AoB is the hardest of them. And its the most convoluted. Angle on Bow is a means of determining course by estimating tyhe angle at while the target is to you relative to your own course. Or as most commonly explained, if you were standing on the bow of the target and looked at the U-boat, the angle that you are looking from the course of the ship is the AoB.
Most people estimate it visually. If you practise in the torpedo mission in the naval academy you can get pretty good at it.
However there is also an equation that works well if you're in the dark and can't see properly.
AoB = (Target True Bearing - Target Course) +/- 180
That works for me.
Also this little wheel works. There's a printable one and an online gif version.
I have the downloadable version, but I lost the link to the gif one.
http://files.filefront.com/AoB_wheel.../fileinfo.html