The first sea lock I took a yacht into, in Brittany, I got almost everything wrong. I arrived at exactly the listed time rather than before it, I had my lines coiled instead of ready, and I drifted across the chamber while a French sailor on the wall watched with the patient pity reserved for visitors. We came to no harm, but I learned that a sea lock is a small set-piece that rewards preparation and punishes improvisation. Once you have the routine, locking into a Breton or Channel harbour becomes one of the satisfying small skills of cruising this coast.
Here is the technique, built around the locks visitors actually meet.
Why these harbours lock at all
Much of the Channel and North Brittany coast has a tidal range so large that a marina open to the sea would dry out completely at low water springs. The solution is to impound the water behind a lock or a sill, keeping a usable depth inside while the tide outside drops away. Saint-Malo is the textbook case: the Bassin Vauban sits behind the Naye lock with more than four metres in the basin, reached down a channel dredged to about two metres. The lock holds the water in regardless of what the tide is doing outside.
That impounding has a consequence for you: the lock only operates around certain times, usually a window each side of high water, and outside those windows you wait. Knowing the times before you arrive is half the battle, and they live in the port pages of the almanac, which is one more reason to be comfortable reading a French pilot book and almanac before you cruise here.
Get the times and the channel before you arrive
The two pieces of information you want are the lock opening times and the VHF channel for the harbour office. At Saint-Malo the Bassin Vauban office works VHF channel 12, while the Bas-Sablons marina nearby answers on channel 09. Camaret, on the Crozon peninsula, works VHF channel 9 with two to three metres at low water in the marina.
The published lock time is generally when the gate opens to the sea. Arrive about 15 minutes before it, not on the dot, so you are positioned and ready when the gate opens and the outgoing boats clear. Call the office on the VHF as you approach to confirm you are coming and to be told where to wait. A short, clear call in basic French or English is fine; they deal with visitors constantly.
The waiting pontoon and the traffic lights
You rarely lock straight in. There is usually a waiting pontoon or a waiting buoy outside the gate, and at busier locks anchoring nearby may be forbidden. At Bas-Sablons, for instance, there is a waiting area with mooring buoys and no anchoring allowed. So plan to lie on the waiting pontoon, or pick up a waiting buoy, while the cycle runs.
The lock signals are the standard red and green light system. The short version: a red light means the lock is not available to you, stay clear or hold; red and green together usually means the lock is being prepared and will open shortly; a single green means the lock is open and you may enter. Watch the lights, do not just watch the clock, because the cycle can run a little early or late depending on traffic. When the gate opens, the boats inside come out first, then the green shows and you go in.
Inside the chamber: lines and position
This is where the technique matters. A sea lock chamber has vertical walls and the water level changes while you are in it, so you cannot simply tie off hard and forget it.
At many French locks the staff on the wall handle the lines for you. The drill is that someone on the lock wall throws down a heaving line with a weight on the end. You bend your mooring line onto it, with a good loop already made up, and they haul your line up and drop the loop over a bollard. You then take the line back to a cleat aboard and tend it. At locks without staff, you take your own lines round the vertical ladders or the bollards set into the wall, and you tend them as the level changes.
The points that make the difference:
- Have both bow and stern lines ready, flaked so they run free, before you enter. Coiled lines stuffed in a locker are no use when you are alongside a wall and the level is moving.
- Rig fenders generously, low down where the boat will bear against the wall, and have a fender board or a spare ready if the wall is rough.
- Once secured, tend the lines actively. As the water rises or falls you ease or take in so the boat stays snug to the wall without being held rigid.
- Keep the engine running and ready until you are properly secured, in case you need to nudge back into position.
A typical cycle has you in the chamber for around half an hour: the gate closes behind you, the water is adjusted to match the inside level, then the inner gate opens and you motor through into the basin.
Going out: the same in reverse
Locking out works the same way around the next opening window. You leave your berth in good time, motor to the inner side of the lock, wait for the green, and enter when the inside boats are clear. You tend your lines as the level drops to sea level, then leave when the seaward gate opens and the light shows green. The one extra thought outbound is the tide and stream you will meet the moment you clear the gate, so on the Brittany coast in particular check the streams before you commit; a foul gate just outside the lock can undo a tidy departure, and the wider picture of tidal streams and the Brittany gates is worth pairing with your lock timing.
Reading the depth and the sill
Some impounded harbours are reached over a sill rather than through a full lock, and the principle is the same: the sill holds water inside, and you can only cross it when there is enough depth over it. The clever bit is that these entrances usually have a lit depth gauge, a board showing the depth over the sill in real time, so you read off the number and decide whether you can go. Knowing how that depth relates to the chart and the tide table comes back to the datum, and a little fluency with French chart symbols makes the gauge and the chart agree in your head.
My pre-lock checklist
I run the same short routine every time, and it has never let me down since that first fumbling entry.
- Look up the lock times and the VHF channel the night before and write them on the chart table.
- Arrive at the waiting pontoon about 15 minutes before the opening and call the office on the VHF.
- Rig fenders low and plenty of them, and flake bow and stern lines ready to run.
- Watch the traffic lights, not the clock, and enter on green after the outgoing boats clear.
- Take or pass the lines, then tend them actively as the level changes, engine ready.
That is the whole art of it. A French sea lock is not difficult, it is just a sequence, and the visitors who look slick are simply the ones who prepared before the gate opened. Do the same and you will lock into Saint-Malo or Camaret like you have been doing it for years.
