prop question, do rpm matter?

[pelican]

Justin, if you're still running that Merc 200, you need to be turning between 5400-5800 rpms at wide open throttle. A lower max rpm will give you just the detonation and internal engine stress that Denny was warning you against. So when you cruise at a lower rpms, you're engine isn't straining so hard. It sounds to me like you still don't have the best prop, but finding the right prop is a process that always involves trial and error.

And just for the record, turning a higher rpm at cruise doesn't necessarily mean you're using more fuel.

very true !

the common line i allways hear is "i never run the engine above 3,500rpm",when i test run the boat,top rpm is 4,100 - the engine's loaded beyond belief - sucking fuel like a jet.
the correct prop,it will allow you to plane quickly,and keep that plane at a lower rpm - it also effects the ride of the boat.load the boat,as you normally use it - duplicate weight- take it out and pin the throttle - record your top rpm number -compare it to what the engine mfg calls for.
selecting wheels - a good general guide is,every inch in pitch will yield approx 150rpm.on many occasions,a 2" pitch change will yield a huge difference,due to the hull being able to lift more,as the hull lifts,rpm increases.example,you're turning 4,800 rpm - your engine is rated @ 5,400-5,800 - you do the math,and you come up with a huge drop in pitch.in reality,a 2" decrease in pitch may give you the results you're looking for - due to what i explained.there's no set rules to this,it's really honestly trial and error - take your best guess,and then sea trial.air temp and humidity can and will cause a big rpm drop - if you're in the northeast area,like nj,the air changes dramatically in the summer - 95% humidity,this can cause an rpm drop of 10% on occasion.
think of propping like this,you're in a vehicle running up hill - by allowing the engine to run a higher rpm,you've made that hill less steep - unloaded the engine...remember,water is 900 x's more dense than air - a boat's only "unloaded" when running down a wave...

correct propping is essential to assure a long engine life...



side note:
2 stroke engines,these are low compression ratio engines - these do not have the octane requirements of 4 stroke engines - using "premium grade" fuel isn't needed.and yes,water in the fuel can cause a detonation.phase seperation is the end result of water in the fuel - not the cause of the fuel - find the source of water intrusion,and you've found the cause of phase seperation...