Difference between prop speed and rpm?
 

Are there gauges for prop speed readouts?

Tachometer/drive ratio is pretty accurate...

Wouldn't the drive ratio determine the prop revolutions? Here is a link to explain.
http://www.go-fast.com/sterndrive_gear_ratios.htm

and gps,,then you get prop slip which will get real speed of prop.

http://www.livorsi.com/catalog/data_tach.htm

We've put these in a few boats in the past, but not sure if they are still offered.

The difference between actual and theoretical boat speed is prop slip/efficiency the lower the slip in percentage the more efficient the hull/wheel combination. Of course trim/sea conditions/wind/load etc will all play into the ultimate result. Typically keep the tabs up to neutral, trim to just under prop blow out and limit the weight of your gear on board to attain your max speed. Then compare actual to theoretical.

Take a look here also http://www.go-fast.com/prop_slip.htm

I just want to point out that the term "prop slip" is a little misleading.

That's a handy chart.
crankshaft rpm ÷ gear ratio = prop shaft rpm

It seems like 'prop slip' is implied through a formula/ratio which is not what I'm after.

I need a sensor that will determine whether prop is in mid-air or underwater. I'm looking for a measure of prop slip over a duration of multiple times a second (e.g. 200hz).

Is there a sensor to measure propeller resistance?

hate to ask, but what for and why. if the wheel is mid-air/out of the water prop. slip is 100%. are you trying to measure blade load as it comes into aeration and re-enters and becomes loaded?


prop slip ratios are not implied they are real. are you attempting to possibly design a wheel with minimal hydraulic resistance? maybe ask the question in the propeller section.