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...
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....
Originally Posted by gn7
....... David 519 is 100% correct........
Originally Posted by fuelinmyveins82
.....I think people forget that racing is supposed to fun. Losing shouldn't be discouraging it should motivate you work on your pile to make it faster.....
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.
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.
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.
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.