Some strategic advice please.

[cardician]

Ok, so this patrol I'm on I decided to go a different route. Turned the realism way down by eliminating the limited air and fuel supplies. So after completing my patrol I decide to head up to that torpedo base NE of England. On the way I come across a couple of British merchants that I down with the giddiness of a schoolboy. Finally I get close to the base and start detecting warships. There are two of them north of me patrolling the area.

So what should I do? What's the best approach here? It's the middle of the night, decent weather. Should I get close to them on the surface or should I creep by low down?

Last time I tried to stalk a destroyer he somehow just "knew" I was around once he got within a certain range and started heading right for me. I have no idea how as I never heard any active pinging and it was night then too so he certainly couldn't have seen my snorkel. I'm wondering if perhaps that's the "advanced" ai. Either way, what do you guys think would be my best approach to get two more threats out of Germany's hair?