All good advice from Steve.
Theres a reason there called "Destroyers" ya know
Anyhow the "short range" thing seems to be a lot less particular than it was in SH3. I have taken some shots from what to me is point blank range (inside 500 meters) which detonated.
There is an option in the difficulty settings that allows/denies duds check that setting.
Ive always played with it on and on average 1 in about 5 torps doink off the hull of the target. Very frustrating but it is an aspect that had to be dealt with in real life.
There is another "malfunction" which happens on occasion is the gyroscope in the torpedo slams to one side and the torp leaves the tube and immediatly takes a turn to one side (usually to starbord) and actually runs in a circle. In this case look the hell out !!! To make a long story short its not one shot one kill, even in "easy" mode.