Im about ready to slam my head into the wall!!!

So Ive decided I need help, the past few careers Ive gotten, Ive been off'd in my first patrol, all because of Convoys. I dont get it, I approach at night on the surface far enough away from any ship and magically destroyers know Im there (this is before radar, I start in 40 to avoid the seas east of Britain)

I approach submerged and bam they still know Im there, (I know theyre not listening because I know they either stop and listen or move and use asdic, and they are always moving unless they know your there and there is more than 1 destroyer after you)

So ive decided I need some good ole' coaching with convoys, can anybody direct me to a thread that has lots of tips strategys or etc on convoy attacks or could anyone suggest a few things to me themselves?? I

Im tired of lone merchants and I want to make the move to the big leagues!