After fiddeling with the AoB dials today i actually managed to sink 4 merchants in a convoy. Mind you that it took several reloads of my initial save, until I realized i had my AoB "up-side-down" and therefor the fire resolution moved in the opposite direction.
First one I took down by letting the convoy move towards me at an angle, second one after avoiding the warships for a while.
Stayed hidden for almost and hour trying to slip away and when I pop back up I find that the two last merchants were behind me. Lucky me i still had my aft torps left, so I took my sweet time and down they both went. =D
Thanks for all the support and for one sweet mod!
/T-Fork