I also had once an encounter with a lone hospital Ship far away in the atlantic. Had a Type IX then, it was 1941 I think. I gave him a shot from the deckgun in front of his bow, and went alongside to him. I did that, because I wanted to inspect the ship if it´s really a hospital ship. So I stayed one hour alongside him, I guess that´s the time my boarding party need´s to check it up.
If my boarding party wouldn´t return - what they did of coourse

or he would send a radio message for help, he would end up as fish food. Nothing bad happend, so I wished him a safe trip home, and sailed a away to continue my patrol.
To put it in other words. I NEVER sink hospital ships.