How do waterflow and propellers interact?

[dioross]

hi everyone!
need some help about propellers..say i have a fixed mounted propeller and a flow of water with known velocity runs through it.. what would be the rpm of the propeller??.. i know there a re lots of things and parameters to consider... i need some good references, calculations ,and basic formulas..thanks!..any help would be appreciated..

i have to design a propeller to be used in water velocity measurements... so i want to have the blade parameters which corresponds to the design RPM of the propeller..

Regards,

dioross

 

[Unrest]

There's all these empirical charts for selecting boat propellers. Here's one I scanned once, can't remember where from.

Tho I wonder if maybe none of that applies when the water's driving the propeller. It would certainly need some rethinking.

I guess you could also do some basic pitch calculations assuming no slippage by treating it like a screw thread. Actually that might be a good approximation since you're not actually transferring power to/from the water. Just make sure the torque is way higher than the friction of the shaft/sensor.

[PLAIN]http://dl.dropbox.com/u/21857463/bpdeltalarge.GIF

 

[dioross]

thank for the reply..
Do you have some ideas about basic calculations?..:)

 

[Unrest]

thank for the reply..
Do you have some ideas about basic calculations?..:)

Yea just imagine it's a screw turning through a block of butter. You can work out the geometry easily if you 'unwrap' the cylinder of the blade tips (or some other point) onto a flat surface, then you find the blade tips trace out a triangle. So it's just the ratio of side lengths that gives you the ratio of fluid speed to blade tip speed. I think that's also the ratio of pitch to diameter.

For the force (and thus torque) applied by the water, look up the 'momentum equation' applied to fluids.