If you are having strategic difficulties beyond the manual targeting woes then you might want to give playing with auto-targeting on or at least Weapons Officer solution plotting a shot (that isn't necessarily cheating becuase sometimes the Weapons Officer did plot the solution based on info provided to him by the Captain). Once you get a feel for how your ship handles and how to approach an enemy with the intent to attack then you can move ahead to the more advanced mind boggling stuff. What I think many new Kaleuns suffer from when trying to play purist style right away is mind overload; suffering from the demand for too many decisions without enough experience to act instictively.
My advice is turn off the Manual TDC and cruise a few patrols until you develop a style of play. Don't be afraid to die a few times or play a risky gambit, thats the beauty of computer games: infinite replayability .
Also you should probably read alot of outside sources on how to play. THis is an excellent guide on how to do many different kinds of manual plotting. Differnt styles for different minds or situations:
http://www.paulwasserman.net/SHIII/#Introduction
ANd just in general experiment. Have fun. Maybe even read some real documentation about U-boats. There is a literal plethora of information out there. If you can immerse yourself in the method of thought in U-boat command then you can start to understand the correct moves in situations. Its tricky but once you get comfortable you'll wonder how you ever missed.