prop question, do rpm matter?

[2182]

it took me quite a while to find the right prop when i repowered a few years back; was looking to max out around 5500 rpm. i got it finally, but one i bought in the process got me fantastic cruise numbers, just wouldnt turn above 5000 rpm. the high end speed is not much different, but at 4000 rpm, smaller prop got 36 mph. larger "correct" prop got closer to 30. smaller prop takes longer to get on plane and like i said, it wont turn more than 5000 rpm w/ an empty boat, but with gas and all that good stuff, i sure like cruising at 36 mph. somebody explain to me why i need to use the "correct" prop please or im going to switch back to the fuel efficient model.

[Blue_Heron]

Is your fuel burn rate the same for both props at 4000 rpm? Since the WOT rpm is different, you're probably giving it more throttle to get the same rpm out of the fatter prop. If my miscalculations are correct, the prop that tops out at 5000 is at around 76% throttle at 4000, the one that tops out at 5500 is at 68% throttle at 4000.

And you probably already know you shouldn't run a prop that won't reach the recommended WOT rpm range with the boat loaded. If you're at 5000 rpm with an empty boat, that alone may disqualify the prop.
Dave

[2182]

good question. i doubt it. and thats a good point. i tore out my fuel management system in an effort to minimize fuel line interruptions recently, and it never worked worth a darn anyway. i remember hearing the 5000 rpm WOT prop was a bad idea in the past, im just unclear on the why.

[eggsuckindog]

It depends on the motor - rudes will work at lower RPM and be fine, Mercs need a bit of RPM. Props can be very different due to design at the same pitch, very aggressive props like Tempest- Mirage - Stilleto need to be much higher to work correctly - now you know why racers are always playing with props. 5000 depending on load is not too bad, that cruise is nice sounding, if you normally don't load it real heavy you could get away with that loaded, or try and raise it some.

[Bushwacker]

. . . i remember hearing the 5000 rpm WOT prop was a bad idea in the past, im just unclear on the why.

If the motor won't wind up to it's max rated rpm at the load you normally run, you're basically lugging the engine, analogous to driving a stick shift vehicle in too high a gear. The result is increased pressure & temperature in the cylinder, which increases loads on the pistons, rings, wrist pins and bearings. This increased pressure & temperature also makes it a lot more prone to detonation/pre-ignition, where instead burning evenly, some or all of the fuel mixture suddenly explodes before the piston reaches the top of the compression stroke. Detonation is a violent event and is the "pinging" you hear in a 4-stroke, which is the valves rattling due to the shock waves bouncing around in the combustion chamber. With a 2-stroke, there are no valves to rattle, so the first sign of bad detonation, other than a slight power loss, is when it blows a hole in a piston!

Outboards have 100% needle bearings and short strokes, and the V-4/V-6 engines have very short rigid cranks, so they love to rev, and the harder you run 'em, the more oil they get, which keeps the bearings happy! But lug 'em down and get a little detonation, which you can't hear, and you can have problems in a hurry, or at the very least shorten the life of the rings and bearings! One day you'll wonder why it has low compression!

Detonation due to low octane fuel is why ethanol gas, which can drop 3 points in octane if you get enough water in it to cause phase separation and remove the octane-boosting ethanol, can be lethal for 2-strokes! If you're gonna run ethanol, it's best to buy at least 90 octane so that you've got at least 87 octane if water takes away all the alcohol!

Before you spend a lot of time/$ fooling with props, make sure the motor is mounted high enough! You should be able to see the cavitation plate above the water when you're up on plane. If not, raising the motor may get you some more rpm with the current prop!

[2182]

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.

[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