As Privateer said. One ship I do know the numbers for - the
Iowa class battleships. With all eight boilers and full power they were rated for 32 knots. Using four boilers (half speed) they could still make 30 knots. There is an optimum speed for every vehicle, and once you've reached it adding more and more power has less and less effect. That's why a 3000-pound sedan making 200 horsepower can go 100 mph or more, and a 1200-pound Grand Prix car making 1000 horsepower can only go twice as fast.
Here is a great explanation of how it works:
http://www.navweaps.com/index_tech/tech-029.htm
And an explanation of why the 50-knot nuclear carrier is a myth:
http://www.navweaps.com/index_tech/tech-028.htm