I don't play with 100% realism, as I have the external camera and auto targeting and no duds on, but I do play with all the other options set.
I managed to score 209730 tons after I came across a 4 battleship convoy with 10 destroyers, 3 light cruisers, and 5 heavy cruisers along with 2 tankers.
In a steller run of luck, I put all 24 fish into Japaneese hulls and sank all 4 battleships 2 heavy cruisers and 1 light cruiser. Even when I missed the ship I was shooting at, I ended up hitting a ship behind it. It was truly a site to behold.
I am kinda against refitting and going back out. IRL, when they refit, it counted as another patrol. (not that I am against anyone else doing that, just how I roll)
Chuck