And knots per hour per hour would be a measure of ever-increasing acceleration, so this is getting out of hand quickly now :)

Sure, but the gif doesn't show 50 knots. The gif doesn't show any speed actually, so I really don't know where the 50 number comes from. But on the tracker the speed was 8.1 knots. Fast for a tanker, but totally believable.

I was under the impression that navy ships could go much faster, but didn't because of wear and fuel consumption. I recall hearing about 60 knots, but I wouldn't place even a small bet on it.

Same for cargo ships, to a lesser extent. If an empty one felt a need to move, I'm sure they could get a little speed to them. But they aren't built for it, and "saving money on fuel" is their prime directive.

Although, as someone noted elsewhere, there don't seem to be any actual measurements of speed. They turned around, and cranked the throttle, but we don't know how far they were going in either phase.