[Sailor Steve]

That's a good point. I think that those kinds of operation usually did take place in warmer climes, since in the North Atlantic they were close enough to home to resupply at a real base.

I think a more important point in the case of the Germans is that those supply subs didn't carry enough fuel or torpedoes to keep the operational boats on station for a whole extra patrol. I'm pretty sure they normally only gave the front-line boats enough fuel to get home, and a couple of torpedoes, just in case.