My advice; stealth. This takes time & patience to do but it makes for mostly sure-kills
ID a target and keep your distance out of his range until you reach a point well ahead of the target's plotted course. Keep lookout for other ships in the area as well because one may be an enemy warship.
Once you pull far enough ahead of your target move to a position to intercept, keeping your target in Uzo sighting distance. This is how wolves stalk their prey.
Once target is close (5k to 6k meters) submerge and use periscope to keep contact with target while plotting YOUR COURSE to cross perpendicular to his course.
Set up your shot and position yourself less than 1000 meters off target's port/starboard. Fire fish when appropriate into target's broadside. Cheer~!
Hope that helps
Good hunting~!
P.S. if a target ship is running dark his ahzz iz mine~!