I think the other thing you may find with regard to dud torps is that if you have twin speed torps you will always get a higher % of duds if you select high speed, which on reflection i think most of us do so we all get it.
if you look through the torpedo files (using S3D) you will see that there is a dud reduction by 50% as standard so you can cut that part of the duds down by using the slower speed.
speed however isnt the main issue here... its the angle of impact of the torpedo on the target, try and keep this figure as low as possible (you can also check the dud possibility angle in S3D).
This angle of impact has been slightly overcome by the NSM4 mod which allows you to use influence instead of contact for keel busting shots
I run with dud torpedo's on. IMO knowing how torpedo's perform will help you plan attacks more effectively.