Hmmm.... lots of variables in bad weather no doubt and it took me a few careers (years) to learn some technique.
In my case, I have a fair success rate in high-seas/high-winds by setting torps to slow, 3 to 3.5 meters depth w/impact. And depending on the type of merchant and draft, I may send two fish simultaneously for good measure.
Also, I attack mostly at periscope depth not more than 600-700 meters distance from target as near as possible to 90 degrees off the bows. This does take a lot of stalking to make it a total surprise attack but how does 75% kill rate hit you? Do-able~!
BTW: I'm at 74% realism w/GWX3.