PavelKirilovich, when moving slow, do not forget to turn on 'silent running' (Z key). This options shuts talking, pumping and loading of torpedoes.
Further, do not pass 100 rpm. There is a dial in the command room (shift+F2).
What I always do is get in the path of the convoy, 2500 meters from the center. Persicope depth 14 meters. You can use plotting the center forward escort to estimate the center of the convoy's tracks. I should then be between the first en second convoy lane; just inside the convoy on a perpendicular course (I mean exactly 90 degrees). That's where I start waiting and let the forward escorts pass by. I use my
'torpedo bearing table' to set up a straight shot at two ships and send them both two torpedoes. Even before I launch I hit the D key to begin a slow silent dive and after launch I set speed to just below 2 knots, making sure the engine rpm stays below 100.
I turn the boat to pass the convoy diagonally (is that a word?). I then maneuvre depending on what the escorts do. Stay out of their path.
I never go below 140-150 meters because there is a point where you cannot stop the boats dive at this slow speed anymore, and you will only descent unless you increase speed, alerting the escorts!