Torpedos running under targets.....

[Dellson]

I'm pretty sure that dud torpedoes are modelled in hsie's patch. While in rough seas you need to set torps deeper or their "failure" will occur. Failure is modelled by torpedo running deeper than usual.

If I recall correctly, the exact formula for minimum dud chance is:
safe torpedo depth >= (wind speed / 2).

You can find it in patch documentation.