You can find a lots and lots of them in large convoys before 1942. They all cruise under american flags, so are simply untouchable. As the 1942 arrives, they almost disappear and are a lot harder to see, but you can still find them in a big convoys, where they are easy prey.
You also have a slim chance to find them standalone in their cruise across atlantic. But they tend to go 20knt or better and chance of succesful intercept is low. But if you already find one, try to get yourself noticed ASAP. They slow down and start evasive maenuvres, which makes them easier to intercept. They make a BIG target, and are not very durable, which makes them easy to sink.
Example:
Passenger liner spotted!
Fast running and going away...
But after a few cannon shells nearby it slows down and lets me fire torpedoes uder it's keel. Liner almost avoids them, but one detonates under the bow, slowing it down. I keep running at flank ahead, nailing it with shells, when suddenly realising, than running full speed straight at a target of a size of passenger liner might not be the best idea
Back emergency!
Finally after a few shots more it goes down nicely!
