Ships weak points and torpedo Detonation/damage

[Turncoat]

So I have had SH3 since 2014 and have made a big comeback to sub sims & subs since the release of the first version of Wolfpack.

I am doing the career in vanilla since I want to complete it before jumping into mods at 74%realism I think(am on patrol while writing this)
I intercepted my first Taskforce in mid-1942 and got curious about the way my fish behaved.

It was just a convoy with some military hardware and light escorts so easy peasy.

on first attack run, I fired 2 tubes each at a C3 and a T3, during which a small merchant got in the way and ate 1 and another missed, so one for each.
The small merchant stayed afloat for hours before sinking, but the 2 huge targets got swallowed by a huge fire and broke in 2.

on second run I remembered to have read that it was a good idea to just use impact detonator and aim for mid hull while in rough seas, so I did that with another tanker and fired a last torpedo at yet a C3.
The one torpedo that hit the tanker, just bounced off like a dud and the C3 got dismantled.



All torpedos were fired at 5m depth since it was rough seas. I do not understand what suddenly makes a single hit, not even under the keel, blow up large targets and let small ones float and I am also confused about the detonator's behavior.

I don't recall setting fires and Yeet things out of the water so often earlier in the campaign, so I am a bit confused.