i'm playing feb 1940, i'm on my 4th patrol in U-36 and my last 2 patrols were both north atlantic convoy routes.
all i've got were single ships tootling along. hell i went into lock ewe and found 1 t2 tanker (and a hell of alot of destroyers!) but not seen a convoy yet.
i have 3 torps left in my bow tubes and i reckon i'll prolly trip accross one any day now
some people seem to only get convoys, others just singles. all depends what year your running.