A tidal gate is a stretch of water you can only sensibly pass during a certain window, because outside that window the stream runs too hard against you, or kicks up seas you do not want to be in. France has some of the best-known gates in Europe, and the way to understand them is not to read the theory in the abstract but to work one all the way through. So that is what we will do here, using the Chenal du Four, the channel between the Brittany mainland and the Ile d'Ouessant that almost every boat heading south must transit.
By the end you should be able to take any French gate and build the same plan. The Four is the obvious teaching example because so many southbound boats meet it first, and the full geography of the Chenal du Four and Raz de Sein passage is worth reading alongside this once the timing method makes sense.
What a tidal gate actually is
The principle is straightforward. In a constricted channel the tidal stream is fast. The Chenal du Four runs up to about 6 knots at springs. A boat that cruises at 6 knots through the water is therefore standing still over the ground against a full foul stream, or worse, going backwards, and the wind-against-tide sea state can be dangerous on top of that. So you do not fight it. You wait for the stream to slacken or turn fair, and you go through then.
The window when the stream is fair, or at least not crippling, is the gate. The whole task is working out when that window opens, and arriving at the entrance at the right moment.
Step one: find the reference
French tidal streams are tabulated against a reference port, and for this part of Brittany that reference is Brest. Every statement about when the Four turns is expressed as so many hours before or after high water at Brest. So the first job is to get the high water time at Brest for your day, from the almanac, and write it down. Everything else is built on it.
For our worked example, suppose the almanac gives high water Brest at 1200 local time, with a coefficient of 70, a mean tide. The coefficient matters because it scales the stream: on a mean coefficient the Four will run noticeably less hard than the 6 knots it reaches at a big spring, which makes the window more forgiving. If the coefficient idea is new, work through reading the French tidal coefficient first, because it changes how wide your gate really is.
Step two: work out when the stream turns
Now the key fact for this channel. The stream in the Chenal du Four turns to the south at around high water Brest. Before that, on the last of the flood, you would be punching a strong north-going foul stream if you were heading south. After high water Brest the stream sets fair to the south and carries you through.
So for a southbound passage on our example day, with high water Brest at 1200:
- Before 1200 the stream is foul for a southbound boat. Do not be in the channel then.
- From about 1200 the stream turns south and runs fair.
- The fair stream then carries for several hours on the ebb.
The commonly given ideal window for a southbound transit is from high water Brest to about high water plus two and a half hours, with a workable wider window of roughly high water minus half an hour to high water plus three. Either way it is anchored to that 1200 high water.
Step three: build the window for your boat
The published window assumes you are at the channel when it opens. You almost never are, so you work backwards from a target time to a departure time.
Say I am lying at L'Aberwrac'h to the north and I want to enter the Four just as the stream turns fair. I want to be at the northern entrance at about 1200, the high water Brest time, ideally a touch after so the stream is already setting my way. The passage from my anchorage to the entrance is, say, 12 nautical miles. At a planning speed of 6 knots that is two hours. So I need to weigh anchor at about 1000.
But I also want the last of the foul stream to be dying as I approach, not at full strength, so leaving a little later and accepting a slightly slower approach against the tail of the flood is often more comfortable than arriving early and loitering. The numbers give you the frame; judgement fills it in.
Step four: sanity-check the sea state
Timing for a fair stream is only half the job. The other half is the wind. A strong southwesterly against a south-going stream in the Four builds a short, steep, unpleasant sea even when the stream is fair, because wind over tide is the issue regardless of which way you are going relative to the ground. So I check the forecast and, if the wind is fresh and against the fair stream, I either wait for it to ease or accept that the gate will be lumpy and prepare the boat accordingly. The wider Brittany picture, where one gate leads to the next, is worth understanding through tidal streams and the Brittany gates, because the Four and the Raz de Sein further south are often planned as a pair.
Putting it together: the worked plan
Here is the whole thing on our example day, high water Brest 1200, coefficient 70, southbound from L'Aberwrac'h.
- High water Brest: 1200. The Four turns south at about then.
- Target entry at the north of the channel: 1200 to 1230, on the first of the fair stream.
- Passage to the entrance: 12 miles at 6 knots, two hours.
- Departure from anchorage: about 1000, adjusted slightly later to let the foul stream die.
- Window to clear the channel: 1200 to roughly 1430 for the ideal stream, with margin to 1500.
- Forecast check: confirm the wind is not a fresh southwesterly that would build a wind-over-tide sea.
That plan gets me through with the stream under me, which on a passage where the stream alone can be 6 knots is the difference between a fast comfortable run and a grinding fight.
Common mistakes that catch visitors
A few errors come up again and again, and all of them are avoidable. The first is timing the gate off your own high water rather than the reference port. The Four turns relative to high water Brest, not relative to high water at the little harbour you slept in; mix the two and your window is wrong from the start. The second is forgetting that the published window assumes you are already at the entrance. Sailors read high water plus two and a half hours, leave at high water, and arrive forty minutes late with the best of the fair stream gone.
The third is ignoring the coefficient. On a big spring around 100 the stream is much stronger and the usable window narrower, so a plan that worked comfortably on a coefficient of 70 may leave you fighting on a 100. The fourth is treating the fair stream as a guarantee of a pleasant passage. A fair stream under a fresh contrary wind still builds a steep sea, because wind over tide does not care which way you are going over the ground.
The last, and the most important, is rigidity. The numbers give you a frame, not a law. If the wind is wrong, or the fog comes down, or you are simply running late and would arrive on the foul stream, the right answer is to wait for the next tide. The gate opens twice a day, every day, and a missed window costs you a few hours, not a hull.
The method generalises
The reason to work it longhand is that the same five steps fit any French gate. Find the reference port. Find when the stream turns relative to its high water. Note the coefficient to judge the strength and width of the window. Work backwards from your target entry to a departure time using your boat speed and the distance. Then sanity-check the wind against the stream. Do that and the famous French gates stop being intimidating folklore and become arithmetic, which is exactly what they are. The chart and almanac give you every number you need, and the only skill is the discipline to lay them out in order before you slip the lines.
