prop question, do rpm matter?

[Fr. Frank]

bushwhacker, thanks for the detailed explanation. sounds like im going back to the bigger prop w/ low cruise speed. i can just run it at 4500 instead. after all, id rather keep my bearings happy than save a few pennies at the gas pump. motor is at the right height by your description. thanks again guys, CSC never fails me.

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.

[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...

[gofastsandman]

Ok I need to come on in to the confessional. I am now running an SST I 15x16. `96 rude 150 compression 87-92 cold. Butterflies were closed. Yeah, I now know better. Engine runs nicely. Normal load and bimini up I only turn 5000-5100.

AV plate is @ 1 1/2 " above hull.

BRP tech support says my motor makes full power @ 5000 and that you are only putting more stress on the motor spinning it higher for no gain. They said it`s not a merc or a yami and was not meant to rev that high. This goes against everything I have learned and been taught. I have not timed the motor nor have I ever done a link n sync.

BRP said when they train their factory techs they tell them to prop motors at the mid point of the range. For me that is 5000.

I can hold plane @ 11 now , down from 17 mph so that is nice. I premix and run rec 90.

Confused,
GFS

[eggsuckindog]

Sandy as I said that is fine on a Rude, Mercs need 5400 minimum which is about all I can get now with that Stiletto I have, I wish I could swap it for a 17 because I like the prop alot. Correction, Rude Loopers need more RPM than the 90'V motors.

I was at the shop today picking mine up and they had a 17 Mirage and a 17 PowerTech, then the son pops up and said he had tried both of them and niether would trim his boat nearly as well as his - Stiletto - dad turns to me and says " maybe you need to stick with what you got"

If bumping around for props, this thing made a world of difference on mine, keep in mind they are aggressive and you will lose RPM against most other props, almost 300 for me but they work damn good on SeaCrafts. I got mine used for 150 so they are out there

[eggsuckindog]

Confused here when you say "smaller prop" the smaller prop should give less cruise speed and more RPM at WOT and should plane quicker???

[pelican]

running the correct pitch prop will allow your boat to plane quicker - think of it like this: you're driving a stick shift vehicle - you attempt to start off in 3rd gear - what happens ? same thing occurs with a boat engine,with way too much prop pitch.too much pitch,the boat's a dog outta the hole - no lift

as far as rpm is concerned:
think of it like this,you're running the engine on the hose - you "rev" it - the rpm climbs quickly -right ? runs and screams on the hose - right ? no load on the engine.for max engine durability,you want that engine to hit it's max rpm,with the boat loaded,as it's noramlly ran...there's a reason an engine mfg puts this reccomended range on an engine - that's where they want it to run.

it's all about "load" ...