Hi Ken,
The speed to RPM is much easier and more accurate with tires on the ground. In boats, the amount of slip is completely variable, with the boat design, prop, speed, RPM, etc. Even the weather (wind) will affect your slip numbers.
Here's a quick excerpt from a previous post that I made last August
concerning prop slip of my old and new props...
"With the old prop and the diesel engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts."
The only way that I could figure the slip was to have an accurate RPM to start with. There's really no way to back into the RPM for a given boat and drive configuration without slip numbers and you can't get the slip numbers without the RPM.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, KEN <rekkamurd@...> wrote:
>
> ... same can be said about reversing the math with a very low range speedometer, ballpark calculating 63360 (mile in inches) dividing prop pitch to arrive at general range rpms.
> miles per hour, rev's per minute.. if you already have a speedometer you can whip up a general mph = prop rpm chart. same for amps vs speed vs range ballpark calculations.
> Â
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