Module 06 · The book and the tide
Reading notes · Module 06 · The almanac
From Skelmar to the Channel: how the almanac is made, the standard ports, the secondary ports and the Solent.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
From Skelmar to the Channel
Two charts
Until now you have navigated on a chart made for learning: Skelmar Sound, with one standard port, Aldern Quay, and one tidal atlas referred to it. On purpose: so that you never had to ask referred to which port?
From here the chart is the real one: the English Channel. 11 standard ports and 40 secondary ports, two countries, two times in the tables, two chart datums, and a book you have to know how to open. Module 02 said it: the almanac gets a module of its own, later: module 06. This is it.
This was Skelmar. One port of reference, and every tidal figure hung from it.
And this is the Channel, the area of the course chart: every standard port with its own tide table, and the secondary ports that hang from them. The page is the first of the almanac.
The same gesture on both
A worked comparison. In Skelmar the stream at diamond C came from high water at Aldern Quay and the range of the day (module 04, part 4). In the Channel the stream in the Dover Strait comes from high water at Dover and the coefficient of the day. The three questions you asked in module 04, from where, at what time, on which day, are the same. What changes is the page that answers them.
Look at the page heading: HW DOVER. Every one of its thirteen pages is an hour before or after high water at Dover.
And look at its foot: how to interpolate between neaps and springs — with the coefficient printed under each day of the tide tables. That is the next chapter; here it is only shown.
What does not change, and where you learnt it
Every procedure you know stays exactly as it was. None changes its shape; what changes is which page the numbers come from, and that now you have to choose the page:
How the book is made
A book with a year
An almanac is bought every year, because its numbers go out of date: last year's almanac is a mistake with a binding. Every page of this one carries its year in the heading.
The heading of the Dover table: the port, its position, the months of the page and 2024.
And its foot, which every page has: where the figures come from, and not for navigation. This almanac is for training: it is made from the course's own tidal constants, and it says so.
Standard ports and secondary ports
A standard port is one whose tide has been measured in its own water: it has its own table and its own curve. It is not chosen for being important. A secondary port has no table: it has a difference on a standard port, and you work its tide from that port's page.
On this page every standard port has the secondary ports that hang from it: Portsmouth has 15, most of them on the Solent's own curves; Dover has 2, Rye and Folkestone; Boulogne-sur-Mer has 1, Etaples; and Calais has 0.
The heading tells you the time and the datum
Before any sum, three questions to the book: which page, in what time, above which datum. The answer to the last two is in the heading of every table.
Dover: TIMES IN UTC · standard (winter) time, on shaded days, summer time: add 1 hour, and heights above the Admiralty chart datum.
Boulogne-sur-Mer: TIMES IN UTC+1, and heights above the Shom's zéro hydrographique. Each coast prints its own winter time. It is what module 02 taught about the zone of a table; here you use it.
And the foot says when the clocks change in 2024: 31 March and 27 October, in red on the page. Between those two dates every day is shaded: add one hour for the clock ashore.
What goes under the date
Under each date: two coefficients — the morning and the afternoon high water — and the phase of the moon on the day it falls. On 19 September Dover prints coef 112 117.
The coefficient is French, and it is not a number of this port: it is worked out at Brest and holds for the whole Channel. It is printed in the English tables too because the Shom tidal stream atlas is entered with it, and that atlas covers both shores. That is the next chapter.
The moon agrees: full moon on the 18th, and big coefficients the day after.
The standard port, with module 04's sums
Dover's page, read like Aldern Quay's
Nothing new: the page of the standard port is read exactly as you read Aldern Quay's in module 04. Four tides a day, high waters in black, low waters in grey, time in four figures and height in metres.
19 September at Dover, in UTC: LW 0650 0.6 · HW 1128 7.4 · LW 1913 0.4 · HW 2352 7.3.
Dover's curve
The curve of the standard port, read as in module 04. The page carries its own worked example in red, and says what it is made of.
The example in red: HW 5.35 m at the top, LW 2.25 m at the bottom, joined; in at +2 h from high water, across to the line and down: 4.73 m.
What the two curves say: the mean range at springs is 6.05 m and at neaps 3.12 m. Those are the two figures you place the day's range between.
And what it costs to read it: 6.7 cm on average over the year and 13.7 cm at the worst hour. A printed curve is an approximation, and this page tells you how good.
Two standard ports of the Channel have no normal curve: Le Havre and Ouistreham print a special curve, hung from low water, because their high water has a stand. They are named here and not practised.
Secondary ports
40 ports without a table
A passage in the Channel nearly always starts or ends at a secondary port. The book does not give its tide: it gives how much earlier or later, and how much higher or lower, than its standard port.
Of the 40 secondary ports of the chart, 25 have their differences on this page; 14, between Christchurch and Selsey Bill, go by the Solent special curves (topic 5); and 1 has no data at all.
It is what module 04 was going to teach and left for a chart that could hold it. The chart is here.
Two rows, read together
Rye, under DOVER. The secondary port's row gives its differences; the standard port's row, above it, says when and at what height each difference holds. Read them together, column by column:
The easy case: the same figure at springs and neaps
Saint-Vaast-la-Hougue, under CHERBOURG: the same time difference in all four columns and the same height difference at springs and at neaps. There is nothing to interpolate.
The row: +0117 on every time, +0.4 on the high waters and −0.1 on the low.
Cherbourg on 19 September, in UTC+1: HW 0926 6.9, LW 0357 0.6.
Add and that is all: Saint-Vaast HW 10:43, 7.3 m; LW 05:14, 0.5 m — in UTC+1, the time of the page you read it from.
When springs and neaps differ: the Admiralty method
Eastbourne, under NEWHAVEN, on 1 September. Its springs and neaps figures are not the same, so you place the day between them: the time by Newhaven's time, the height by Newhaven's height.
The two rows. NEWHAVEN: high water at springs 0000 · 1200, at neaps 0600 · 1800; low water at springs 0600 · 1800, at neaps 0000 · 1200; MHWS 6.9, MHWN 5.2, MLWN 2.2, MLWS 0.8. Eastbourne: −0005 · −0001 on high water, +0021 · +0007 on low water; +0.8 · +0.6 and +0.2 · 0.0 on the heights.
HW Newhaven: 10:16, 6.1 m, in UTC.
Time: 10:16 lies between the neaps column, 06:00, and the springs column, 12:00 — 4 h 16 min past the first, of the 6 h between them: 71 % of the way to springs. Eastbourne's difference goes from −0001 at neaps to −0005 at springs: -4 min, so HW 10:12.
Height: 6.1 m lies between Newhaven's MHWN, 5.2, and its MHWS, 6.9: 53 % of the way to springs. Eastbourne's difference goes from +0.6 at MHWN to +0.8 at MHWS: +0.7 m, so HW 6.8 m.
Low water has its own columns and its own levels. LW Newhaven: 04:13, 1.5 m, in UTC. Time: 04:13 lies between the neaps column, 00:00, and the springs column, 06:00 — 4 h 13 min past the first, of the 6 h between them: 70 % of the way to springs. Eastbourne's difference goes from +0007 at neaps to +0021 at springs: +17 min, so LW 04:30. Height: 1.5 m lies between Newhaven's MLWN, 2.2, and its MLWS, 0.8: 50 % of the way to springs. Eastbourne's difference goes from +0.2 at MLWN to 0.0 at MLWS: +0.1 m, so LW 1.6 m.
Beyond MHWS: extrapolate the height
19 September at Newhaven: a big spring tide. Brighton, from Newhaven:
HW Newhaven: 11:45, 7.4 m, in UTC.
Time: 11:45 lies between the neaps column, 06:00, and the springs column, 12:00 — 5 h 45 min past the first, of the 6 h between them: 96 % of the way to springs. Brighton's difference goes from +0001 at neaps to −0002 at springs: -2 min, so HW 11:43.
Height: Brighton's difference is −0.1 at MHWS and at MHWN: HW 7.3 m. (The tidal stream does the same above coefficient 95 — next chapter.)
The same for every level: beyond MHWS or MHWN, or MLWS or MLWN, the height difference is carried on in the same proportion. The time difference never leaves its columns. The next practice needs it.
Between high and low water: the standard port's curve
A secondary port has no curve of its own: it uses its standard port's, going in with its own times and its own heights.
Brighton on 19 September at 10:40 UTC: its HW is 11:43, 7.3 m, and the LW on that side 0.5 m. 10:40 is 63 minutes before its high water.
On Newhaven's curve: Brighton's two heights on the scales, joined; in by the time, across and down: 6.7 m. The same figure the checker gets.
The small letter on the right
Each row says where its differences come from, and the foot of the page says how far to trust each kind:
M, measured from a tide gauge at the port: Brighton misses the water by 16 cm and 5 min. R, from the Shom's published levels: the error grows with the miles to the standard port. F, from the FES2022 model: 42 cm and 21 min to be expected.
A commercial almanac does not say it; this one does, and a careful skipper reads it.
The Solent, which is not like the others
A high water that lasts for hours
In the Solent high water is a stand of hours and its time says little; low water is sharp. So the curve is hung from low water, and between Christchurch and Selsey Bill the secondary ports page gives no differences: it sends you to the Solent special curves, referred to low water at Portsmouth.
Lymington: the double high water of the western Solent and of Southampton Water.
Portsmouth: the long high water of Spithead.
Christchurch Bay, from the Bournemouth gauge.
And Dover's, for comparison: a normal curve, hung from high water, with a sharp top.
The page's own example, all of it
The Lymington page carries its worked example in red: 19 September, the low water nearest midday, and the height three hours later. It is done with the steps you know, starting from the other end.
Portsmouth on 19 September: LW 1717 0.4, in UTC.
Lymington's differences on low water are printed on its page, and they are interpolated exactly as on the secondary ports page, with Portsmouth's columns and levels. Time: 17:17 lies between the neaps column, 12:00, and the springs column, 18:00 — 5 h 17 min past the first, of the 6 h between them: 88 % of the way to springs. Lymington's difference goes from −0013 at neaps to −0021 at springs: -20 min, so LW 16:57.
Height: 0.4 m is below Portsmouth's MLWS, 0.8 m: beyond the levels, so the height difference is extrapolated — carried on in the same proportion. From its MLWN, 2.0, to its MLWS, 0.8, is the whole way; 0.4 m is 133 % of it. Lymington's difference goes from −0.4 at MLWN to −0.1 at MLWS; carried on, +0.0 m, so LW 0.4 m.
Its high water is the next one: 3.2 m, at 00:26 on the 20th — past midnight, so the date changes, and on Portsmouth's page it is on the next day's line. That is the high water the example joins its low water to.
At 19:57, +3.0 h after low water: in by the hours from LW, across to the line, down: 1.86 m. It is the red example on the page.
Ports with no series of their own
Ports that share a body of water share a curve.
Cowes has no series of its own in this training almanac: it uses Lymington's curve, and the page prints it in red, borrowed curve. A port in red borrows the shape of its tide from its body of water, and the page says so.
Christchurch is not in the Solent: it uses the curve of its own bay, from the Bournemouth gauge. Read against Christchurch Bay's water, that curve costs 8.3 cm; Lymington's would cost 22.4 cm.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.