Another thing I just thought of, when you have the ship locked up in the scope, click the word ship in the top right corner of the attack scope. This will bring up the ship recognition guide, and the exact ship you are locked onto. The guide will tell you displacement as well as the draft of the ship. (how deep the hull goes under water) If the ship has a draft of say, 9.8M set your torpedoes manually to a depth of 10.8-11M deep.
