Prop Calculator - How accurate?

[dg22]

I found this prop calculator on the web. Interesting to see how accurate this for calcuating your top speed (compared to your actuals)?

Go to Link http://www.rbbi.com/folders/prop/propcalc.htm

Enter your Engine RPM (Eg. 7500), Gear Ratio (Enter 1 for 1:1), Prop Pitch (Eg. 12), Prop slip (I'm guessing to enter 10), Leave SPEED emty and click the Compute button.

[PopPop]

Props

Slips the trick!!
Particularly with surfacing lower units.

[Jack Stotts]

I calculated mine using best known info on slippage and found the system to be in error as compared with GPS readings. The formula came up with 64.3 and my GPS reads 78.2. I moved the slippage to .01 and got 70.7 so still way off. Have no idea what the true slippage would be.

[mac19f]

Works out pretty close on my Yamato and my Dmod at 10% slip. Don't know why it didn't work for Jack. I've chased Jack around the course a few times so I don't doubt his speeds.

[champhotrod]

Pitch

How are you guys measuring your prop pitch? This may be the varible in the formula!

Cooper

[dg22]

Prop Pitch

Your prop should have 2 numbers stamped on the side of the hub. For example a D prop 7" x 12", the smaller number would be the prop diameter and the larger number would be your pitch.

[sam]

Your prop should have 2 numbers stamped on the side of the hub. For example a D prop 7" x 12", the smaller number would be the prop diameter and the larger number would be your pitch.

Yes, BUT exactly where on the prop the 12 is measured affects what the real pitch is. Just because a prop has a number stamped on it or because a "pitch meter" shows 12 at some particular point .... doesn't mean the prop actually works at 12" pitch

Cooper is right ... exactly how and where the measurement is taken affects the accuracy of the reporting of the pitch ... introducing yet another variable to the question

This would be a good subject for a prop guru like Dewald or Ron Hill

[dg22]

Good point Sam...I understand where Cooper's coming from now...sorry about stating the obvious. I'm a little new at this.

I noticed with this caculator you can compute any variable you're not sure about. So for example, if you have accurate readings on RPM, Speed, gear ratio and a rough idea of prop slip, you can calculate pitch. Not sure how accurate but that's for you decide.