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Speed & Horsepower Formula

Old 04-27-2020, 02:00 PM
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Default Speed & Horsepower Formula

I apologize in advanced if this question has been asked before, but Im nearing the end of a project and need a little help.

I have a 2000 Awesome Cat 260FX, in the "Awesome" section a magazine did a review of the boat and with 470hp it ran 85.1 mph with a 502 Mag if I remember correctly. I had a supercharged 540 built for it and its in the final stages before we splash it. So if the boat ran 85.1 with 470hp, is there a way I can get an idea of what speeds I'll see with 1000hp?

Ive read its add 1.5mph per 10hp, Ive also been told its 1.5mph per 25hp all by outboard guys...

Any help is appreciated
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Old 04-27-2020, 02:06 PM
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Adjust the "constant" to your current weight, speed and horsepower its usually relatively accurate.

https://www.go-fast.com/boat_speed_predictions.htm
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Old 04-27-2020, 02:12 PM
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Thank you!!
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Old 04-27-2020, 03:31 PM
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MPH increases with added HP are not linear. The faster you go, the more HP it takes to see gains in speed.
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Old 04-27-2020, 04:06 PM
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Iíve always used 20hp per 1mph, but itís the law of diminishing returns. Drag is a product of speed squared, so the faster you go, drag increases exponentially.

Originally Posted by Keith Atlanta View Post
Adjust the "constant" to your current weight, speed and horsepower its usually relatively accurate.

https://www.go-fast.com/boat_speed_predictions.htm
I like that calculator! Pretty accurate within a couple mph from various engines ive had in my boat.
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Old 05-01-2020, 09:05 AM
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Originally Posted by Keith Atlanta View Post
Adjust the "constant" to your current weight, speed and horsepower its usually relatively accurate.

https://www.go-fast.com/boat_speed_predictions.htm
It took some time playing with number to make it seem like a "reachable" speed. Initially putting in the information I thought was accurate it put the boat going 160+ which is not realistic, So I adjusted the weight and horsepower as tested in Boat Magazines Review of the boat until it matched the speed they achieved with 470hp.

After calibrating the calculator to the article, it predicts the boat will see the 120's which is little more inline with what I thought. Thank you for the information
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Old 05-01-2020, 06:44 PM
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I just today put an updated version of the go-fast site online. That site has been online with few changes for over 20 years! And way past due to move to a more modern platform. It will take a day or so for it to "propagate"

Everything should work the same and there is some new content.

So I am all ears as to any bugs or stuff that is missing.

Thanks!!
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