Other shipping affected by bad weather?

[kalkulator]

My 6th patrol was a storm from start to finish. 15mps wind and I almost got seasick myself watching it go up and down like that. Worst weather I have seen yet in SH.

The sonarman picked up some engine-noises while being 10m deep in between the waves (I didn't have to put myself to periscope depth). And when I started following them, at full flank speed, I had a hard time catching up.

IID max speed is 13knots, but obviously not during these rough seas. I made an unsteady 10knots, dropping to 6 in huge waves. Are other ships affected like this in the same manner?

I did catch up on a passenger-ship, having 1 torp left. Being 800m away, I fired it.
After 400 metres it exploded?!