Where I went to school they called it "square root"

explains probably why they also taught you to say " do you want fries with that ?"

Square root is the inverse of x^2 (x^2 is "x squared")

Cube root is the inverse of x^3 (x^3 is "x cubed")

using the formula as stated, my 24' liberator will go from 42 mph at 270 horse to 70 mph at 400 horse.

using the same formula substituting the cubes for the squares, I get 74 mph.

Boy, I sure hope either one is right!

Whoops...my bad. I guess I slept through more math than I thought.

As far as the "fries with that" statement, I think I'll just refrain from responding.

actually neither....

V = 42 mph = 61.6 fps
V cubed = 233,744

270 = drag x 233 744

270/ 233 744 = .00115 so the coefficient for the total drag of the boat is .00115 .... based on your top speed as given vs hp as given... w/ 400 hp

400 = .00115 x v cubed

400/.00115 = 346 288

v cubed = 346 288 so do a cube root and get to 69.92 fps

divide by 1.46 to get to mph = 47 mph

so another 130 hp buys you 5 mph if the data you started w/ is accurate... but the big concept to grasp is that while 130 hp buys you 5 mph at 42 mph it might buy only 1 mph at 50 mph or 10 mph at 30 mph.... the faster you go the nececessary power to go faster is NOT linear... doubling the power does NOT mean you go twice as fast. also.. all this presupposes that the boat is at minimum drag already... for instance if the boat didn't have enough power to get on plane but an increase of just 25 hp WOULD get it on plane the increase in speed would be HUGE because of the drag reduction of being on plane vs not.

for the record , that drag number seems really high... my top gun at 10,000 lbs has a drag number of .0009 based on gps speeds and published hp numbers for the box stock 502's i have.... the total drag is made of many factors and, as such is difficult to calculate as an entity ... but a GOOD top speed number combined with GOOD hp numbers will give you a CUMULATIVE drag number that will give you a VERY accurate prediction of increase in to speed. the math is a given and is used to calculate any load (with slightly different parameters) generated in any viscous medium. maybe you have less power than you think or your speedo ( if not gps ) might be wrong... or maybe the boat is just "draggy" reducing the drag buys you REAL increases immediatly.

someone here pointed to a web site w/ a plug in calculator done diffrently and the answers still came out essentially the same.

Ahh, I forgot to divide by 1.46.

I sure hope 130 HP gives more than 5 mph. The other calculator you speak of is

http://go-fast.com/boat_speed_predictions.htm

It shows my new motor giving 51 mph, which is more in line to my gut feeling.

I never drove the boat before, I am going off the previous owners #'s

The hull was dirty when I bought it, and I know from experience that a crusty hull can cost severe mph. With that said, I hope for 55 mph, but expect 50-52

proped rite your 130 hp will give you 7 to 9 mph

just out of curiosity... if you plug your numbers as you know them now into the calculator, does it give you 42 as the predicted top speed of the boat w/ 270 ? i.e will it validate what the prev owner told you ?

Problem is I haven't ran it on the water yet. I got the motor running, and had to adjust a valve. In doing so, I had to remove the riser (stock style) and must not of buttoned it up right because I got milkshake. I am also getting the intake decked to port match, so it'll be next weekend before I find out, weather permitting.

On my Whipple install it went from the 500efi to 750 Hp on the dyno at 7 lbs of boost.It was running about 80-81 and with the Whipple it went to running 98-99.8MPH and that was at 6000 .I did no prop testing or anything. This year Now I am at 850HP and I don't think its going to go much faster ,I bet I am at the limit of the hull???I know of others like my boat with the 800plus HP and 100-104 is it.So I think putting more HP to it is only going to help acceleration, and hurt MPG and dependability.I am going to do some testing with the new B-Max's and props at the 850HP level and probably back it back down to 750HP and still run at 100MPH.

Rob

I think Rich is close to or. a little above 800 Also his hull is efficient and he does do testing trying to get more speed out of it.That makes a lot of diff getting the right set up.Good luck Rich and be safe