prop x rpm x 0.0009469 x slip

Result from above divided by gear ratio = theoretical speed.

tt

tt

ANSWER: NO, we can't finish this?

We give up..... what's the answer?

Thanks, Tim Bostic
Hoss Marine Props

Speed = (RPM/GearRatio)/60 * (Pitch/12) * slip * 0.68

Where

Speed in mph
RPM in rev/min
Pitch in inches

If you take all the constants together, that gives you 0.00944, which is just rounding errors different from what was originally posted.

The original post was missing the gear ratio factor.

this was a formula to figure out speed and slip 31pitch x 5800 rpm x 0.0009469x 0.90 slip devided by gear ratio 1.50= 102 mph .. I can hit 100. 0.90 is 10% slip so my slip is about 11.5% after posting this question I remembered the fomula ,I have not use for sometime and Im getting old HA HA

Here's the formula that I use:

RPM/gear ratio * pitch / 1056 = Theoretical Speed
Actual Speed / Theoretical Speed = Slip

So using your info:
5800/1.5 = 3,866.66 prop rpm
3,866.66 * 31 = 119,866.66 (inches per minute)
119,866.66 / 1056 = 113.51 (converts to miles per hr)(Theoretical speed)
100 / 113.51 = 88.1% efficiency or 11.9% slip

I use this formula because I can remember it (I'm getting old too!! haha) and do it on my phone while I'm at the lake testing props. This works good when you are doing back to back COMPARISONS on props.
You have to assume the speed is accurate.... the rpm is accurate.... and the pitch is accurate?????..... if any of the info is in error, then your answer is off too.

I use GPS, calibrated tach, and actual prop pitch.
When testing new designs, I have a "shop" prop that I run first to verify engine performance and conditions.... then test the new props. If testing lasts most of the day, I'll finish with a final run with the "shop" prop to verify that nothing had changed throughout the day.

Thanks, Tim Bostic
Hoss Marine Props

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