There is a reason the cruisers who can sail whenever they like so often choose September. It is the month that keeps the best of summer and quietly drops the worst of it. The sea is still warm from three months of sunshine, the holiday hordes have packed up and gone home, the prices have dropped, and the harbours that turned you away in August are wide open. If August is the month to survive, September is the month to savour.
Warm water, no crowds
The defining September number is the sea temperature. The Mediterranean holds onto its summer heat well into autumn: at Nice the September average sits around 24 degrees, warmer than it was in June and only a touch off the August peak. The sea has all of summer stored in it and releases it slowly, so swimming off the boat in late September is still a genuine pleasure. The Atlantic and Channel cool faster but stay swimmable through the early part of the month.
While the water stays warm, the crowds collapse. The French grandes vacances end with the rentree, when the country goes back to school and work at the start of September, and the coast empties almost overnight. June and September are the two months consistently flagged for lower prices and thinner crowds, and September has the edge on water temperature. The anchorage that held fifty boats in August holds five. The marina that wanted a fortnight's notice waves you straight in.
The Atlantic tells the same story a touch cooler. La Rochelle holds around 19 to 20 degrees through early September, still firmly swimmable, and even north Brittany stays in the high teens for the first half of the month before the autumn chill sets in. So the warm-water argument is not a Riviera-only one: the whole French coast keeps its summer heat into September, just on a falling curve as you head north and west.
Prices follow the crowds down. You drop from peak rates back to shoulder-season pricing, often a large saving on the August berth fees, and you can cruise the trophy ports on spec again. The contrast with the peak is laid out in my comparison of the shoulder seasons for spring and autumn in France.
What you give up
September is not a free lunch, and being honest about the catches is what separates a good September cruise from a disappointing one.
The days are getting shorter, and fast. The autumn equinox falls around 22 September, so the month begins with daylight close to balanced and ends with the nights drawing in noticeably: by the last week the south coast is down to around twelve hours of usable light, well off the June solstice high of about 15 hours 30 minutes at Nice. That is still plenty for sensible day hops, but the long, lazy evenings of midsummer are gone and you plan passages with a closer eye on sunset. The further north you sail the more pronounced the loss, because Brittany and the Channel shed daylight quicker than the Mediterranean. Arriving in the dark becomes a real possibility on a longer leg, so the casual late departures of high summer need more thought.
The shore side starts to wind down. The seasonal-berth window runs roughly mid-April to mid-September, so towards the end of the month some marinas begin shifting back to reduced hours, and the harbour restaurants that reopened in spring start closing again from early October. You are still in season, but the back end of September is the start of the wind-down, not the middle of the party.
Weather: the autumn shift
September weather is the real reason to pay attention. On the Mediterranean it is generally a lovely month, often settled and warm, but autumn brings two things to watch. The first is the return of stronger mistral and tramontane episodes as the seasonal pattern starts to break down again; they are not as relentless as in spring, but a September mistral can blow hard, and you plan your Gulf of Lion and Provence legs with the bolthole in mind. My guide to the mistral and tramontane Mediterranean winds covers reading them.
The second is the autumn thunderstorm and the risk of heavy rain. September is when the Mediterranean can serve up its most violent storms of the year, the warm sea feeding intense cells and the occasional torrential downpour. They are less frequent than August's afternoon storms but can be more severe, so you take the weather warnings seriously and avoid being caught at anchor in an exposed bay.
On the Atlantic and Channel, September is the turn towards autumn. The first proper depressions of the season start tracking in on the westerlies, the settled summer pattern breaks up, and Brittany begins serving longer spells of unsettled weather. Early September is often glorious up there; late September starts to feel like the season closing. Brest's average wind is still modest in the first half of the month, around 9 to 10 knots, but the gusts that come with the early frontal systems are the thing to plan around, not the mean. You build more weather margin into Atlantic and Channel passages than you would in July.
Where September shines
For the Mediterranean, September is the month to go back to the places August ruined. The Lerins, the Hyeres islands, Porquerolles and the calanques are warm, swimmable and suddenly half empty. It is one of the months the best month to cruise Cote d'Azur guide singles out, precisely because you keep the summer sea without the summer scrum. Corsica in September is superb too, with summer water and a fraction of the boats, and the best month to cruise Corsica makes the case.
For the Atlantic, aim for early September and the south Brittany cruising grounds while the weather holds. The Gulf of Morbihan, Belle-Ile and the Glenan archipelago are excellent and quiet once the holidaymakers leave, and the south Brittany cruising guide gives you the route. The early part of the month is the window; by late September you are watching the forecast harder.
September is also the classic month to start positioning a boat for winter, to make the last big passage of the season, or to begin a run south before the autumn weather sets in properly. If those are your plans, the French sailing season and when to go where helps line up the timing, and if you are weighing the trade against the peak, cruising France in August lays out exactly what September lets you skip.
What is still open in September
The shore side holds up better than the daylight does. Through most of September the seasonal-berth window is still running, fuel pontoons are staffed, and the capitainerie offices keep summer-ish hours, though the very end of the month is when some marinas begin trimming back. The harbour restaurants that reopened in spring largely stay open until early October, so a September cruise still gets a meal ashore and a working fuel berth without the rationing of high season. The thing to confirm ahead is the smaller, seasonal-only harbours, a few of which start closing facilities from late September, the mirror image of the patchy early-season picture in cruising France in April. Plan the back half of the month around the larger ports that stay fully open, and you keep all of September's warm-water advantage with none of its wind-down friction.
Why the connoisseurs choose it
The case for September is the case against August turned inside out. You keep the warm sea, you keep the long-enough days, and you lose the crowds, the queues and the peak prices. What you take on is shorter daylight and the first stirrings of autumn weather, which is a fair trade for having the French coast handed back to you at summer temperatures. Sail the first three weeks, build in weather margin, respect the returning mistral and the autumn storms, and September gives you the cruise that August promised and could not deliver.
