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math formula for speed????

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Old 12-11-2003, 08:41 PM
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Default math formula for speed????

I know I saw something somewhere on how to compute probable top end speed in a boat....any math experts here? I want to know how much power it will take to move a certain hull to a certain top end. Can it be done via a formula? This may help in determining what engines to decide upon in any particular boat..... any takers?
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Old 12-11-2003, 09:01 PM
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$ + $ = MPH
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Old 12-11-2003, 09:03 PM
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ROFLMAO!!!!!
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Old 12-11-2003, 09:04 PM
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Here's a starting point: rpm X pitch X ratio X .00094697 divided by slip. Now comes the hard part. How do you know how to calculate slip? There is a different factor for every kind of hull; V, displacement, cat, etc. Its a very black mystery as is propellor technology. If you get 100 answers here, you'll get 100 different solutions. Remember, there is never too much horsepower or too many cubic inches or too many dollars...
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Old 12-11-2003, 09:08 PM
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ooooooooooooooh, that's how you do it...
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Old 12-11-2003, 09:10 PM
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http://go-fast.com/prop_slip.htm
this is pretty good
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Old 12-11-2003, 09:21 PM
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Actually, try this for speed predictions, the formula is there if you want to do it long hand, but I put a calculator online.

http://www.go-fast.com/boat_speed_predictions.htm
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Old 12-11-2003, 09:37 PM
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The formula we use has proven very accurate;
100 hull efficiency = RPM/gear ratio x pitch/12/5280 x 60

The tricky part is to have the % of efficiency value of your boat (less than the 100 value that your hull is) You would then at the end of this equation multiply the result by this percentage to receive the speed.

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Old 12-11-2003, 09:47 PM
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interesting, thanks for the help....only question now is how do I know what the hull efficiency is??????
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Old 12-11-2003, 10:32 PM
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Don't confuse prop slip calculations with predicting speed.

Once you already know how fast a boat goes, the prop calcs are fine for fine tuning or predicting RPM changes for different props and ratios.

Speed predictions need to take into account hull efficiency, weight and power. Nothing to do with prop and ratio. The prop / ratio stuff is after you figure out how fast it might go.

The speed prediction part is very difficult, as the hull efficiency can only be guessed at (can be a very educated guess) until the boat is actually tested. Picture 2 of the same hull, optimize drive height / setback on one and it might be 5-10 MPH faster than it's more conservative twin. Different efficiency.
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