Module 04 · Tidal streams
Reading notes · Module 04 · Tides
Set and drift. The diamonds on the chart, and the atlas.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
What moves sideways
The same tide, two questions
The last three parts asked one question: how much water is there? This one asks the other: which way is it going, and how fast?
While the tide rises and falls at Aldern Quay, the water of Skelmar Sound runs one way and then the other. That is a tidal stream: the same tide, seen sideways.
The arrow in the middle of the Sound is the water at one place, four hours before high water. In a few hours it will be pointing the other way.
Thirteen hours of one place
Diamond C, in the middle of the Sound, hour by hour. Above, the height at Aldern Quay on 1 May 2026; below, the water at C.
Four hours before high water: 095 at 2.3 knots — going east. One before: 049 at 0.5 — almost still. Three after: 271 at 2.2 — going west.
Two words carry it. Set is the direction the water is going to; rate is how fast, in knots.
And the one that decides a passage: the slackest water is not at high water. At C it comes 1 hour before it.
Move the hour
Drag the hour from six before high water to six after. The height climbs and falls on the curve; the arrow at C swings round.
Watch the two together: they do not keep time with each other.
The tidal diamonds
Ten places where the stream is given
A magenta diamond with a letter marks a place on the chart where the stream is given hour by hour, in a table at the side of the chart.
They sit where the water does something: A, the gap north-west of the Sound, between the islet and the north shore; B, the western Sound, south of the gap; C, the middle of the Sound, off Fladda; D, the eastern Sound; E, the way out to the north-east; F, the southern channel, east of the precautionary area; G, the passage north of Auskerry; H, the channel west of Skelvoe; I, the bay south of Skerry Steps; J, the approach to Kirkvoe Haven.
And the table says which port it is counted from: referred to HW at Aldern Quay — a port that is not on this chart. A stream is counted from the high water of one standard port, wherever that port is.
The table, column by column
Rows: 6 before … HW … 6 after. For each diamond, three columns: Dir in degrees true, Sp the rate in knots at mean springs, Np at mean neaps.
Diamond C, row 3 after: 271 · 2.2 · 1.4.
Which row?
On 1 May 2026 high water at Aldern Quay is at 0937. At 1230 it is 2 h 53 min after it, and that is rounded to the nearest whole hour: row 3 after.
At C: 271, 2.2 at springs and 1.4 at neaps.
Between springs and neaps
The day is hardly ever springs or neaps
Where a day falls is said by its range — and it has to be the range of the tide you are working in: the high water you count from, and the low water next to it on the side of your time.
At 1230 on 1 May you are after the high water of 0937, 6.8 m, so the low water is the next one, 1522, 2.0 m: the range is 6.8 − 2.0 = 4.8 m. Between 4.2 at mean neaps and 6.4 at mean springs, 27 % of the way to springs.
Before a high water it would be the low water before it: the rising tide.
The sum
The rate moves from the neaps figure towards the springs figure in the same proportion as the range.
At C at 1230 on 1 May: row 3 after gives 2.2 at springs and 1.4 at neaps; the range is 4.8. So the stream is 271 at 1.6 knots.
The tidal stream atlas
Thirteen pages, one for each hour
Where a diamond gives one place, the atlas gives the whole chart: an arrow every 0.9 miles. Every arrow is the same length; its thickness is its strength. This page is 3 hours before high water at Aldern Quay: the Sound runs east.
And 3 hours after: it runs west. Each arrow carries two figures in tenths of a knot, neaps and springs: 15,22 is 1.5 and 2.2 knots — not 15 and 22. The foot of every page carries the sum for the day.
Which page?
The same sum as the row of a diamond: high water at Aldern Quay on the day, and the time you want, rounded to the hour. On 1 May at 1230: the page headed HW ALDERN QUAY +3 h.
Read an arrow, and measure it
On that page, the arrow at 46°14.9′N 5°53.1′W: figures 15,22 — 1.5 knots at neaps and 2.2 at springs.
Its direction is not written: you measure it, with the plotter you used in module 02. Drag the edge along the arrow, the arrowhead of the plotter the way the water goes; turn the rose until its grid lies north–south; and read the index. This one is 276. Measured like that, it is good to 3°.
Between the arrows, step by step
The place you want is hardly ever on an arrow. At Y, 46°15.3′N 5°54.0′W, on page +3: take the three arrows round it.
Measure how far each one is, with the dividers — the ones you have used since module 02. Click on Y, then on the middle of the arrow: that is the span. Drag the dividers to the latitude scale at the side of the chart and read it: one minute of latitude is one mile. Never the longitude scale along the top. Here: 261 · 2.0 kn at 0.54 M, 276 · 2.2 kn at 0.76 M, 288 · 1.6 kn at 0.77 M.
The nearer an arrow, the more it counts: each one weighs one over its distance, and the three weights are shared out so that they add up to one: 1/0.54 → 42 %, 1/0.76 → 29 %, 1/0.77 → 29 %.
Then the set and the rate are each the three figures mixed with those weights: 273 at 1.9 knots at springs. The water at Y runs 272 at 2.1: reading between arrows is good to a few degrees and a couple of tenths, and that is the margin this chapter accepts — 5° and 0.2 knots.
Mixing sets like this only works when they are close together. Across north it goes wrong: 350 and 010 both point north, but their average is 180 — due south. So when the sets you mix lie on both sides of 000, add 360 to the small ones first, mix, and take 360 off if the answer comes out over 360.
Diamond or atlas
At a diamond they agree
The atlas has an arrow on every diamond. At C, page +3: 14,22 at 271. The table of C, row 3 after: 271 · 2.2 · 1.4.
It is the same water, measured once.
Away from it, they may not
In the southern channel, 0.6 miles south of F, where it narrows, one hour before high water. Diamond F says 038 at 3.7 at springs; the nearest atlas arrow — in the narrow part — says 31,48: 4.8 knots. The water at X runs at about 5.
Use the source nearest to where you are, in the same channel. A diamond in another channel is no use however near it is.
And when two disagree, which figure you plan with depends on what the stream does for you. When it is against you, or when it decides a gate, plan with the stronger: that is the one that can stop you. When it is with you, do not count on the stronger to work out when you arrive: it gives a time that is too good, and you get there late with the plan saying you are early.
Tidal gates in Skelmar
The strongest water on the chart
The gate is worked where the water limits you: the narrow part of the southern channel, not diamond F, which sits in wider water and reads short. The atlas gives that place hour by hour — its arrow, page by page.
At springs it reaches 4.8 knots, 1 before high water, going 017.
Kittiwake does 5 knots through the water. Head-on, you subtract: against 4.8 she makes 0.2 over the ground. From the side it is no longer a subtraction — that is module 05.
The gate, in its table
Going south, the stream is against you whenever the narrow arrow runs north. With a rule of no more than 2.5 knots against:
at springs the channel is shut at 2 before, 1 before, HW, 1 after; at neaps only at 1 before and HW.
The gate is wider at neaps.
Each row is an hour
A row of the table is not a moment: it is the hour centred on its time. On a day with high water at 1526, the row 1 before covers 1356 to 1456, the row HW covers 1456 to 1556.
So a gate made of shut rows opens half an hour after the time of the last one, and shuts half an hour before the time of the first. The line below is that day, row by row.
What a gate does not promise
A gate worked out on paper says when the water lets you through. It does not say you will be there.
You may arrive late — a slow leg, a late start. Getting through takes time: the gate has to be open for the whole of your passage through the narrows, not just when you enter. And wind against the stream raises a steep, breaking sea in exactly the places where the stream is strongest.
So have the other plan ready before you go: where you wait, or where you go instead, if you miss it.
All of it, on your own
What you can do now
Where, when and which day: a stream read from a diamond or from the atlas, for one hour, with its set and its rate.
On 1 May at 1230, at C: 271 at 1.6 knots. That is exactly how module 05 hands you a stream — «271 at 1.6 knots for this hour».
What you do with it — draw it, see where it takes you, work out the course to steer — is module 05.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.