The most common reason for misses playing stock and auto targeting is failure to open the torpedo tube doors BEFORE you shoot. Use the Q key to open all your doors first.
Next most common is range, either too long or too short.
Allow at least 450 meters arming distance as mentioned earlier. Try for ranges between that and 1500 meters, any longer the target will see the torpedo wake in good weather. Bad weather and night time are your friends for long range shoots. Escort destroyers are very hard to hit as they see the torpedoes much earlier than merchants and also are much more maneuverable. For a long and healthy career avoid DDs as much as possible. Bad weather also helps you evade the escorts.
If you're playing the standard V1.4 as suggested get the U boat Missions expansion that is V1.5 and improves the Fleetboat game a great deal and adds some U boat action.
This game is a strategy game more than a shoot em up so keep that in mind and with a little practice you can have some success. Take your time and set up good shots, wait for dark or heavy weather. Once you start planing out a good attack and sink some ships I think you'll enjoy the game. As far as very deep diving goes it is not all that necessary in stock as the escorts are pretty easy to evade. Silent running and very slow speed will most often get the job done. Just takes practice.
Read the sub simmers Bag of Tricks thread by Rockin Robbins and you'll pick up a lot of good information.
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