Calculating Approximate Pitch

[72SideWinderSS]

I see a lot of questions about " How can I figure out what pitch I have "

Here is one solution :


One way to find the Pitch of an unmarked prop

With your prop sitting on the bench (Prop-nut side up), place a square
from the bench up at the trailing edge of the prop at 5 (five) inches from the centerline of the hub.
Make a mark on the trailing edge of the prop and another mark on the bench, measure the height of the trailing edge and write it down as H1.

Do the same on the leading edge of the prop and write it down as H2.

Now measure the distance between the marks on the bench and write it down as X.

Subtract the Leading edge height (H2) from the Trailing edge height (H1), write it down as Y.
(H2 - H1) = Y

Now divide Y by X, this gives the slope of the line write it down as S.
Y / X = S

Multiply 31.4 By S this will give you approximate pitch.
31.4 * S = pitch

Y / X = S

(5 * 2) * pi = 31.41 (radius * 2) * 3.141 = 31.41

31.4 * S = pitch

 

[Dhadley]

Re: Calculating Approximate Pitch

Not coming out right for me. Is that taking into account progressive pitch, IE will that be the approx pitch at 5" out?

 

[guy74]

Re: Calculating Approximate Pitch

Tried it on two known props. On the 14X13 Hustler prop, I came up with 14.9, seems alittle off. The 14X17 OMC prop came out 17.1, so It worked out pretty close on it.

 

[72SideWinderSS]

Re: Calculating Approximate Pitch

Dale,

The Theory (or formula) is the blade does most of the work at 70% of the radius. I chose the 5" measurement to be 70% of a 13" dia. wheel (for V4's) and 66% of a 15" wheel (for V6's), and to keep the math simple. Progressive pitched wheels need to have measurements all across the blade face to average out the true pitch.

If the blade is not cupped, it should be very accurate. If the blade is cupped one needs to allow for the depth of the cup. This is how we pitched wheels for the big ships (tankers) and tugboats. I have not worked in a prop shop since the late 70's, but this how it was done back then.

Most all prop mfg.'s claim their pitch numbers are an average of the actual pitch, on progressive pitched props.

Guy74 and Dale,

All of the Turning Point Props I have been around actually measure greater pitch than advertised. Even the ones I have tested show very small slip numbers for the advertised pitch. i.e 3 - 5% slip. If you plug in 1 -2 inches greater pitch the numbers for slip become more realistic.

I use a Fluke 88 with an RPM adapter for RPM measurement and a GPS for Speed (averaged over the course in both directions).

 

[guy74]

Re: Calculating Approximate Pitch

Thanks for the info, Jimmy.
I need to dig up my mystery prop that I used to use, and see how it measures up. It was supposed to be a 13.75X15 prop, but it seemed to be closer to a 14 in pitch. It worked great for skiing with my underpowered setup, but ventalated too easy in turns since it wasn't cupped.
Thanks again for the education,
Brian