Also, a ship that is fighting it's way through high waves and stormy winds in a storm will take in far more water when hit than ships in calm waters.
I have managed to sink the Royal Oak with a single torpedo in rough seas, as storms deals a slight portion of damage to the ships itself.
Earlier i always avoided engaging ships in storms, now i tend to like it as the high waves make it easier to kill, but harder to aim, so get reeeealy close and comfy to them