This war ship is in a Force 9 gale based on the Beaufort Scale. This would have been what the rough weather D.A. Rayner and his fellow sailors faced in the North Atlantic in World War Two. Look at the motion of the ship and imagine trying to eat or sleep or stand on an open navigating bridge. And a gale like this could last for days.
As bad as this is, Rayner would not have thought this appalling or atrocious weather. Those terms were reserved for Force 10, 11, or 12 gales. The last being a hurricane. A Force 10 gale has winds up to 63 mph and waves up to 41 feet. In weather that bad, Rayner would have put the bow of his ship into the waves to keep it from being beaten to pieces by the force of the sea. In such a situation, speed of five or six knots would be maintained just to keep the ship from being pushed backwards and broaching. But the ship would not make any forward motion over ground.
I found this ten second clip on the internet. It was part of a collage so I don’t know the source. This is a modern day warship, probably a frigate, not a World War Two escort ship which would have been smaller.