Go to your nav map and draw a line thou your boat and the target ship, now where the two line's intersect grab your protractor tool and place it at the intersection and drag it down to your sub and click, now from that point go to the center of the target ship and click, now look at your sub there will be a degree by it, that's the AOB.
Some time's when it's small angles you have to count back from 90 degrees.
http://www.hnsa.org/doc/attack/index.htm
I'll make a short movie this weekend and post the link here.