I was returning from my latest patrol with one torp left, and in my stern tube at that, when I stumbled upon one of the new "large cargo" ships at about 11,600 tons. I knew I wasn't likely to get her with my one torp if I fired it first, so I surfaced and attacked with my deck gun (fortunately the weather was still borderline tolerable - 7m/s winds). I figured I wouldn't be able to sink such a large ship with my little 88, but I was hoping I could at least slow her down and maybe bring her to a halt, so I would have a real easy shot at her with my last torp.
Sure enough, after firing about 70-80 shells (all my AP and all my remaining HE) into her bridge and waterline/engine/boiler room areas (minus the 30-35% or so of wild misses, I guess due to the relatively rough seas), I finally managed to get her dead in the water and listing slightly to port. Then I was able to line up a real easy stern shot from about 600m - broke her in half and sent her to the bottom - a sweet way to finish off my best patrol so far this career - sure made me glad I had saved that last torp for the return trip