some tips that I think may help
- Never go faster than 256x time compression
- Make sure your watch crew effectiveness is 100% by checking the bar - should be max green
- Never take on the aircaft - sometimes they come in packs and then you got a real fight on your hands - remember u-boats are to sink ships not shoot down aircraft
- Once you get spotted, stay submerged until dusk if possible.
Hope that helps
Good Luck