I just sunk a Southampton Light Cruiser (10,000+ tons) travelling in the middle of a convoy outbound from Gibraltar in June '41. Grid CG77 or thereabouts. (I do have to brag a bit about that patrol....that was a 52,000 ton patrol for me. I must have sunk about 8-10 ships, the majority of them being large cargos. I was having a streak of luck hitting them in the fuel bunkers. You can sink a heck of a lot when you're only using 1 torpedo per ship

)
I think they become a lot more common in convoys as time goes by. Just my general impression, I could be wrong.