Can anyone provide me with the formula for calculating MPH using RPM, gear ratio, and prop pitch? I use to know this but have forgotten it over the years.

Thanks

Scott

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- 01-26-2020, 05:50 PM #1
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## Formula

Can anyone provide me with the formula for calculating MPH using RPM, gear ratio, and prop pitch? I use to know this but have forgotten it over the years.

Thanks

Scott - Sponsored Links Remove Advertisements
- 01-26-2020, 06:02 PM #2
- 01-26-2020, 08:05 PM #3
I do not remember the quicker formula but this will get you there

rpm X gear ratio (if 15% overdriven use 1.15 if 15% underdriven use 0.85) to get prop revolutions/minute

Multiply that number by 60 to get prop revolutions/hour

Multiply that number by the prop pitch to get theoretical inches/hour

divide that number by 63,360 to get theoretical miles/hour

multiply by 0.8-0.85 to account for a more realistic slip ratio on a flatbottom

so the numbers on a boat with 15's at 5000 rpm and a 16" pitch prop would look like this

5000 X 1.15= 5750

5750 X 60= 345,000

345,000 X 16=5,520,000

5,520,000/63,360=87.12

87.12 X 0.85(15% slip)=74.05 mph which is probably right in the ballpark for a true flat with those specs.

Runner bottoms would likely be a little more efficient and hydros even a little more with less slip. V-bottoms would be a bit less efficient and have more slip. - Sponsored Links Remove Advertisements
- 01-26-2020, 08:34 PM #4
Try:

RPM x Pitch x OD x 60 / 5280= MPH with no slip.

Pitch is in feet (16" pitch is 1.33333')

OD is overdrive in box; 25% over is 1.25

60 converts minutes to hours,

divided by feet in a mile...

My Canyon:

16 pitch prop, 29% gears, 7500 RPM

7500 RPM x 1.3333 x 1.29 x 60 / 5280= 146.6 MPH

That combo typically nets me about 128 MPH or about 13% slip. My Canyon is pretty efficient, meaning it cleans up pretty well. Rides on the center pad and the chines dry. I've played with the numbers of some pretty fast flats and they're in that ball park. I'd guess realistic slip numbers for a hot lake boat (flat) in the 15%-20% range but YMMV.. Also, I've never messed with numbers for hydros, wackers or I/Os but the math is pretty much the same...

A couple other factors. Drag increases at the square of the speed. Power to overcome that drag goes up at roughly the cube of the speed (for the engineers out there, yes there's some simplification here, but it's close enough for the girls we go with ) Again and as always, YMMV.... - 01-26-2020, 08:35 PM #5
- 01-26-2020, 08:58 PM #6
- 01-27-2020, 08:38 PM #7
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- Jan 2010
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Thanks Guys.........great info. That's not exactly the numbers I remember using, but I'm sure they're very close. I'm running a 21' Tunnel, I have a Gaffrig GPS speedometer in my boat so I can do some comparison this summer during gear and prop testing. Once again, thanks for your input.

Scott - 01-27-2020, 08:53 PM #8
With a tunnel when your at top speed and packing air, 10% slip is possible if the set up is good

- 01-27-2020, 10:01 PM #9
And every prop and combo are different. We have one prop that has more slip than the rest, and yet it is the fastest. As far as the formula goes, these guys NAILED IT! But yes, the slip is a guess until you have a time slip or GPS numbers to verify it. At least it can get you in the ball park.

Paul - 02-04-2020, 11:43 AM #10
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- May 2017
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- 1

## Slippage Conversion Chart

How about the easy way since they all like it the hard way, https://bblades.com/propeller-slip-calculator/, I can also give you weight variances and Deep V/Cat conversions as well, let me know what you need, https://www.facebook.com/drscottazink/?modal=admin_todo_tour.

Scott

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