Using the sub's periscope. you determine range and bearing of your target for several separate observations a few minutes apart, using the maneuvering board to correct for your boat's speed and course, you can then derive the position, course and speed of your target.

Then, knowing the speed of your torpedo, you solve the vector triangles so that you can aim your boat so the torpedo and target arrive at the same place at the same time. The "solution" is triangles drawn on the MB that you can measure with dividers and parallel rulers to get the necessary angle off the bow. The length of the vectors can also give you time-to-impact.

Torpedo away! Take her down to 200'!