[stephenf555]

Interesting.
So why then the return, or continuation at least, of designing boats that were much slower submerged, with deck guns and AA and all that? Did the fast running submerged boats not work strategically for some reason?

I suppose that it may have just been down to battery characteristics...if you can only stay running submerged for a few hours and it takes a few hours to recharge on the surface, then you're going to have to spend a lot of time surfaced, and therefore you require AA and deck guns, and hence you go slower when submerged.