20 knots - possible with a displacement hull boat?

Question

Just wondering, if length defines the speed, couldn't I just try to build one really long boat and get it up to 20 knots without needing much more horsepower? Or does that linearity end somewhere?

Sorry, I'm a car designer and live by the sea. I know how to make cars, boats are a new thing to me, but I want to get into it.

Answer 1

An aircraft carrier must go fast enough to create 30 knots of wind over the bow for launching and catching planes. So the speed varies with the wind, but 30 knots, with no wind. Carriers are about 1000 feet.

Square root of the waterline in knots = the hull speed
Given enough horsepower, any boat can partially plane.(can exceed hull speed)

Answer 2

you're on the right track that, all other things being equal, a longer hull "drives" more easily and needs less horsepower to obtain a given speed, and yes indeed, that curve is hyperbolic/parabolic and the returns drop off rapidly....

there's a lot of fudge to this basic equation, but to start, figure the maximum speed of a displacement hull is the square root of the waterline times about 1.2........after this speed a hull just puts energy into creating a bow wave, rather than speed; in theory it can create a big enough bow wave that the boat just drives itself under.

If you're wanting 22 knots; that would be 22 / 1.2 or 18.34; 18.34 squared is 336 foot waterline.......from my study of war and merchant ships, you'd need about 45,000 hp to do this....

now, to get 22 knots out of a planeing hull or a semi displacement is a piece of cake........there's thousands of small ( like 25-40 feet )power cruiser designs that will do 20 plus knots with 200-500 horsepower

Answer 3

If you make it long enough, absolutely. Think of the rowboats with 4, 5, 6 rowers, they use for racing

Answer 4

its possible but with a lot of power. Aircraft carriers have 2 nuclear reactors that produce upwards of 280,000 hp each, thats why it goes 30kts.