*In response to your PM*
I'd go for a Type VIIC in 44, you'll end up with something like U-621 'U-Flak' in the picture below.
Everything Steve has posted is true but as for enhancing the damage shells cause you would be wise to use S3Editor.
Be advised....any changes to a shell calibre for example 20mm will mean the shell has the same destructive force by whichever side is so equipped.
The game engine will not allow you to pick one side only.
