Prop slip...

[C-Webb]

What does it mean for a propeller??

[pyro]

It means how much % of thrust loss is experienced compared to what the prop would do in "theory" based on pitch, rpm, gear ratio, and resulting speed. Less is better.

It represents the prop's (lack of) efficiency. 9-15% is common.

Run the calculations here:

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

[Techno]

A 26" prop will advance like a screw through jello 26" per prop revolution.
Since they don't do this in water the % slip is the amount it really advances. If it advanced 13" for 1 revolution it would be 50% slip.
Or 50% thrust.
90% thrust/10% slip.

If the slip seems very large it could be due to a bad tach or speedometer.
I've heard trailers can be the cause too:p

[C-Webb]

pyro and techno, thank you very much for your answers

[Techno]

In the end prop slip don't mean much. Since the props usually have a varying pitch and are stamped with the median pitch you don't know which part is doing the propelling. Also since the fastest speeds usually result in a high slip....
Low %# do help economy though;)
Its a nice thing to check on but not to get hung up on.
I don't know how many times I've run the 28-32" through the calculator:D To choose which one I need.