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post #45 of Old 12-19-2012
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Re: overpropped vs underpropped

Originally Posted by srah1953 View Post
The only data that I can find in the manual is the maximum output rating of 39hp at the crankshaft at 3,000rpm. It gives a propeller speed of 1,293 rpm and a reduction ratio of 2.32. Can I multiply one by the other to get max rpm (which would make 3,000 rpm)?
No you shouldn't be multiplying anything by your gearbox ratio. Your tachometer, if correctly calibrated, reads your CRANKSHAFT rpm, not your propeller shaft rpm. It's max RPM at the crankshaft at WOT that you're after.

My engine has at least 3 variants as well. One governed at 3000rpm, one at 36000rpm and one at 4000rpm. Mine is the 4K version.

The way my mechanical surveyor found this out for sure, was he used a sight gauge to verify that my tachometer was reading correctly as they often are not. Mine was off by 1000rpm EDIT: this should read "of by 100 rpm at 3000rpm"at 3000rpm. Once the engine was WELL WARMED UP to operating temperature after running under load for a while, he had me put it in neutral and advance the throttle to full throttle. He took a reading on my tachometer, and another from his strobe sight tach thingy, and I put the throttle back to idle.

In short, if you want to know for sure where your engine is governed, then floor it in neutral once it's warmed up. Where the govener stops you is the RPM that you should be able to reach (or get near) at WOT.


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Last edited by MedSailor; 12-19-2012 at 11:10 PM.
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