I have also succesfully made a few nighttime surface attacks, same method as BBW mentions
important note, if you do this, and you do manage to fire your eels get away from the spot you fired from asap because the nearest escort will head straight for it, also do NOT get caught in searchlights if this does happen you have only one option, crashdive
oh and don't even think about it after 1940, or when there's a big warship in the convoy, unless you're happy to have the muzzleflashes of 9 14' guns be the last thing you see
