How to improve one-torpedo sink rate ?

Well, most of the time, i hit a tanker with two torpedoes and she is still somewhat able to stay afloat. I have to use my deck gun to finish her. I guess i am making bad shots.

My questions are :

+ Where is the best place of ship to hit ?
+ What is the optimum depth for torpedoes ?
+ Does torpedo speed affect the damage it inflicts ?
+ Does range affect damage ?

Thanks in advance for replies.