How good is the formula for calculating new speed with an engine upgrade? The formula I have seen thrown around is (desired speed/current speed)squared x current hp = hp needed to achieve desired speed. Does this formula account for the change in rpm's to achieve your new speed? What have others found?

Have you tried this

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

none of that is correct or even close... the prop calculation is certainly valid as analogous for a "gearing" calculation on land IF you have a " good" slip number... but at anyrate....

hp to overcome drag is the cube of the speed. the formula is :

hp = .00000289 (v cubed) this calculation is based on a cd of 1 which we know is not the case but it gives you a direct relative indicator and using the medium density ( rho) of air.

the formula for calculating drag in lbs and then being able to covert to hp is A x .00256 v (squared) x cd ... this is calculated WIND resistance... the density of water is obviously different as well as there being about 500 other variables to consider BUT the good news is " who cares ?"

if you have a GOOD absolute terminal velocity for your boat and you KNOW the hp required to sustain it , then you can back into the total drag number constant that you need for the hp calculation by doing this...

hp you THINK you have divided by the speed ( in ft/sec) CUBED = your drag constant...

for example... my boat has 850 hp ( 2 x 425 or so it says) it goes 67 on the best day of its life... so

850 / 97.82 (cubed) is 850/ 936,015 = .000908 as my drag constant

so... if i want to go 77 mph ( 112 feet per second) or another 10 mph i CUBE that 112 which is 1,420,792 and multiply that by my drag constant .000908 which is 1,275 hp so the figures imply that to get another 10 mph out of the boat i would need about another 200 hp per motor at that speed... or 625 hp motors instead of 425 s.

now... these are just numbers working from a single isolated data point... in order to REALLY get a good drag constant ide have to do a hand full of back to back tests etc but i have to tell you that the answers doe NOT seem that far out of line from what i absolutely know to be true in air on the cars...

drag never sleeps. and even the slightest configuration change that ADDS drag will slow you down like crazy and the slightest drag REDUCTION is like putting nitro methane in your motor.

BIG gains in drag reduction... sunstantial gains in ACCELERATION with more hp but not a lot of gain in top speed.

Take a look here:
http://go-fast.com/boat_speed_predictions.htm

Its working for me.very close on the #'s.

Here is another one to try, www.boatramp.com. There is a link ( Boat Performance Analyzer ) on the left side of the screen for speed calculation and/or prop slip calculation. Give it a try.

plugged in my numbers and came out to 66 mph essentially right on. plugged in 1200 hp ( from my example) and came out w/ 78 also essentially right on... so their " factor" essentially the same drag calculation et al as the math provides.

the math never lies.

I like the prop slip calculations - unfortunately, everything is based on existing results, algorithms that will work better for some boats than others.

Another thing to consider in the drag equations - the type of hull and outdrive, as well as X dimension will affect transition from laminar to turbulent flow to both the hull and to the lower unit. There are so many dynamic elements to determining the speed of your boat, unfortunately, tons of assumptions are made.

In a boat that goes 67, adding horsepower will probably bring the outdrive closer to the transition to turbulance. Could just hit the same ~80 mph wall with 1600 hp than it would with 1800 hp.

In other words, good luck!!!

G.P.S.