There are two tidal gates standing between North Brittany and the Bay of Biscay, and they have a reputation that goes ahead of them. The Chenal du Four runs north to south along the inside of the Finistere coast, between the mainland and the offshore islands of Ushant and Molene. South of the Pointe du Raz lies the Raz de Sein, a narrow gap between the mainland and the Ile de Sein where the whole tide of the bay squeezes through a slot a mile or so wide. Get the timing right and both are straightforward. Get it wrong and you can be carried backwards over breaking overfalls in a foul spring stream, which is not where anyone wants to find themselves.
I have been through both gates a dozen times now, in everything from a flat summer calm to a brisk westerly that I should have sat out. What follows is how I plan the day, not a substitute for the chart and the almanac.
Why these passages matter
The west coast of Finistere is the corner of France. To get from the cruising grounds of North Brittany round into South Brittany and the Bay of Biscay, you have to pass it, and the only sensible inshore routes are the Chenal du Four and the Raz de Sein. The alternative is a long detour offshore round Ushant, which adds miles and exposes you to the shipping lanes and the open Atlantic swell.
Both gates carry serious tide. The Raz de Sein runs up to 6 knots at springs, and close around La Vieille lighthouse on the worst of the rock the water moves faster still. The Chenal du Four is gentler but still reaches 4 to 5 knots in places. A boat that does 6 knots through the water has no margin against a 6-knot foul stream, which is the whole reason the timing is non-negotiable.
Everything hangs on HW Brest
The single most useful thing you can do is reference the entire day to High Water Brest. The tidal atlas for the region is built around it, and the slack water times for both gates are quoted against it.
For the Chenal du Four, the slack water windows fall at roughly High Water Brest plus 5 hours and High Water Brest minus 1 hour, with the south-going stream beginning around High Water Brest minus 1. For the Raz de Sein, slack comes at roughly High Water Brest plus 4 hours and High Water Brest minus 2 hours. Different pilots quote slightly different figures, and the honest answer is that the published times are approximate, so treat them as a starting point and watch the water itself. I always cross-check against the tidal stream atlas for my exact date and coefficient rather than trusting a single remembered number.
The classic southbound plan links the two. You want to carry the last of the south-going stream down the Chenal du Four and then arrive at the Raz de Sein near its slack, so that the new south-going stream sweeps you through and on towards Audierne or the Bay of Douarnenez. Done well, you ride a fair stream almost the whole way. Done badly, you arrive at the Raz against a building foul stream and a head sea, and you turn back.
Read the coefficient before you commit
The French tidal coefficient, that number between 20 and 120, tells you how big today's tide is, and it changes the character of both gates completely. At a coefficient of 45 the Raz is a benign stretch of moving water. At a coefficient of 100 it is a different animal, with much stronger streams and far more violent overfalls if there is any wind against the tide.
My rule is simple. For a first passage, or with nervous crew aboard, I wait for a coefficient under about 70 and a settled forecast. Experienced and in the right conditions I will go on a bigger tide, but I would never take a high-coefficient Raz with wind against the stream. If the coefficient system is unfamiliar, our guide to reading a French tidal coefficient explains it, because every bulletin and marina office uses it and the gates demand that you understand it.
Wind against tide is the real danger
The tide on its own is manageable if you time it. What turns these gates dangerous is wind against the stream. A westerly Force 5 over a 6-knot south-going Raz stream raises short, steep, breaking seas that can be genuinely hazardous to a small boat, and the same wind over the overfalls around the Pointe du Raz creates standing waves that look like a tide race because they are one.
The cautious approach, and the one I follow, is to pass at slack water in settled conditions only, and to sit it out in harbour if the wind and tide are set to oppose. There is no prize for forcing a tidal gate, and L'Aberwrac'h or Camaret are pleasant places to wait a day. If you are arriving from the north, plan the wider North Brittany cruising guide route so that you reach the western abers with time in hand to pick your weather window rather than arriving committed to a fixed date.
The cautious route, step by step
Here is the shape of a typical southbound day, the way I run it.
I start from L'Aberwrac'h or, if I have already worked west, from inside the Bay of Morlaix. I time my departure so that I enter the Chenal du Four to carry the south-going stream, which means leaving with the tide rather than at a fixed clock time. The channel is well buoyed and the transits are clear in good visibility, but it is no place to be in fog, so I will not start unless I have reliable visibility and a forecast that holds.
Down the Four, past the Four lighthouse and the Pointe de Corsen, I keep to the channel and let the stream do the work. As I clear the southern end I aim to arrive off the Raz de Sein near its slack water, around High Water Brest plus 4 or minus 2 depending on which window I have built the day around. Through the Raz I hold the recommended track, give La Vieille and Tevennec a respectful berth, and watch for the overfalls that mark the edges of the deep water.
Once south of the Pointe du Raz the sea usually eases, and I have the choice of Audierne, the Bay of Douarnenez, or a longer run to Benodet and the Odet. From here the cruising opens out into South Brittany, and the gates are behind you.
A few hard-won notes
Carry the largest-scale SHOM charts for both passages and have the tidal stream atlas open on the chart table. A plotter is fine, but the atlas gives you the whole tidal picture at a glance in a way a moving boat icon does not.
Brief the crew before you start. Everyone should know that the plan is built around slack water and that turning back is a normal, sensible option, not a failure. I have aborted a Raz approach once when the wind freshened earlier than forecast, and we had a fine afternoon in Camaret instead.
Watch the actual water as you approach. If the overfalls are breaking and the sea looks worse than the forecast suggested, believe your eyes over the printed slack-water time. The gates are predictable enough to plan, but the Atlantic always keeps the right to surprise you.
Do all that, and the Chenal du Four and the Raz de Sein become what they should be: a satisfying, well-planned passage that opens up the whole of South Brittany, rather than the bogeyman of the pilot books. Before you tackle the corner, it is worth reading our L'Aberwrac'h guide to the first French port, because it makes an ideal base from which to wait for your window and stage the passage south.
