surface and start shooting your deck gun at the lead destroyer. make sure you are going flank speed directly at him. as he gets closer start to go port or sb of him until he sinks. then start sinking ships with your gun and torps on the surface. keep an eye out for the two side destroyers. head to the port or sb of the convoy and use the same deckgun tactic on that destroyer. as you await the destroyers continue to sink merchants with your deckgun. keep lots of spare crewmen ready to take your dead gunnery crew's spots. if you are being double teamed by two destroyers are usually go to periscope depth and fire off a mag torp at one then go down to 160m. wait it out then come back up and follow then wounded convoy.
