Try the strait between Orkney and Shetland, or north and north west of Shetland.
In my current (3rd) career, 3 rd patrol on a type VIIB, December 1939 I sunk three ships: a passenger cargo and a small freighter both with deck guns, and 1 large merchant (one torp and deck gun, hey I'm a casual skipper
) all ships sunk north of Shetland, in less than a day.