There is a moment in the Raz de Sein, if you get your timing wrong, when the wind is against the tide and the sea stands up into short, steep, breaking walls that throw the boat about like a toy. I have been through it at the wrong time once, by a margin of about forty minutes, and I have no intention of repeating the experience. The Raz, like the Chenal du Four to its north, is a tidal gate, and a gate is a thing you pass through at the right moment or not at all.
For a visiting cruiser working down the west coast of Brittany, these two passages are the rite of passage, and the whole game is timing.
What a tidal gate is
A tidal gate is a stretch of water where the tidal stream runs so hard that you cannot, or should not, fight it. The flow constricts around headlands, islands and shallows, accelerates, and reverses every six hours or so with the tide. Pass at slack water, or with the stream behind you, and it is straightforward. Pass against a spring stream, especially with wind over tide, and you face a dangerous, sometimes impassable sea.
Brittany's west coast has two famous ones in quick succession. The Chenal du Four runs between the mainland and the islands off the northwest tip, and the Raz de Sein lies around the corner to the southwest, between the Pointe du Raz and the Ile de Sein. Most boats heading from the Channel coast towards south Brittany and Biscay pass through both, often on the same tide.
How fast they run
Fast enough to matter, and faster than many cruising boats can motor.
The Chenal du Four streams reach up to about 6 knots at springs as the flow squeezes around the underwater features. The Raz de Sein is similar and, in a wind-against-tide chop, considerably more violent for its short, nasty section. The tidal range in this area can reach around 8 metres on a big spring, which is what drives such strong horizontal flow.
Put that next to your own boat speed. A typical cruising yacht motors at 5 to 6 knots. If the stream is running 6 knots against you, you stop or go backwards. Even a 3 knot foul stream halves your progress and burns fuel for nothing. This is why you do not fight these gates, you time them.
The size of the stream depends on the coefficient. On a small neap, say a coefficient of 45, the streams are far gentler and the gates much more forgiving. On a 100 spring they are at their fiercest. Before I plan a transit I always check the French tidal coefficient for the day, because it tells me how hard the gate will be slamming.
Timing off HW Brest
Here is the practical heart of it. The streams in both gates are referenced to high water at Brest, the great tidal benchmark for this corner of France. You work your timing off HW Brest, not off your local harbour.
The standard pilotage, drawn from the usual cruising references, runs roughly like this.
- In both the Chenal du Four and the Raz de Sein the stream turns to the south at about HW Brest minus 0030, in other words half an hour before high water at Brest.
- For the Chenal du Four, slack water occurs around HW Brest plus 5 hours and again around HW Brest minus 1 hour.
- For the Raz de Sein, slack is around HW Brest plus 4 hours and around HW Brest minus 2 hours.
These are guides, not gospel, and you should cross-check them against your own up-to-date tidal stream atlas and pilot. But the principle is fixed: find HW Brest for the day, apply the offset, and you know when the gate is open.
Linking the two gates on one tide
The classic southbound run links the Chenal du Four and the Raz de Sein on a single fair tide. The trick is that a south-going stream that carries you down through the Four also carries you down through the Raz if you time your departure so the flow is going your way through both. Boats heading south typically aim to take the south-going stream, leaving the Four area so that they reach the Raz while the tide is still running fair and ideally near its slack, before it turns foul against them.
Get this right and the two most feared passages in Brittany become a pleasant, fast day's sail. Get it wrong and you arrive at the Raz with a foul spring stream and a building sea, which is exactly the situation to avoid.
Wind against tide, the real danger
The streams themselves are manageable. What turns a gate lethal is wind against tide. When a strong stream runs into an opposing wind, the waves shorten, steepen and break. The Raz de Sein in a fresh westerly against a spring ebb is genuinely dangerous for a small boat, far worse than the wind alone would suggest.
So my rule is two-part. First, time the transit for slack or fair stream. Second, if the wind is strong and against the direction the stream will be running, wait, even if the timing is otherwise perfect. The sea state, not just the speed of the water, is what hurts you.
My checklist before a gate
The routine I run before either passage:
- Find HW Brest for the day and apply the standard offset to find the slack and the fair window.
- Check the coefficient: a big spring means stronger streams and a narrower comfortable window.
- Check the wind direction and strength against the stream direction. If it is wind over tide and blowing hard, postpone.
- Build in a margin. I aim to arrive at the gate early and slack rather than late and foul, because forty minutes late at the Raz is a long forty minutes.
- Confirm my contingency: where I can wait, anchor or divert if the window closes.
If you are completely new to thinking this way, the Atlantic tides crash course covers the underlying ideas of range and stream before you tackle the gates themselves.
A word on the alternatives
Some visitors ask whether you can simply avoid the gates altogether. You can stand well offshore and go round the Chaussee de Sein, the long shoal that extends west from the Ile de Sein, giving the whole tidal mess a wide berth. In settled weather that is a perfectly reasonable choice and many do it, especially if their timing for the inshore passage has slipped. The cost is distance and exposure: you trade the short, sharp tidal section for more open water and a longer day. In a strong forecast I would rather take the inshore route at the correct slack than be out in the open off the Pointe du Raz in a blow.
The other honest point is that the two gates are not equally fierce. The Chenal du Four is longer but generally kinder, and a little foul tide there is usually a nuisance rather than a danger. The Raz de Sein is shorter but far nastier when it turns against the wind. If you have to compromise your timing, compromise it in the Four and protect your slack-water arrival at the Raz, not the other way round.
How the tide here compares
If your tidal education so far has been on the Mediterranean, the scale of all this can be hard to credit. The Med barely tides, so the idea of water moving at 6 knots is alien. The numbers in western Brittany are at the extreme end of what a cruising sailor meets anywhere, and they are why the official forecast and the right charts matter so much here. Carrying current and accurate charts for French waters is not bureaucratic box-ticking in this corner of the coast, it is the difference between knowing where the shoals are when the stream is sweeping you towards them and guessing.
Worth the respect, not the fear
None of this should put you off. The Chenal du Four and the Raz de Sein are passed by hundreds of cruising boats every season, and the great majority do it calmly because they did the arithmetic. The gates reward preparation absolutely and punish improvisation absolutely. Learn to time off HW Brest, watch the coefficient and the wind, and these two stretches become a highlight of cruising western Brittany rather than the thing you lie awake worrying about. The sea here keeps to a timetable. Keep to it with her and she lets you through.
