Hi Bubblehead ive done some diging on the internet and found this hope it helps some
https://www.history.navy.mil/researc...elligence.html
when i play the GWX Steel Sharks Campaign than run the gauntlet running on the surface in the bay of biscay i would dive to periscope depth as soon as i had enough depth under my keel.
U flak info below
https://uboat.net/types/u-flak.htm
normal crusing speed of a type VIIB,C-IXB,C would be 7knots that was deemed the most economical for fuel consumption and distance traveled