When attacking on the surface at night, keep it slow like Nemo said. I also try to run decks awash-set depth at 7m. Your watch crew stays up and it makes you harder to see although your speed drops off, so make sure you're in good position before you go decks awash.
If you run around the convoy at flank speed and show a large silouhette(:hmm: spelling don't look right), you'll get spotted.
@Nemo:
Looked at your picture. Hafta start calling you "Silent Otto" for getting inside the convoy like that.