The best tonnage grabbing areas I've found so far:
Northern half of Gulf of Cadiz, Draw a line from Strait of Gigraltar to edge of the shallows off Cape Sao Vicente. -- Be ready to crash dive, lots of aircraft even early in the war.
East -west line 20 kms. south of nordern edge of AM51.
North-south line just east to Cape Verde Islands.
Just East of Belle Isle Strait (near Scapa)
BF 14
Good hunting