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Thread: Max RPM

  1. #1
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    Max RPM

    Fellas,

    So I finally got my 87 Donzi ZB21 running well and went for the first WOT test run. A little background: The engine (Merc 350, afb carb, edelbrock intake) was a rebuilt last year that suffered a bad intake gasket allowing water to get in. Caught it early, did about 4 oil changes and all new top end gaskets and now she's running great.

    So I took her out to some smooth water with myself and my wife in the boat and put the hammer down. With the trim up to a safe position I can only achieve about 4100 rpm. I believe the prop is a 23' clever and the original prop on the boat. Any ideas on the limited rpm? I think it might be running a little rich but otherwise runs great. My old merc 3.0's could muster 4700 rpm, but is this normal for this kind of engine combo? Outdrive is an alpha.

    Thanks,
    Matt
    87' ZB-21 "She Gone"

  2. #2
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    I don't know about the alpha but think the motor is ok with 44-4600 rpm Glad to hear it is running ok hopefully I will see you out there one of these days
    When the sky is grey,look out to sea.
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  3. #3
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    Too much pitch cut back to a 21 if you are only getting 4100 I would say 5000 WOT

  4. #4
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    Maybe I missed something--what'd it do before last year's incident? The power and drivetrain were the same then, right?
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

  5. #5
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    Last year it ran the same with the same power and drive. I'm going to double check the prop pitch. I would also think that motor should turn 4600 and maybe even top out at 5000 super trimmed. What are the advantages of a clever prop? Would a more typical Mirage prop be better? And does less pitch equal less speed or less planing power? Or Both? I've always had 3.0's with the stock Merc prop.

    Thanks,

  6. #6
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    If everything's good with the engine , then you've always been running over propped.

    A personal example . When I had my old Merc stuff I ran a 21'' prop and got perfect factory recommended top RPM
    One day I decided to invest in a 21'' Mirage to get a bit better performance. Well , I never really paid any attention to the tips that a 21'' Mirage performs more like a 23'' regular prop and voila !
    I lost about 800 RPM at wot. No to mention about 20mph too !

    So I went back to the standard 21'' prop . You have to get it propped for top factory RPM and an over sized prop just isn't going to do it .
    Just because something's old doesn't mean you throw it away !

  7. #7
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    +1 on what the other guys have said. I feel your pain on prop selection, sometimes it seems like black magic. You really want that SBC turning around 4800 light and no less than 4600 with the boat loaded for bear.
    "Speed's fine, but accuracy is final."
    - Bill Jordan

    1998 Donzi 21LXR
    1971 Boston Whaler 13

  8. #8
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    Matt
    some rules of thumb
    for every inch of pitch = 200 rpms
    for every inch of diameter = 200 rpms
    if the prop is cupped add an inch of pitch to the number
    in a perfect world/vacuum a 23 pitch prop should travel 23 inches in one revolution


    but the world is not perfect so each prop has slip a good prop has 10% slip or less so that means the prop really travels only 90% of the 23 inches or a little more than 20 inches


    so if your motor is 100% dropping to a 21 pitch should give you an increase in rpm of about 400 bringing you to 4500

    finding a prop that handles and performs is a task sometimes but choices are many for a merc

    you need to fill in the blanks on this prop Calc to see how you are doing

    http://www.rbbi.com/folders/prop/propcalc.htm


    this is just the tip of the black magic iceberg that is the world of propology
    rake cup stern lift bow lift
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
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  9. #9
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    Adding to what some others have said, pitch is basically a form of gearing. Less pitch = lower gear. If the boat labors out of the hole, less pitch could be part of the answer.

    Cleaver props I *think* are a fairly old technology that I don't seem to run into in the new models. Maybe they are still being made in large numbers, but I don't know of it. I *think* they are typically pretty good for top end, but slip a lot at low-mid RPMs and thus are inefficient on fuel for cruising. Perhaps terrible to useless for wakeboarding (AKA: trying to hold plane at 20 mph, where a 4-5 blade or duoprop would shine). BTW, this is speaking about the reputation of cleavers as I've heard it, not from direct experience with one.

    And those are all very broad, general statements. Specific real-world results can make your head spin. As was noted, welcome to the black art of propping.
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

  10. #10
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    cleavers are usually seen on setups that run near or at the surface. OBs that run high or surface type drives so you don't normally see them on a normal I/O setup now a days. the blades are very sharp and have very thin leading edges and very fat trailing edges like a meat cleaver the sharp end is thiner than the thicker back edge. making them cut into and grabbing aerated water. you may find that you are just not getting the prop high enough near the surface for it to find the type water it likes to run in. a mirage type prop may be better , bow lift is not a feature on most cleavers so that may be an issue too not getting the boat up and out of the water

    http://www.mercuryracing.com/propell...adecleaver.php

    http://www.mercuryracing.com/propellers/te-cleaver.php

    http://www.islanddesign.com/temp/JJC_8464.jpg


    http://www.mercuryracing.com/_media/...leaver_med.png
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

  11. #11
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    LOL The black art of propology. Oh boy. Well I think I'd like to look into something different just to try. Should I sat stainless or can I run aluminum? I wouldn't call my boat a big power boat at about 300hp but that might be in the stainless realm at that. With older boats we always used to like aluminum incase of a knick. Sacrifice the prop and not the outdrive, but with a stainless, the prop is the hardest thing in that equation. Perhaps I'll try going down a size and getting a cupped prop... however that sounds like it will just cancel itself out. lol

    http://www.islanddesign.com/temp/JJC_8464.jpg This image looks exactly like what I have. Nasty and sharp!

  12. #12
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    I'm thinking you could use a 19" with some cup maybe.
    "I don't have time to get into it, but he went through a lot." -Pulp Fiction

  13. #13
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    props

    Well I am interested..... I have 18 with a 350MPI,'96, with a mercury vengeance 48 16320A45 23P and barely get 4000rpm's with the alpha. Anyone got recommendations?

  14. #14
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    Quote Originally Posted by ted z View Post
    Well I am interested..... I have 18 with a 350MPI,'96, with a mercury vengeance 48 16320A45 23P and barely get 4000rpm's with the alpha. Anyone got recommendations?
    i'm a thinking something else is holding you back, that combo should have no problem spinning a 23 to at least 4500-4700
    Charter Member - WAFNC, SBBR, KWOSG
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  15. #15
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    props

    I don't know what...It had a 350 EFI throttle body engine in before and performed the same. The boat seems fine and gets a lot of regular use.

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