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Originally Posted by Ghost Dog
So to manage 22.8, doesnt seem far-fetched at all.
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It does to me, but with qualifications. The difficulty I've always encountered is that speed/horsepower calculations are never a straight line. The example I like to use is a car which can make 100 mph on 100 horsepower. The law of diminishing returns comes says that doubling the power will give half the return. If you double the horsepower to 200, the car will not go 200 mph. The drag will increase exponentially and the car will peak at around 150 mph. Double it again to 400 and your top speed will be around 175 mph. Double it again to 800 and you'll end up with 187.5 mph. Of course you can play with the gearing and trade acceleration and top speed, but not by a whole lot.
Back to ships. The power needed to overcome hydrodynamic drag increases as the cube of the speed. By the time you reach the indicated top speed you would have to double the available horsepower just to get that extra two knots, and that is simply not possible.
HOWEVER...
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Official sources for those kind of things are always below the actual speeds anyways.
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That's the qualification, and that could well be the answer I was missing before. "22.8 knots" might well have been closer to the actual top speed of the boat, and "20.25 knots" could have been the "do not exceed" safe speed. If that's the case then hydrodynamic calculations would have to be made from a basis of a 23-knot top speed, not 20 or 21.
And that, as you said, no longer seems far-fetched to me.