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Old 12-22-2003, 06:59 PM
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Question speed formula

Can somebody please post the formula/equation used to calculate top speed? I know it's prop pitch times rpms minus something but I can't find it anywhere. Thanks....
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Old 12-22-2003, 07:35 PM
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This may help.
http://www.go-fast.com/boat_speed_predictions.htm
Go to speed calculator on the bottom.

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Old 12-22-2003, 07:45 PM
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This has always worked for me:

rpm X pitch X ratio X .00094697 minus slip. If you are using a bravo type drive, be careful with your ratio number, example; if you have a 1.5 gear, the multiplier is .667, if you have 1.36 then the multiplier is .735. Your big problem is determining slip. Thats another whole story. Slip is horrendous as a boat accellerates. ie, a freiends K boat turning 7600 with 54% over drive and 16 pitch prop, calc's out 177 but was radar'd at 145 on a
short straight. At approx 12% slip it may actually go over 155. Good luck...
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Old 12-22-2003, 09:37 PM
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Thanks guys....
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Old 12-22-2003, 11:11 PM
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Another way is RPM / gear ratio x pitch / 1056 - percent of slip.

So if you are turning 5500 RPM with a 1.5 gear and a 32 pitch prop with 12% slip your formula is as follows

5500/1.5 x 32/1056 x .88 = 97.78 MPH

The best way to find your slip number is to go run the boat and use the above formula without the slip factor just stop after you divide by 1056 then take the actual speed of your boat and divide it by the number from the formula.

Say it ran 97.78 and the number is 111.11

you take 97.78/111.11 = .88 which is 12% slip

Hope this helps

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