Karl Townsend wrote:
Sounds like you need to run a much lower pitch prop. If it is cavitating,
you are just wasting energy. Are you running it in a tunnel? Pitch?
Diameter?
I need to run lower RPM to get rid of the cavitation. I'll need a higher
pitch to still move enough water. My question is do I need a larger motor
and VFD.
Cavitation comes about because the pressure on the lifting surface of
the prop blades falls below the vapor pressure of water*, and the water
"boils".
For a given pressure differential across the prop, a higher pitch prop
is going to have to generate more lift, because it'll be working at a
less advantageous angle.
All else being equal, if playing with pitch and speed without adjusting
diameter will get you there, a lower pitch, higher speed prop should be
the way to go. Theory says you should be able to do this with a larger
diameter prop, to get your volume at a lower pressure differential
followed by a nozzle to get the pressure up -- but this is where the
fluid dynamics part of my brain starts flashing a red light next to a
sign that says "ask a real hydrodynamicist". So I may try it myself at
home, but if I were going to take your money to try it I'd be playing
with scale models at a wet bench -- and I'd start by advising you that
you'd spend your money more effectively with a real hydrodynamicist.
I'd suggest that you consult a boat racing site or expert, because they
deal with cavitation all the time. But expert racers can have notions
that are quite divergent from reality; myths and legends that were the
best that science could come up with in 1908 somehow get carried forward
to 2008 as if hundreds of physicists and mechanical engineers hadn't
spent their entire careers improving the state of the art.
So, consult a boat racing site or expert, but bring your salt shaker.
* Ya, ya -- I'm simplifying. So sue me.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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