Whew, you are passing up some easy tonnage early war with convoys.
Sometimes they only have a single escort, or two. Take out the escorts and you can surface and use your deck gun to wipe them out since the sim does not make the convoy disperse and early war they won't have deck guns of any kind.
A few times with a Type IX I have completely annihilated entire convoys of a dozen ships or more. Not realistic but it is fun to look at your map and see nothing but a big smear or red sunk ship icons over 20 km!
Steve