Another factor - you can throw 100* the horsepower on a given hull - but there is a limit on how fast that hull can be pushed through the water. There are diminishing returns after a certain point - very little gain for lots more energy expended.

Speed on a displacement hull is more a factor of length at waterline than of horsepower.

Is it primarily the length (long ways) or the width (the size of the pushing surface) that determines the max speed? I feel like it would be a function of both, but weighted more heavily on the width. I'm new to these things.

Kb

thanks:D

It's called the Fineness Ratio, and it's governed by length versus width (or beam as it's called in a ship). For any given width, the longer the hull the faster it can go. Of course battleships have much longer hulls than destroyers but are always slower. An Iowa class battleship is 887 feet long and has a beam of 103 feet. The best speed any Iowa recorded was after the war, when all the AA guns had been stripped off, and it was just over 35 knots. During the war they made about 32 knots. A Fletcher class destroyer is 376 feet long, but the beam is less than 40 feet, and on a good day they could make 38 knots.

Also, concerning horsepower, even in cars, as HP goes up the speed gained is a function of the square root of the optimal HP. If you have a car that goes 100 mph on 100 HP, doubling the horsepower to 200 will only give an increase to 150 mph; doubling it again to 400 will give about 175 mph. This is why 800 HP Formula 1 cars still top out at about 200 mph. Of course that can be changed with proper gearing, but then the more top speed you get the slower the acceleration will be.

Same is true of ships.

Edit: Just saw SS's far more complete answer.