Hitman's Optics Mod. This was already in the NYGM mod but now has a version for GWX 3.0. Its a must have as it is true to how range was actually determined.
If your new to subsims. Reading Wazoo's manual will save you a lot of frustration on manual targeting and intercepting. The game does not have a good tutorial on either of these.
I agree with what was said earlier about starting with stock. While your learning I'd leave the dud torpedoes off and use the external camera from the view of the target ship to see where the torps are missing. Its kinda fun watching the bubble trails.
I'd also recommend this thread for a crash course in interception and targeting.
http://www.subsim.com/radioroom/showthread.php?t=88961
After reading that thread my tonnage totals tripled.
Oh and on your keyboard the Q key will open your torpedo doors. You have to select the tube and press Q for each tube individually. If you don't open the doors first there will be a 2 second delay in firing. I bet I played a week before I knew this.