I never ram targets cause it is more probable your boot going to sink rather damaging the hull of the ship.
Furthermore i dont find hard to destroy tramp steamers or small passenger/cargo ships hard to sink with a single torpedo, though usualy they take quite a while to sink.
However if you encounter single ship (unarmed ofcourse) i would advice to gun them down with the deck gun rather wasting a torp.20-30 round around the waterline are usualy enough.
For bigger targets (if they are in convoy and above 5-6000 tons) i use 2 torps, this is usualy enough.As mentioned in preveous posts deph of the torp is esential. I prefer aiming from 4-7 meters running depth of the all because if they run too deep they could hit the curved part of the hull and it gets frustrating not to blow
I hope i was useful.