What makes those speeds so hard for the crews to endure?
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Ships and subs, unlike land vehicles, do not have shock absorbers.
Imagine driving a car down a road with pot holes everywhere.
Now imagine you are driving your car at 46 mph (40 knots) over the same road - pretty bumpy, eh?
Now imagine driving that same car over that same road at the same speed without shock absorbers for hours on end.
Now imagine driving that same car over that same road at the same speed without shock absorbers for hours on end, while moving about the car doing various tasks, many calling for precision, focus, accuracy and concentration.
Now put the car on autopilot and grab some bunk time, while the straps on the bunk keep you from hitting the ceiling ...
That’s why.
Actually if you drive faster on a wash boarded road it will smooth the ride out. I do it all the time in the outback.