There is a particular kind of satisfaction in a Four-Sein day. You round Brittany's northwest shoulder through the Chenal du Four, cross the Iroise Sea, and slip through the Raz de Sein, all in a single tide, and by evening you are anchored off the Glenan or tucked into a south Brittany harbour with the whole exposed corner of France behind you. Done right it is one of the great day passages in European cruising. Done wrong it is hours of foul tide and a fight through overfalls at the worst end of the day.
The two passages are usually treated separately, and the Chenal du Four and Raz de Sein passage is the obvious place to read each one in detail. What follows is about linking them, because the timing that makes one easy can make the other impossible, and the trick is fitting both into the same tidal cycle.
The two gates and their numbers
Both passages are tidal gates, and both reference their slack water to high water at Brest, which is the standard reference for this whole corner of Brittany.
- Chenal du Four: slack water occurs around HW Brest plus 5 hours (for about 30 minutes) and HW Brest minus 1 hour (for about 10 minutes). Streams in the channel run hard, several knots at springs.
- Raz de Sein: slack water occurs around HW Brest plus 4 hours (for about 30 minutes) and HW Brest minus 2 hours (for about 10 minutes). The stream here reaches up to about 6 knots at springs.
Look at those numbers and the problem reveals itself. The two slack windows are close together but not identical, and the passages are roughly 25 to 30 miles apart by the time you have crossed the Iroise. You cannot be at both gates at slack. So the day is not about hitting two slacks. It is about hitting one gate at slack and arriving at the other with the tide working for you rather than against you.
Southbound: the classic plan
Most people run this north to south, and that is the easier direction to time.
The strategy that works is to go through the Chenal du Four on the last of the south-going ebb, near the end of the favourable stream rather than dead slack. You then cross the Iroise Sea against weak north-going currents, which are gentle in open water, and time your arrival at the Raz de Sein to catch the next ebb stream. That gives you a fair tide through the Raz and a favourable current carrying you across the Bay of Audierne for the next three or four hours, towards the Pointe de Penmarc'h and on to south Brittany.
In practice that means leaving your overnight port, often L'Aberwrac'h or Camaret, with enough time to reach the top of the Four around the start of the south-going stream. The L'Aberwrac'h as a first French port makes a logical jumping-off point at the northern end, while Camaret sits closer to the Four and shortens the first leg.
The Iroise crossing in the middle is the bit nobody talks about and the bit that makes the day work. It is open water, the currents are weak there, and it gives you the slack you need between two hard gates. Use it. Do not rush it. If you are early for the Raz, slow down and arrive on time rather than punching the last of the foul stream.
Northbound is harder
Running south to north, the timing is trickier because you are trying to take the flood through the Raz and then arrive at the Four before its favourable window closes, and the geometry does not line up as kindly. It can be done, but you have less margin, and a slow boat or a foul wind can leave you arriving at the Four against the stream. If you are northbound and the day does not obviously work, split it. Stop overnight at Camaret or Douarnenez and take each gate on its own tide. There is no shame in it and a great deal of comfort.
The hazards in between
The Chaussee de Sein is the thing to keep in your mind the whole time. It is a reef that extends roughly 15 miles west from the Pointe du Raz out past the Ile de Sein, and the Raz de Sein itself is only a two-mile-wide gap near its inshore end. The seabed there is scattered with rocks and shallows. You are threading a gap, not crossing open water, and the streams set you sideways towards the rocks if you are not watching your transits.
Wind against tide is the killer at both gates. When a fresh breeze blows against the stream the overfalls build steep and confused, and a small yacht can be thrown about badly. The standard advice, which I follow without exception, is to take both passages at or near slack if there is any wind against the tide, and to abandon the linked day altogether if it is blowing hard. Brittany also gets fog on this coast more than visitors expect, and the tidal streams in the Brittany gates become a lot more frightening when you cannot see the marks. If the visibility is poor, do not attempt the link.
What it is like in the middle
The part of the day I remember most is not either gate, it is the Iroise crossing between them. After the focus and concentration of the Chenal du Four, with its marks and transits and hard stream, you come out into open water and the pressure drops away. The currents are weak, the sea opens up, and for a couple of hours you are simply sailing across the Iroise with the Pointe Saint-Mathieu astern and the Pointe du Raz somewhere ahead in the haze. It is the breathing space the day is built around.
Use that space to get ready for the second gate rather than to relax completely. Check your timing against the Raz slack, eat something, brief the crew on the next set of transits, and adjust your speed so you arrive at the Raz exactly when you want to, not early and punching the last of the foul tide. The boats that have a bad afternoon at the Raz are almost always the ones that treated the Iroise as a rest and arrived at the gate without a plan.
My checklist for a Four-Sein day
Before I commit, I want all of these to line up:
- The HW Brest time for the day, with the two slack windows worked out from it.
- A realistic boat speed that gets me from the Four to the Raz inside the tidal cycle, usually meaning I need to make good at least 5 to 6 knots across the Iroise.
- A wind forecast that is not against the stream at either gate during my window.
- Good visibility, or at least a confident fallback if fog rolls in.
- A bolthole identified, Camaret or Douarnenez, in case the second gate slips out of reach.
One more honest note. The published slack times are a guide, not a guarantee. They shift with the coefficient and the actual tide on the day, and several experienced sailors will tell you the book times do not always match the water. Check the stream atlas, watch the sea as you approach each gate, and trust your eyes over the almanac when the two disagree.
Get it right and the Four-Sein day is the passage you will talk about for years. You take the two most serious tidal gates on the French Atlantic coast, link them across the quiet Iroise, and arrive in the gentler waters of south Brittany having done a proper piece of pilotage. Plan it around the tide, respect the wind, keep a bolthole in your back pocket, and it is one of the most rewarding days a cruising yacht can have in France.
