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  1. #1

    Question

    Mercury's website offers this advise:

    o Adding 1 inch of propeller pitch will reduce full-throttle RPM by 150 to 200.
    o Subtracting 1 inch of propeller pitch will increase full-throttle RPM by 150 to 200.
    o If you're moving from a 3-blade to a 4-blade propeller, a 4-blade generally turns 50 to 100 RPM less than a 3-blade propeller with the same pitch.

    My question is, "What's the paradigm for prop diameters? All other things equal, what's the RPM change for one inch increase in prop diameter?"

  2. #2
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    There is no direct correlation between diameter and rpm as there is for pitch. That is why you will not find any recommendations relating to it vs rpm. Diameter and rake affect different boats and different set ups differently. Thats part of the black art of props. Trail and error, test, test, test...there is no way around it.

    On smaller lighter boats sometimes you can try going down a little on diameter with an increase in pitch for more speed. Doing that would give you more slip, but there is a CHANCE (no gaurantee)of more speed.

  3. #3
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    Going back to the original question: what is the theoretical change in rpm with a change in diameter .... ZERO!

  4. #4

    It's gotta go somewhere

    Mark75H,

    You mentioned slip...

    If at 6500 RPM and a 1.87 gearbox running 26" prop at 75 MPH ends up with a .123 prop slip

    Then a .08 slip, if attainable should give me an RPM of 6200.

    Maybe I'm looking for a change in prop diameter in inches/slip relationship. Still there's got to be some sort of positive relationship.

  5. #5
    On F1 boats diameter makes critical differences in RPM. Our prop diameters change in 1/8 inch increments because of the extreme sensitivity in RPM changes. I'm not familiar
    with large heavy boats but It would seam to me that the smaller diameter and greater slip factor would equate more RPM per speed in any case.

  6. #6

    John,

    Exactly! So, would you care to hazard a guess?

    If I'm looking to pull 500 RPM off my top end (what, through the greater drag on the prop from surface area), how much larger over 13.75 would you go?

  7. #7
    What Sam (Mark 75H) and John both said is right. It's all in the setup and will vary with all the different factors. If you've got more diameter than you need, then you've just got extra baggage. What you're trying to assume is that extra diameter in itself adds efficiency and that's not always true. In the case of John's Formula 1 stuff, there are several factors that make comparison with your setup difficult. First, he's running a lot smaller diameter, a lot lighter load. He's turning his motors 9 grand give or take, and doing it on a gearcase that's turning the blade at close to 90% of the engine RPM's or maybe 8000 prop RPMs for the sake of general comparison. He's doing all this in relatively clean water too, because there's nothing in front of the gearcase but the tunnel and the gearcase is very small. You on the other hand are turning a blade roughly 50% of engine RPM, or maybe 3000 to compare to his 8000 or so, on a heavy load, behind a fat gearcase, in super dirty water churned by the boat bottom and the aforementioned fat gearcase, which may even be running partly sideways because the poor thing is crabbing trying to swing 14+ inches of blade, turning in one direction and maybe half out of the water. So as Sam said, your queston can't be generally answered without a lot of trial and error. Unless your slip #'s are high, generally speaking I'd think pitch is what you're looking for to lower RPM's while increasing speed. I'd think of pitch as volts and diameter as amps.

    Another way to think of it is in exagerated sizes. If you were to go up to say, a 6 foot diameter prop at a given pitch, it may very well get in to a very high percentage of efficiency, but remember it's working in a very large column of water and the blade drag would be so great that I think you see the boat speed would be extremely low comparatively. So given this line of thinking, the smallest column of water that the prop can work in with a reasoble level of efficiency will produce the best speed per RPM. Again this is where load, bow lift requirements will play a major role in the requirement.

    [Edited by Raceman on 08-18-2001 at 08:27 PM]

  8. #8

    Point of clarification

    I don't fall into the normal profile of this forum's participants. I'm not remotely concerned about improving the top speed of my boat. Quite the opposite.

    The fact is, my hull (and its tender ride) and the nature of Keystone Lake Oklahoma wave action (often windblown and choppy) really doesn't reward efforts for top speed in a small boat. I've settled on an acceptable speed in the low 70s which is what a 26" prop should roughly target. That said, I'm willing to trade 500 or so RPM (approximately the amount over redline that the engine is capable of) and the associated speed to increase the pulling capability of the boat.

    I would expect that with an increase in prop diameter that there would be a corresponding reduction in top speed and RPM while maintaining my holeshot.

    Again, anyone care to offer their experience on effect on RPM/increase in diameter?

  9. #9
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    Hunter,

    If I understand your post you want a prop that pulls like a small pitch and is speed and RPM limiting on the top. I suppose you already know that the easiest way to accomplish this would be with a small pitch prop and the throttle. My XR runs with relatively small pitch props 99% of the time. Lighter load on the motor will also make it last longer. Works for me since I have no need to fly around at 90 plus most of the time.

    Barry

  10. #10

    Yep!

    Yeah, I'm looking to fix my top speed in the low 70s.

    I currently am capable of maybe 6700 - 7000 RPM depending on the load.

    I'd like to get the RPM at full throttle down to 6200 with my max load.

    With a 26" prop of 13.75 diameter, what diameter prop would accomplish that goal?

  11. #11
    I just don't know how the question can be answered any differently, because there is no exact answer. If you want someone to tell you that by increasing diameter, it'll drop the RPM's, it probably will. Will the prop hold better on the holeshot, probably. Will you decrease the acceleration, maybe, maybe not, there's more drag, or hydrodynamic resistance for lack of a better term. But again, there's no way to come up with a formula because it varies greatly with boat and set up and for that reason it may not happen at all or it may happen a some. In any case, adding or subtractin 1/4" of diameter on a boat such as yours isn't going to make as much difference as playing with the design of the blade or pitch. Also, you generally don't have a lot of diameter choices without playing with different brands or custom work. It's just not that important on what you're running. I think you'll get better advice on your question if you say: My boat is a XXXXXX, I'm runnin a XXXXX on it with a XXXXX blade, turning XXXXXX and XXXX setback and the propshaft is XXXXXX below the bottom. I want to keep or increase my holeshot while dropping the RPM by XXXXX. Otherwise your question becomes engineering theory that requires testing to veryify. You may even find rake and definately blade design to be more critical to your requirements.

  12. #12
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    I don't think a larger diameter prop will accomplish that. I suppose a poorly designed prop might. Running at reduced throttle will. It will also save fuel and engine wear.

    Barry

  13. #13
    Another factor is some times a smaller prop diameter with more slip will accelerate better by putting the motor in its power range. As long as slip is controlled before blowout.

  14. #14
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    Well said, John.
    CC

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