Prop Speed Calculation
 

Does anyone know how to calculate prop speed.Now a stupid question.Why when using a 1.27 ratio lower compared to a 1.5 ratio why do we have to drop prop pitch.With the 1.27 ratio the prop shaft is actually turning slower so i would think a bigger pitch prop would be required.

The gear ratio refers to the number of engine revolutions for each prop shaft revolution.

If the engine turns 5000 rpm with a 1.5 gear ratio the prop shaft rpm is 3,333.

With a 1.27 ratio at 5000 engine rpm the prop shaft is turning 3,937 rpm.

So in theory you will go faster with a higher (numerically lower) gear ratio with the same prop at the same engine rpm.

Explained perfectly.Thanks

If you go to a higher gear ratio (lower numbers), it will take more power to turn the same pitch as previously used at the same rpm as previously turned with the lower gears (higher number).

If you can do these things, turn the same rpm with the same pitch with the taller gears (lower number), you will go faster.

Just changing the gears dose not do this. You change the gears because you ran out of prop pitch and you are on the rev limiter or over reving the engine. (or other reasons we won't go into)

In theory which will go faster if you even out the numbers say. A 1.5 with a 28p or a 1.36 with a 26p. I was always told a 1.5 with a 28 would be faster than a 1.36 with a 26.

V = 0.000947 * Engine RPM * (Pitch / Ratio) * Efficiency

V = Speed in MPH
Pitch = Prop pitch in Inches
Ratio is Drive Ratio
Efficiency = ( 1 - Prop Slip)

In Theory, it would be best to turn the largest prop you can as slow as you can. You engine speed's going to be a given, so the faster your transmission is spinning, the more heat and frictional losses you will incur. Same with a prop, the faster you're spinning it, the lower your efficiency will be because the thrust to drag(torque) ratio starts to decline.

The negatives: The bigger the prop, the more torque it will generate, and acceleration suffers.

Finding that perfect delicate balance is Throttle Up's Specialty!
:)

When you get into it it's a delicate balance.How many people would be out there that are searching for every little bit but are missing out due to a simple factor like gear ratio or pitch.Take a reasonable size vee,mid weight.It would need torque more so than a comparably size cat i guess.Remember the post on here about Fountain using 1:65 ratio Bravo's.Were they achieving something by turning a big prop slower?

Agreed Larger pitch at slower rpm is faster. I guess thats why the racers years ago favored the 1.68. It should accelerate faster.

Z Zuperboat , in your case it seems a ratio change is in order 35 pitch props tend to look like paddle wheels, I would calculate ratio to end up near a 29.
Jim