The best way to kill a Black Swan is to ram into it's side, disabling it, with your U-boat stuck in it's flank. Then quickly board it with your crew, punch the captain in the face - John Wayne style, then poke him in the eyes a few times and make the rest of the officers bunk with Bernard for the rest of the voyage. Sonar operators, make them listen to heavy metal at full volume for a few hours = end of their careers.
