[Pohl]

I definitely say that what makes a U-boat a unstopable machine it's the technology inside her, I did got my hands on a XXI in 1939....but I had no hydrophones, radar or any worthy piece of technology to detect cargo ships, the result is I ended trying to dive in the middle of a 3-4 destroyer task force on their way to Norway, which used me as a sitting duck in those vital 25 seconds.

So next time I got the XXI enabled all the top tier tech in 1939 and I found that XXI lack the stern tubes, which are sometimes vital to get rid of annoying destroyers doing DC runs, which the IX has 2 stern tubes giving you a total of 4 shots to take down any annoying destroyers (I sank HMS Hood with 2 stern torpedos) If I was using the XXI I would had to order full ahead and hard to port to be on a weird angle and be able to torpedo it.

I think subs will "fit" in this order:

Slow and weak: Type II
Fast and maneuverable: Type VII
Heavy weapons: Type IX
Convoy killer: Type XXI