[SubConscious]

You're encountering realistic & historic problems with the torpedoes. The early war torpedoes ran deep and the magnetic pistols were far too sensitive (i.e., they'd explode if they passed over an iron ore deposit). The torpedoes were so bad, in fact, that they held a court of inquiry and imprisoned the persons responsible for testing them.

If you don't want to experience the same level of frustration as the historical U-boat commanders, there is a workaround for this problem: Simply go to your Weapon Officer's station (right click on the icon for his station), which will allow you to manually adjust the torpedo speed, pistol type (i.e., you can now use impact pistols), and running depth. If you do this, you'll override the historical features and your torpedoes will perform almost perfectly.

Good luck!