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.

Sounds like what you need is a strain gauge to measure the loading on the drive. Reach out to Mike Clesceri (MDC DriveGuardian) he can help you.

An auto-throttle system that will plug-in to existing throttle linkages.

So to apply the concept: actuator, cable, 3DOF accelerometer and ECU

ECU logic:
1. Program RPM (6000rpm)
2. RPM Sensor (compares current rpm to 6000rpm)
3. Are we above set rpm? (If yes, release throttle cable/If no pull throttle cable)

The above method that would only work for hull exit/rpm redline, but it's missing the re-entry portion. That's because rpm is a secondary data point. The primary data I need is what you just mentioned.

Quora: What components are needed to create a marine auto-throttle system (throttles to zero when hull launches off wave)?

Very cool! Never heard about that product before. I'm looking for a similar system for twin outboards though. Heard of any?

Source:
http://www.offshoreonly.com/forums/g...ear-later.html
http://www.boatingmag.com/gear/drive...imiting-clutch
http://www.marinedesigncorp.com/our-products.html

Re: strain gauges, these guys were at PRI

http://tecatperformance.com/

But it might be easier just watching RPM and controlling electronic throttles . Logic would be something like:

1. If RPM suddenly spikes upward, reduce throttle to maintain the RPM that occurred just before the spike
2. When RPM suddenly spikes downward increase throttle to maintain the RPM that occurred just before the spike
3. Throttle movements by the throttle man always override computer
4. Computer control is over ridden if the boat isn't flying level

This logic isn't tied to a specific rev limit and would be applicable to rough conditions that prevent top speed/RPM. Not bouncing off the limiter either. I would think that any ECU is sampling RPM at the rate you need.

There are aftermarket traction control systems that do something similar but they use ignition retard instead of throttle movement.

http://www.moretraction.com/Products_Drag_EFI.htm

Ignition retard might react faster than the actuator in an electronic throttle, but for long "flights" throttle control is definitely preferable.

Good luck with your project!