I have not claimed the U-20 sank a hospital ship, I merely stated that the captain of the U-20 deliberately fired at a hospital ship while in the full knowledge of its status.I think we're stuck in an endless loop here...
The U-20 (SM U-20) that sank the Lusitania also sank the Hesparian, those were the only two passenger ships in it's prize listing.
Both were sunk during it's time of service between 1913 and 1916.
So when you keep referring to a hospital ship, that would be the only possible candidate, because the hospital ship you referred to by name was sunk after the U-20 was out of action and by an entirely different submarine.