwards and backwards.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
The page
The page you actually open
Everything in this block comes off one page. Here it is: two months of one port, day after day, and for each day the times and heights of its high and low waters. Your day is somewhere in there, and finding it is the first thing you do.
That is all an almanac is being asked to do here. How the book is arranged, and how you find a port you have never heard of, is the module on the almanac — not this one.
Three things to find before you read a single number: which port, which day, and which hour those numbers are in. The third one is the one that puts boats on the mud in summer.
You already know that last one: it is the line you met in the module on time. Here you only have to notice it.
Which port, which day, which hour
The port, at the top. A page is printed for one port and no other, and the figures below belong to it alone.
The day, down the left of each block: the date in big figures, and under it the day of the week. One block, one day — and the block carries everything that happens to the water that day.
Its tides run down the block, in clock order: four lines, one to a tide — high, low, high, low — each with the time it happens and the height it reaches. A day with only three tides leaves the fourth line empty.
Top left, in its box: STANDARD TIME (UT), and under it for Summer Time add ONE hour in non-shaded areas . Now look at the background of the blocks: it changes part way down. Before that Sunday the page and your watch agree; after it your watch says one hour more, and a tide read an hour late is a tide you have missed.
The curve
Four moments, and everything in between
The table gave you four moments of the day — two high waters and two low ones — and the water was doing something at all the other hours too.
Almost nothing you ever want to know happens at one of those four moments. You want to know at half past nine, because that is when you were thinking of leaving.
So the almanac prints a second thing beside the numbers: a picture of the shape the water makes between one turn and the next. Not for a day — for the port.
This screen is about what that picture is. Reading a height off it is the next thing, and it is easier once you know what you are reading.
The picture the almanac prints beside the numbers
Along the bottom, hours before and after high water — not hours of the day. High water sits at zero, and everything is counted from it. That is why the same picture works for any day.
Up the side, from low water at the bottom to high water at the top. Not metres: the fraction of your own range . And it is marked in tenths, so that a height you have worked out can actually be found on it.
Now the curve for a day of big range. Follow it from the left: the water climbs, turns at high water, and falls away again.
And the one for a quiet day. Same axes, different shape — a small tide is not just a big tide made shorter.
They are furthest apart on the way up, about three and a quarter hours before high water. On a big day that gap is the red figure, and it is why the almanac bothers to print two.
Height at an hour
The four marks that give you a height
Two boxes, side by side, and they share the same up-and-down: low water at the bottom, high water at the top. On the left, metres. On the right, the curve you have just met. That shared up-and-down is the trick — a line drawn across at one height crosses both boxes at once.
First mark: high water, on the top edge of the left-hand box, at its height in metres. That comes straight off the page you read in topic one.
Second mark: low water, on the bottom edge. Now the box knows how far the water is going to move today.
Third: join them. That sloping line is your day , and here is what it is for: it turns a fraction into metres. A quarter of the way up, halfway, three quarters — the line tells you what each one is worth today. Everything after this is reading, not deciding.
Now go in at the hour you care about, on the bottom of the right-hand box. It is measured from high water, not from midnight.
Up to the curve. That is how far up the tide has come at that hour, as a fraction of the day’s range — and the curve you are reading is the one interpolated for today, between the printed springs and neaps.
Across to the line — the sloping one, the one you drew. Where the two meet is your day’s water at that hour, and it is sitting directly above the metres scale.
And down to read it. Where you land on the bottom scale is the height of tide at that hour. Four marks and two lines — that is the whole method, and you will use it for the rest of the module.
Try it before it counts
The next screen asks you to do this for real. This one does not: the sheet is yours to play with. Drag on the top edge of the left-hand box to put high water where the table says, on the bottom edge for low water, and along the bottom of the right-hand box for the hour. Nothing here is scored — move things about and watch what happens.
Watch what the sloping line does when you move either end: a bigger range tips it flatter, and every fraction of the curve is suddenly worth more metres. That single line is the whole difference between one day and another.
And notice that the height does not change at an even rate as you move the hour. Around high water and around low water you can drag a long way for very little; halfway between, an hour costs you more than a metre. That is the whole reason the curve exists — if the tide rose and fell at a steady rate a ruler would do.
When, not how much
The question you will actually ask
You can now take a height off the sheet: give it an hour, and it gives you metres.
Nobody ever asks that. Nobody stands on the pontoon wondering how much water there is at three o'clock.
What you ask is the other way round: at what time will there be enough? Because what you are deciding is when to leave.
Same sheet. Same four marks. Walked the other way.
The same sheet, walked backwards
Here is the question you will actually ask, and it is not the one you have been practising. Nobody stands at a chart table wondering how much water there is at half past nine. They wonder how late they can leave it.
High water first, as before — same box, same top edge. Nothing about the sheet changes when you walk it backwards.
And low water. Careful here: a window crosses high water, so it touches both of the day’s low waters. You draw the line to the lower of the two — that way your window comes out a little shorter than the truth, which is the error you want to have.
Join them. Same line, same job: it turned a fraction into metres, and now it is going to turn metres back into a fraction.
Now go in at the height you need, on the metres scale, and drop down to the line. This is the one figure on the sheet that is not printed anywhere: it came out of your own subtraction.
Across to the curve — and look what happens. The line you have drawn meets the curve twice. Of course it does: the tide passes every height on the way up and again on the way down.
So there are two hours to read, not one, and you take them both down to the scale. And they are not mirror images: the second crossing sits 23 to 35 minutes nearer high water than the first one is early — measured across every stretch on this page. That is two quarter-hours of the scale you are reading, so working the second one out by subtraction puts you two marks wrong.
That is your window: from the first to the second, and everything in between is water enough. Neither one on its own answers the question.
Springs and neaps
Two curves, and your day is neither
The sheet has been printed with two curves all along: one for a day of big range, one for a quiet one.
Your day is almost never either of them. Measured over a year at Aldern Quay, 90 days of the 365 fall between the two — neither springs nor neaps.
So the two curves are not a pair of boxes to be sorted into. They are the two ends of a range, and the day you are working sits somewhere along it.
This screen is about landing in between.
Landing between them
Start where you always start: the range of the day, high water less low water, straight off the table. That single number decides the shape.
The printed springs curve was taken from a day of 7.6 m. Hold your day's range against it.
The neaps one from a day of 3.1 m. Now your day's range has one of the printed ranges above it and the other below.
And that is the whole of it. However far your range sits between the two printed ranges, your curve sits the same distance between the two printed curves. Not the nearest one — the one in between.
Without the sheet
The one you can do without the book
The curve you have been reading is printed for a standard port — and at a standard port a yacht usually has water to spare. The depth under her was never really the question.
Where you do put her keel at risk there is no curve printed at all: a cove, a small harbour, a bar, an anchorage off on its own. Nobody prints one for those.
What you have there is what the book gives for high water and low water — when they turn, and how high they get. And that is enough.
You have learned the sheet. This topic is for the water that has no curve: not a shortcut for anyone who would rather not draw one, but the only method there is where there is none to draw.
The rule of twelfths
You will not always have a sheet in front of you. Take the stretch from one turn of the tide to the next and cut it into six — those are the «hours» of the rule, and they are sixths of however long that stretch actually is, not hours of the clock.
And here is what the rule is standing in for: the curve you have been drawing all block. This is not a second method. It is the same method done in your head.
First hour: one twelfth of the range. The tide has only just turned and it is barely moving.
Second: two twelfths. Getting on with it now.
Third and fourth: three twelfths each. Half the range in two hours — this is the middle of the tide, and it is where the water moves fastest.
Fifth: two again, easing off.
And the sixth: the last twelfth. One, two, three, three, two, one — that is the whole rule, and it is worth knowing by heart.
The red bar marks where your staircase leaves the curve furthest behind. That gap is the price of doing it in your head instead of on paper.
And against the real tide — not the curve, the water — the rule is out by 0.17 m on average and 0.48 m at worst. Worst in the middle of the tide, where the water moves fastest and a mistake costs you most; best at the turns, where it barely moves. Knowing where it lets you down is what makes it safe to use.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.