[U-96]

In 1939, the Brits did not fully understand the U-Boat threat at this point and most of their shipping was rather open and care free, Alot of them used the english Channel. just keep pacing back and forth at the mouth of the english channel (Don't enter it cause there's usually destroyers about there) But right at the mouth there's always lone merchants, even an occasional C2 transport (6000 tons of merchant, that a single fish under it's keel can sink)

Save your renown for a type VIIB while still in 1939 and go there again to really wreck havoc this time with your deck gun. And more fish stored. You just have to be patient.