[UKönig]

That was a theory of mine.
Some ships seemed to be more or less "indestructible".
They are put there so as to make you waste your ammo by firing more shots than are strictly necessary. If you take the bait, then it's the same net effect as if you had dud torpedos (or other failures) enabled.
If I have to put 3-4 torps into 1 merchant, then I conclude I was being baited and I hit "alt-f4" to quit and get my weapons back, then go off looking for an easier target.
If I set 6m depth against a small/coastal merch. I can usually get 1 shot, 1 kill if aiming at the 'midships' mark. I know it's bait if that same ship can take 3 hits before sinking or blowing in half.
It also seems that if you do a lot of damage in a short space of time, 2 torpedo strikes within seconds of each other, vs 3-4 separate hits over 10-15 minutes, you are more likely to sink it quickly, than let the damage control teams try to repair and restore. That is, the longer it takes you to sink it, the more likely it will be that you will have to fire another shot. ...and another...