I have gotten so much help from this forum...now it's time to give something back.

To calculate your boat's top speed you have to 1st make some assumptions.

1. Add 1" to your prop's pitch if it is cupped.
2. Assume a prop efficiency of .9 (a 10% slip).
3. It's assumed you are operating in the correct RPM band for your boat and prop.

The formula:

Formula ONE
(RPM x Pitch x Efficiency x .0009469 ) divided by (Gear ratio) =
Your projected speed in MPH.

(the .0009469 is a conversion factor to make RPM's, Inches, Time, Miles....etc - spit out the answer in units we want - MPH).

How much slippage does your prop actually have?

Use Formula One but skip the Efficiancy.

Formula TWO
(RPM x Pitch x .0009469) divided by (Gear Ratio) = Perfect Speed.

Now - using your recorded numbers - take your (Actual Speed) and divide it by your earlier calculated (Perfect Speed) which will = Efficiency. 1 - Efficiency = Slippage in %.

Now you know your slippage or efficiency of your prop. But - this number will change at different speeds so take that into consideration.

Hope the members of this forum can use this and if anyone is interested I can also provide a formula to calculate Center of Gravity....

TJ

This site Prop Slip Calculator works fine to...

Dang ....So to go to 80mph .... I need to from my 23' to a 28' .... Is there a way to calculate how much HP is needed ?

15-20hp per engine for every 1mph gain for a standard V bottom.

is that for a single or twin screw?
or does it make a diff?

storm, read it again; per engine

YES there is!
This assumes all else is equal.
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Here is the formula:
Desired speed / current speed = n.
n * n = m.
m * current hp = hp needed to get to desired speed.
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An example:
My old power was a 7.4mpi at 310 PSHP. My best top speed was 63 on a perfect day. I want to go 78.

78 / 63 = 1.238
1.238 * 1.238 = 1.533
1.533 * 310(old hp) = 475hp to go 78.
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This formula is very close.
I installed an HP500 and have hit 78.2 on the gps. The only other change was from a 24p Bravo 1 to a 28p Bravo 1, both stock.
I hope to hit 80 once I start playing with props!
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I got this formula from Bob at Full Throttle Marine.
Kent

Holly hell it works

thanks DK, i must be losing it.

So, last yr my boat ran 64.3 w/ 310hp.
Now I should be around 570hp.
I should be close to 80mph.

The calc gives me more speed, but not really knowing what pitch i'll be running.
old: 1.5ratio/23pitch @ 4800rpm @ 64.3, mph = 8% slip.
new?: 1.5ratio/27(?)pitch @ 5500rpm @ 8% slip = 86mph.

it calculates out pretty close for current speed .... 68mph .....
my 454 mags .... using 375hp would need to bump to 519 ...