I grew up sailing the Solent, where the tide is real but rarely terrifying. The first season I took my own boat across to north Brittany, the tide stopped being a footnote and became the thing that ran the day. Off Saint-Malo the range on a big spring can top 12 metres, and water that has to rise and fall that far has to move horizontally at a fair lick to do it. That moving water is the tidal stream, and learning to read it is the difference between a relaxed passage and an afternoon spent crabbing sideways wondering why the headland will not get any closer.
This is the beginner's version. No vectors triangles for now, just enough to read what the chart and almanac are telling you and to plan around it.
Stream is not the same as height
People muddle two different things. Tidal height is how deep the water is, and it goes up and down. Tidal stream is how fast the water moves horizontally, and it flows one way then the other. They are linked but not the same. You can have plenty of depth and a vicious stream, or a slack stream over a drying bank. On the north Brittany coast the streams are what dominate your planning, because they routinely run faster than a cruising yacht motors.
If you have not yet got your head round the height side, my piece on the French tidal coefficient explains how the French quote spring and neap strength as a single number, and that same coefficient tells you roughly how hard the streams will run on a given day.
Where the numbers live
Two sources give you stream data. The first is the tidal diamonds on the chart, lettered capitals inside small diamonds, each keyed to a table in the chart margin. The table lists, for each hour before and after high water at a standard port, the direction the stream sets and its rate. Rates come as a pair: the mean neap rate and the mean spring rate.
The second source is a tidal stream atlas, a booklet of small chartlets, one per hour, with arrows showing the flow across the whole area. On an Admiralty atlas the arrows carry tiny figures in tenths of a knot, written as two numbers separated by a dot. So a notation reading 12,23 means a neap rate of 1.2 knots and a spring rate of 2.3 knots, and the position of the dot marks where the measurement was taken. Once you know that convention, a page that looked like clutter becomes a clear picture of where the water is going.
The "hours before HW" trick
Every stream figure is tied to high water at a standard port, often Saint-Malo or Brest for this coast. The atlas pages run from 6 hours before HW through to 6 hours after. Each tidal hour is centred on the clock hour and runs from half an hour before to half an hour after. The chartlet labelled "3 hours after HW" is valid from HW plus two and a half hours to HW plus three and a half, not from HW plus three to HW plus four. Get that half-hour offset wrong and you can plan to leave on slack water and instead motor straight into the full flood.
So the workflow is simple. Find HW at the reference port for your day. Work out which tidal hour your passage falls in. Read the arrow or diamond for that hour. Note direction and rate.
Why this coast makes you respect it
The reason this coast is the place to learn properly is that the gates here run hard. The Chenal du Four and the Raz de Sein further south reach up to about 6 knots at springs, faster than most boats motor. I have written a fuller account in tidal streams and the Brittany gates, and if you are heading down the west coast you should read it before you go. But even on the gentler stretches around Saint-Malo and the Cotes d'Armor, a 2 to 3 knot stream across your track will set you a long way sideways on a 10 mile leg.
Putting it to work without the maths
Here is the beginner's planning method I still use for a straightforward passage.
- Decide your departure time so the stream is with you, or at worst slack. A fair tide of 2 knots added to a boat speed of 5 turns a 30 mile day into a six hour hop instead of ten hours of grinding.
- For each hour afloat, write down the stream direction and rate from the atlas. You end up with a little column of arrows for the day.
- If the stream sets across your track, expect to point your bow up-tide of your destination to hold the line. As a rough beginner's rule, for every knot of cross-stream against a 5 knot boat speed, aim off by about 10 degrees. It is crude, but it keeps you roughly honest until you learn to draw the proper vector triangle.
- Re-check your GPS track against your plot every half hour. The stream is the part of the plan most likely to surprise you.
Springs, neaps and why the same plan changes weekly
The streams you read off the diamond are quoted for two states, neap and spring, and the day you sail falls somewhere between them. Springs come twice a month, a day or two after the new and full moon, when sun and moon pull together and the range, and therefore the stream, is at its greatest. Neaps fall between, when the pulls partly cancel, the range is small and the streams are gentle. On this coast the difference is not subtle: a gate that runs 6 knots at springs might run barely 3 at neaps, and a crossing that is white-knuckle on a big spring is a pleasant motor on a neap.
The French express this strength as a coefficient running from about 20 at the deadest neap to 120 at the largest spring, with 70 as the average. A glance at the coefficient for the day tells you, before you even open the atlas, whether the streams will be fierce or forgiving. I plan demanding passages, anything near a tidal gate, for the smaller coefficients when I have the choice of date, and I keep the big spring days for runs where a fair tide will simply give me a free push down the coast.
Don't forget the height while you watch the stream
A trap for beginners is to fixate on the stream and forget that the height is changing at the same time. In north Brittany, where the range is huge, a bay that floats your boat at high water can be a drying sandflat four hours later. Plan the stream for your passage and the height for your destination together, because arriving on a fair tide is no comfort if the harbour has dried out under you. The two move in step but they answer different questions: the stream asks how fast you travel, the height asks whether there is water under your keel when you stop.
The day it clicked for me
I was crossing from the Iles Chausey towards Saint-Malo on a coefficient around 90, a strong spring. I had timed it to carry a fair flood and the boat was making over 7 knots over the ground on an engine that normally pushes us at 5. The cliffs came up fast and easy. Two hours later, mistiming the return against the ebb, I watched the same boat claw along at under 3 knots. Same engine, same water, opposite result. That contrast taught me more than any textbook: on this coast you do not fight the stream, you book your passage around it.
Once you can read the diamonds and the atlas confidently, the natural next step is timing an actual tidal gate, and the worked example in your first tidal passage walks through one start to finish. Learn it here, on the forgiving stretches, before you take it to the Raz.
