o84 and rpm

[txriverrat]

I am trying to learn as much as I can about these engines, it might me my choice for the second boat. So bear with me if I ask some dumb questions.
The rpm yall are turning has me puzzled, If I understand correctly these are 3600 rpm motors and that is what they make the most horse power at.
Are you turning 2500 to 3100 because thats all they will turn the props or is this to match the maximum prop speed because of length ?
Ron

[Papee]

If the prop is matched to the engine correctly the prop will "govern" the engine and hold the rpm down to what the props max efficiency is. The 2500-3100 rpm you are seeing are direct drive numbers. The engine can turn more rpms with a reduction but I would not run it much past 4000 because of the oil bathed rod bearings. I know they have been run at a higher rpm but not continuous. The harder you push them the less reliable they will be. I choose direct drive and lower rpms over a little more speed. This is my sixth or seventh season running mine relatively trouble free.

[Doozit20]

Yeah as papee said there are a few boats being built right now and running with both reduction drives and direct drive as you know. Typically when you see a number as 3600 or higher itll be a reduction drive boat but at the same time those boats will run there engines down to the 3000 rpm range to cruise and save a little on gas and still have that little extra power if needed. I as u know run direct and the 2800 or so is all I am able to get from my setup due to the prop not allowing it to spin up any faster. Having said that people say these engines are 45hp however at 3000 wouldnt it only be around 32-35 hp? I know if you sit at a prop thrust calculator and punch numbers thats what it comes out to anyways. Also if you go to the ultra-prop website and look at there charts the 59×10 at 2800 rpm that I can spin is only at about 35 hp according to there charts.