I have come upon very good DD and not so good DD. I also found out the AI merchants will ram you as well. I do not remember merchant AI in SH3 attempting to ram me. DD did for sure but not the merchies.
Last night my torps missed a merchie, I surface 800 yards off for cannon action. The SOB stopped and then headed right at me
I was in emergancy reverse. He scrapped along my side. I was able to sink him and then I blasted the life raft out of the water because I was PO'd he did that. Thus far I'm satisfied with the DD.