Buy this book about uboat tactics, it's very handy and helped me a lot!!
http://www.amazon.com/U-Boat-Command.../dp/0939631210
It's packed full of methods for surface and submerged day and night attacks, includes every from stalking, where to get positioned, on and on. You'll love it! Pretty cheap too at 9 bucks, I bought it at the u505 museum in chicago.