1: Your in '41 so I guess you are still using the T1 torpedo's (steam)
as you know they leave an trail of bubbles on the surface so when firing those at DD's or BB's will not work they spot that trail and speed up to evade them.
I always carry some TII's Torpedo's I always use those on DD's or BB's
2:

Naughty Boy dont shoot at hospital ships
3: Aim for the weakspots they are easy to find.