You could try what I managed last night:
I torpedoed three ships in an eight-ship convoy from periscope depth (8200 yards for the furthest one, which is probably my best shot ever I think). They were escorted by three destroyers coming into the channel to approach Hiroshima (which is fortunately quite deep in places). One sank since it got hit by four torpedoes, but the other two kept going at about 2 knots. So I sneaked in under the thermal layer and surfaced between them (took bloody ages, even on accelerated time). The two crippled ships shielded me from the destroyers and I was able to finish one of them off with the deck gun. Then it was back down to 250 feet, silent running on and thank-you very much and goodnight. And yes, that was on manual TDC and all the hard settings.
Good old Gudgeon, she never lets me down
On the subject of which, Gudgeon (the real one) apparently, attacked a ship once on the surface and then sat there whilst it sank, and the crew were allowed up on deck to watch. Not something the backroom boys normally got to see.