How to convert prop pitch to MPH

[dwsatt]

What is the formula for calculating MPH given a certain pitch prop and RPM?

Check out this formula:

Given: 26" pitch prop, 6,000 RPM 1:87 gears

Multiply 26 inch pitch x 6,000 R/M X 60 Minutes/ Hour X 1 foot/12 inches X 1 mile / 5280 feet X.87 Excluding a slip factor.

Do the math and you should come up with 128.52 MPH. I don't go that fast in my dreams, but the formula looks correct. My actual speed would be closer to 95 MPH.

To reduce theoretical speed to actual speed (95/128.52) would require a slip factor of 0.739. That's a lot of slip!

Can someone explain what's going on? If you use a different formula, please provide the deriviation with units of measure.

[Fish]

dwsatt, I am not smart enough to use the formulas, but I do know where a couple of online prop calculators are. I actually came up with 71.1mph with a 26, 1.87 gears @ 6000, factoring a WAG prop slip at 10%. Perhaps yours is more effiecient, but many use 10% as a ball park here (9% puts me within 3/10th's of a mph on what I actually have seen on my boat).

Here is the one I just used, it is very simple but seems to work:
http://www.rbbi.com/folders/prop/propcalc.htm

Here are a few others:
http://hillpropellers.com/propcalc.html

http://www.go-fast.com/Prop_Slip_Calculator.htm

[dwsatt]

Fish,

I think the problem with my formula is the lower unit ratio. on a Merc 280 Sportsmaster is the ratio 1:87 or 1:1.87?

[dynobo]

Yea, your ratio is incorrect. You were factoring that off of flywheel speed (6000rpm) not prop shaft speed (6000x.53) which would be approx. 3180 rpm.
The gear ration is 1.87:1 which would equate to propshaft speed being .53 as fast as flywheel speed. You won't be anywhere near 95 mph turning a 26p prop at 6000rpm (flywheel ) with 1.87 gears.

[BenKeith]

Download this one and try it. I like it better than any I've tried http://bassboatowners.com/speedcalc.html

Depending on the boat but on my heavy bass boats 6,000 rpm with a 26" raker gets me in the low to mid 70's

[pyro]

Prop pitch is an inconsistent measure, it's only a rough guideline. Too many factors to determine MPH from pitch alone.

[Prof. O/B]

I use a digested form of this calculation:

RPM X PITCH / GEAR RATIO / 1056 (A CONSTANT) X .90 (EFFICIENCY) = SPEED, Example:

7,000 X 26 / 1.87 = 97326.2

97326.2 / 1056 = 92.165

92.165 X .90 = 82.948 MPH

This is easy for me to remember, simple and has served me well........Prof. O/B

[The Big Al]

Are just enter your info here and you have it.


Free prop calculator just click on link below!

http://www.rbbi.com/folders/prop/propcalc.htm

[Fish]

Are just enter your info here and you have it.


Free prop calculator just click on link below!

http://www.rbbi.com/folders/prop/propcalc.htm

looks a lot like one of the three I posted above Big Al;)

[pyro]

chill out dude. Flaming Al for being TOO helpful???

I use that calculator too. Good stuff.

[Fish]

chill out dude. Flaming Al for being TOO helpful???

I use that calculator too. Good stuff.

No one flamed anyonepyro, I know Big Al, I was kidding with AL, See the smily wink thingy at the end of my post??? Who really needs to chill here?:rolleyes:

You are right, it is a good calculator and the other two I posted with it are also pretty good too.

[capnzee]

I have a REALLY SIMPLE METHOD! Take the 6000 RPM and divide it by 1.87 which is 32085561. multiply this by 25 (prop pitch minus one)and the answer is 80213902, or 80 mph. The first two numbers equal the theoretical speed at 100 % efficiency. Throw in 10 percent of slip (if you are lucky) and your speed should come out to about 72 mph. Simply speaking; take the RPM of the prop, then subtract one (1) from the pitch of the prop. multiply this number times the RPM of the prop and the first two numbers will be the approximate speed at 100 percent efficiency. Couldn't be simpler and you can do it quick. Just not up to an engineers standards but close enough for most, considering tach error, speedo error, pitch error. Before you knock it, try it! Just about the same number that Professor O/B came up with (82) but a whole lot easier and quicker. The calculator is great if you have your computer handy! Capnzee

[j_martin]

Huh, this engineering chit is killin' me. I might go back to drinkin'