Thread: Under propped?
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Old 03-15-2014, 10:29 AM   #12
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City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 4,969
The norm is to prop so the engine gets a little OVER the rated RPM at full throttle with the boat FULLY LOADED. This approach ensures that you can run the boat at any RPM without overloading the engine. That's what all the engine manufacturers specify and require for warranty conformance.

If you want to diverge from that, there are lots of tricks that a small number of people play. One is to load the engine more heavily at lower RPMs (like your friend is doing), but that will overload the engine at higher RPMs so it needs to be accompanied by a way to reduce your max RPMs so you stay out of the overload RPM zone.

This is playing with the difference between the load that a prop presents to an engine, and the load that the engine can sustain. At lower RPMs, the engine can sustain a higher load than the prop presents. However at higher RPMs, that same prop will overload the engine. So you have two choices.

1) You can prop to reach max allowed load at max RPM, which allows for engine operation that is within specs at all RPMs. This is what all the manufacturers recommend/require.

2) You can prop for higher load at lower RPMs, but need to simultaneously limit MAX RPM so you never go into overload on the engine as you increase RPMs.
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