When i search for any freighters and convoys im doing zigzag formations on the map. When i spot a convoy nearing my position
i try to go submerged on silentspeed next to them.
This way i can hit freighters from the side, when its night i attack at a very close range about 200/300m away from the target

hitting as many ships i can, then i go to a depth of 150m standard below the convoy and shutting everything off and staying put for a while.
Destroyers are getting frustated searching for me when all those freighters stand in the way

i try to avoid coastal waters because there are to many airplanes patrolling the area
the waters are too shallow, the Canal of Dover is a real nightmare.
Just give me the good old Atlantic