Suggest, as well, that with the added length (and added displacement) the longer time to submerge would counteract any and all advantage to the extra horsepower. What made the type 7 boats so successful was the fact they were small and numerous. I believe there were a few made it over this side of the "puddle" and created a lot of havoc especially off the US coast where there was NO blackout or nav aids (bouys and lights) removed.

Cheers, Tom Hirst
Halifax, NS