One thing to keep in mind is that if you are used to using the "fast 90" technique in the Pacific, using it in a U-boat involves a slight change.
Where as in a Fleet Boat you set the AOB at 90 Degrees plus or minus 10 degrees (for slow targets), in a U-boat you will set the AOB at 90.
Check out this thread for some smart people telling what I did wrong when I went from U-boats to Fleet boats.
http://www.subsim.com/radioroom/showthread.php?t=147428