Propeller Slip ????

[gjohns6896]

I am trying to figure some numbers out for my Donzi, but am ignorant as to what this term means.

My prop is a Viper 14.25 X 21" powered by a '92 350 with OMC King Cobra drive.

My speedo said I was hitting 58 MPH at 4100 RPM. Is my gear ratio also 1.43? Where do I find that number on the engine?

Cheers,
George

[HyperDonzi]

do you have a prop calculator? (someone put a link up) enter everything into the fields then leave prop slip blank. it will fill it in. 10-15% i think is average?

[Formula Jr]

a prop is a screw in the water. But water is less than a solid, so there is a theoretical speed based on your prop pitch, RPM, gear ratio etc., and then there is actual forward motion. Prop slip is the difference of Ideal and actual.

[Greg Maier]

Here is a link to a prop calculator:
http://www.chadhillpropellers.com/propellercalculator.html

[gjohns6896]

Thanks, BUT I used that one yesterday, and had NO IDEA what to put as my "Propeller Slip" number. I assumed the other number on the prop, but that did not work either.

So, I just used 10% and that put me right at the speed I was going, almost. Something like 57.6 MPH or something, while my speedo displayed 58. But I am not directly in front of the gauges, so it could have been the way I was reading it. Anyhow, thanks all.
I am all straightened out now...I THINK!

Cheers,
George

[blackhawk]

Leave the slip blank and fill in the rpm, ratio, pitch and speed fields and it will calculate your slip.

[tailwind]

Your numbers are indeed close to mine: 1991 with the 350 Spitfire: 62.5@4200 rpm.

Accordingly, do not trust the calculators online. Must recalibrate our tach accordingly. We are both close herewith but not absolute! Our point herewith: let us all re callibrate our tach, with a known animal.

Randy

[gjohns6896]

That is all well and fine, but how in the hell do we recalibrate our Tachs? What tool is this?

Cheers,
George

[Ralph Savarese]

www.boatpropellers.com/propcalc.html (http://www.boatpropellers.com/propcalc.html)