I did it both of the last 2 years in a 2ft 6" draft Snapdragon. No probelms at all. You should be all right at 3'6" but if it's really 4' take some stores or water off! The chef du port in Dinan will tell you what's the situation when you get there. He'll drop your mast for you too for about £12! In any case if there's a difficult patch it's the very beginning, the first 10 k after Dinan. After that there's plenty of depth. So even if it seems like hard work, don't presume the hard work will last for all 60 odd locks. It's a great trip.
Very interesting.
Stayed in Dinan for 3 days last summer. Loved it.
Took tender up river through 1st lock, and it all looked quite delightful.
A friend was asking about this same trip, through to W. Brittany coast, last weekend and I said forget it.
Too tall and too deep.
He draws about 1.3 mtrs, with air draft about 3 mtrs (min').
He had one of the French Navicarte books which showed min' depth in one section of 1.2 mtrs. Can't remember the min' airdraft.
He would love to do the trip. Perhaps you could advise.
It is a motor boat 50 x 14 x 4+. Atlantic 50, rear deck style steering position, folding radar mast and folding windscreen.
I agree with you. If he was 25 ft single keel sailing boat I'd say go for it. The really shallow bit is only about a mile long at Evreux, I think it's called. But 1.3 on a big boat like that, and then you have the problem of the cill depth as you climb locks. I think it would be impossible. He could always rent a little penichette in Redon and have a lovely time cruising the canal, worry free. Or the skipper of one of the cross channel ferries has a couple of very nice English style canal boats which he rents out at Hede.
John
