It takes a while to get used to the trigonometry involved in getting the torpedoes where you want them. To simplify, you want to be exactly perpendicular to your target's course. When you are within 10 degrees of this, get a scientific calculator and take the target speed and divide it by the torpedo speed. Once you have this number, hit Atan (or tan-1) on the calculator. The number is the lead angle for your shot.
With the tutorial, the targets are at 10 knots and your torpedoes default at 30 knots. The calculation comes out to roughly 18 degrees. That means we need to add or subtract this from 0 or 180 (straight ahead or straight back) to know where to shoot. Since the tutorial has the ships crossing starboard to port. (right to left facing north) we will add 18 degrees to 0 and when the ships cross the 18 degree mark on the periscope, we fire. A hit is almost guaranteed.
