If Bismarck reaches Saint-Nazaire

What happens if Bismarck survives the Swordfish attack and reaches a French repair port?

She is patched at Brest and moved to Saint-Nazaire, while Britain uses bombing and a reinforced submarine watch to contain her.

The counterfactual begins with the Swordfish attack failing and Bismarck reaching a French port. She would receive temporary work at Brest before making a fast transfer to the only suitable large dry dock at Saint-Nazaire. RAF attacks might damage the area but, judging from the experience of Scharnhorst and Gneisenau, would not reliably stop repairs. The Royal Navy would probably supplement Coastal Command surveillance with two submarines so that one boat could recharge while the other maintained the watch. The historical Channel Dash surveillance gap explains why Bismarck's destruction of Hood would make the Navy unwilling to rely on air patrols alone.