After many failed attempts, I gave up from trying to sink her with one eel, and started to use two.
On my last encounter with Passenger/Cargo, one torpedo missed, and the other one hit a bit infront of the stern mast. I was about to make a second approach when my crew informed me that the ship is sinking.
I used electric torpedo, impact detonator, I *think* the depth was set to 5m.
Looking at the screenshot, I guess I was lucky the torpedo didn't bounce of the hull.