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20' pontoon 40hp rpms at WOT?

Most 2 strokers are spec'd to run 5000 to 6000 max RPM, top end, regardless of LU gear ratio and prop. Your job as the owner/operator is to adjust your prop pitch, taking into account your LU gear ratio, whereby you can achieve that RPM level at a light load. As the gear ratio decreases, the prop shaft rotates faster for a given RPM so the prop pitch has to be reduced accordingly to maintain the desired values.

Dropping below 5000 RPM. regardless of boating circumstances or load, is detrimental to the health of the engine and reducing propeller pitch to get on up into that recommended range on your most used load factor in the boat is a necessity. On a wide disparity between the loaded (boat's full of folks and gear) RPMs and unloaded value (just you alone), where the engine wants to rev. above the recommended range for your engine, just cut the throttle back until you don't exceed the engine's max RPMs recommended.

The gear ratio and propeller pitch are part of a fixed gear train to the twisting of the engine's crankshaft....only other part is "prop slip", the difference in a "screw" twisting in a semi-solid/solid...wood.... and a screw twisting in a moveable environment like water which is measured as a % of calculated performance reduction in the moveable medium. The heavier the load, and the less the efficiency of the hull to water relationship, the higher the slip. A light, fast boat might run in upper single digits in percent of prop slip. A tub will run 20-30%.

That translates to a light, fast, boat, for a given engine HP, will run a much higher pitched prop and maintain recommended RPMs whereby a tub would need a shallow pitched for the same reason.
 
This is a 40 EFI four stroke. Recommended max rpm is 6000. It has a four blade Michigan 11.5x10. It will only turn 3000 max at WOT with just me in the boat. There has got to be something else going on that I have been trying to find for days. The shop manual recommends a three blade 12-1/4 x 9 for a pontoon. But I can't believe that is going to make a difference of 3000rpm?
 
52wrench said:
This is a 40 EFI four stroke. Recommended max rpm is 6000. It has a four blade Michigan 11.5x10. It will only turn 3000 max at WOT with just me in the boat. There has got to be something else going on that I have been trying to find for days. The shop manual recommends a three blade 12-1/4 x 9 for a pontoon. But I can't believe that is going to make a difference of 3000rpm?
Click to expand...
Rule of thumb on here varies from 150-200 RPM change per inch of prop pitch change depending to whom you are speaking.
 
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