Most torpedoes fired into a single ship

[Hene]

So, after this one badly gone attack I'd like to know what are the highest amounts torpedoes that you have fired at a single ship, of which all have hit. My current record is 10 without sinking the ship. I had set up an attack against a T3 tanker after an earlier badly gone attack against the same convoy. I fired a salvo of three torpedoes at a range of 800-1000 yards, first hit "Torpedo is a dud sir!" second one, also a dud. The third one exploded, and I quickly fired a fourth torpedo at the tanker. After a while I get again "Torpedo is a dud".. Having emptied all of the front tubes I turn my sub around and fire one of the rear torpedoes (fifth one), also a dud. I fire the sixth torpedo at the ship, dud again..
I turn the sub around again and wait for the front torpedoes that i have left to load. After the first one has loaded i fire it, with the same results as before, a dud (7). I turn around once again to fire the loaded rear torpedo, which also was a dud(8). I turn around once more to fire my last front torpedo, which also fails to explode (9). with only one rear torpedo left I set the tanker to my rear and fire, waiting, waiting, and finally, "The torpedo is a dud sir!"(10). after spending all of my torpedoes i go behind the tanker to avoid the nasty cannon, surface and finish it of with my deck gun. after that I returned immediately to port to write a complaint about the sub-par quality torpedoes that we had received.