You must dock at a German controlled port that has facilities for U-Boats. Right? I went to Willhelmshaven when I was based at Kiel and refuled - reloaded. This was after getting pounced in the North Sea and wasting my eels (all five in a Type IIA
). It saved a lot of transit time, and got me some renown in the long run.
I don't know if you can sail into Gdansk, or Bergen for instance and get fuel and torps on demand.