The Dick O'Kane targeting method is a method submarine commanders can use to hit a target with a torpedo. What's great about it is that it only requires two things:
That's it! You don't need to know target range, or angle on the bow, or anything else. For tutorials/explanations, see two great posts by 'Rockin Robbins', one in the sobsowespac forum, or this one in the subsim.com forum.
But, how do you figure out how much you want to lead the target by? The posts I've seen usually give simple rules of thumb, like "10 degrees for a slow target, 20 degrees for a fast one", or "about 1/3 of the target's length", but I'm anal and like to know exactly.
The lead angle is a function of two things: the speed of the target (Vtarget), and the speed of the torpedo (Vtorp), and it's simply equal to atan(Vtarget, Vtorp). The calculator below lets you figure it out, for example it will show you that if the target is going 10 knots, and your torpedoes go 36 knots, the optimal lead angle is 16 degrees: