formula for prop calcs?

[Wee Hooker]

I'm looking for the formula to calc prop slip, speed etc using RPM, gear ratio, etc. I've seen it around here but a search isn't producing.
Also two other questions:
a) What is a reasonable expected prop slip?
b) Is there anyway to ID the pitch of a prop if it has no markings?

 

[Maclin]

Re: formula for prop calcs?

This is a web page I found that has a calculator. I do not know the mathematical formula behind it, but this one does seem to produce good numbers. It works with these 5 factors:

Engine RPM
Gear Ratio
Propeller Pitch
Propeller Slip
Speed in MPH

You give 4 of the 5 required factors and it will give the fifth factor. They key for it to calc missing any factor is to have exact readings for the other 4 factors.

For prop slip, 10-20 percent is a normal range. It can vary by speed for any given application. I use 15% if I need to calc something and do not know the slip.

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

 

[hwsiii]

Re: formula for prop calcs?

Hooker. the formula for prop slip follows.

Theoretical speed=(Pitch*Shaft RPM)/12)*60)/5280
Real speed = Input Speed
Motor RPM = Input RPM
Pitch = Input Pitch
Gear Ratio =Input Ratio
Shaft RPM = Motor RPM/Gear Ratio
Real Prop Slip = 1-(Real Speed/Theoretical Speed)

I personally use a shorter formula but this is the complete formula.

Prop Slip for a good aluminum prop on a lighter boat should be about 10 to 12% at WOT RPM

Prop Slip for a good stainless prop on a lighter boat should be about 6 to 8% at WOT RPM

But these are just generalities, when you get into heavier cruisers prop Slip can be as high 25% to 35%

There is a way to ID prop pitch without taking it to a prop shop, but the formula for doing so is very mathematically intense and time consuming, but it also takes into effect any theoretical pitch builtin to the prop such as cupping and progressive pitch


H

 

[45Auto]

Re: formula for prop calcs?

BAM Marine has a nice calculator:

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

 

[Wee Hooker]

Re: formula for prop calcs?

Thanks much Gents!