For the 'Arcade mode' as it were, open the TAI options in game and set 'round numbers on torpedo track' (or words to that effect). Then when you lock a target, just line up the matching numbers on the TAI by moving the periscope and fire when they match.
You can also lock a target, aim the periscope where you want the torps to hit, then unlock and fire straightaway - for some reason this auto-plots the torps, and it's possible to hit from several thousand metres with that trick.
If you want to give manual a try (with New UIs with TDC you can just switch to manual regardless of your difficulty setting), my rough written guide to AOB, Speed & Stadimeter is here:
http://www.subsim.com/radioroom/show...57&postcount=5
and the full no map contacts manual tutorial here:
http://www.subsim.com/radioroom/showthread.php?t=164917
