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.