Or if you don't want to take any chances, drop back to +/-5000 meters and get in some long range target practice. Figure the range, or if you're lazy like me just ask the WO for it, and then direct your shots into the waterline at a safe distance. Merchants are usually terrible shots at that range, (though later war they might be more accurate) and after a few shots you'll figure where to aim. Mind you this takes far more time than another torpedo, and I wouldn't recommend taking too long close to shore
