"Be more agressive!"
(Bad things always happen to me when I let my batteries get below 50%....but for a shot at that......I'd take it like the others stated, go deep and get out of the neighbor hood quietly,,,,,,comeback later with a reload and some battery juice.....20% will last longer than you think, but not much room for error.....ahhhhh, I'd love to have this problem! I'm in the middle of weeks of 15 metter wind, rain and no visibility.....sitting next to a wounded merchant who just will not sink, I have no torpedos left and only a few deck gun shells......sooner or later the wind will slow down and I can sink this bugger and go home!)
Geeze...............I love this game!