lo mate
i too am currently patroling similar areas to you and ive found quite a dangerous but busy area to the west of scapa flow...in shallower waters granted ....right round to the north west point of scotland...seems to be a steady stream of freighters for me...and a couple of warships too mind!!
thing is this early in the war the british arent at their best in anti submarine tactics so its possible to evade.
hope this helps
p.s you asked about maps showing convoys.....you do know there is a map hiding at the top of your F5 screen in game showing common convoy routes around the british isles and elsewhere??