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  1. #1

    Default Run DuoProp Rig with Only 1 Propeller

    I have a 1998 Volvo Penta 5.0 Gi (PBYCCE) engine with a DuoProp (DP-SM) outdrive in a Monterey 242 mini-cruiser. Although VP specs say it should get up to 4800 RPM, it maxes out at about 3700 and top speed of about 40MPH. Its running with F7 propellers, and I've been advised to try a smaller prop (F6 or F5). One friend suggested I test by running without the back prop at all and see if RPM improves. Another lent me an F5 back prop, to try with the F7 front prop.

    Are either of these ideas worth trying? Or could doing so have negative long-term impact on the outdrive?

  2. #2
    Join Date
    Mar 2009
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    Portland, Oregon,
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    Default Re: Run DuoProp Rig with Only 1 Propeller

    Not recommended!
    Run both matched props, and perhaps drop down a size or two.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  3. #3

    Lightbulb Re: Run DuoProp Rig with Only 1 Propeller

    Or could doing so have negative long-term impact on the outdrive?
    I've learned the hard way that using equipment as it's designed to be used really reduces the need to repair unintended damage from not doing so. Conventional wisdom says put matched F4s on and see what the RPMs do ...

  4. #4
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    Default Re: Run DuoProp Rig with Only 1 Propeller

    To further this discussion:

    Quote Originally Posted by Solarity View Post
    Another lent me an F5 back prop, to try with the F7 front prop.

    The Duo Prop propellers are designated FWD and AFT.
    The larger diameter and less aggressive propeller is the FWD prop.... the prop closest to the transom.
    The smaller diameter and more aggressive propeller is the AFT prop..... the prop furthest from the transom.



    Are either of these ideas worth trying? Or could doing so have negative long-term impact on the outdrive?

    The Volvo Penta engineers designed the Duo Prop unit with several things in mind..... the foremost being increased hydro propulsion.

    With the Volvo Penta DP, you'll notice two different diameter and two different pitch props being used.
    This is what gives the Volvo Penta Duo Prop an advantage over that of the BIII, for example.

    Inside of the lower gear unit you'll see one "drive" gear and two "driven" gears.
    Each of the two "driven" gears must share an equal torque load.
    By design, the unique and matched Volvo Penta DP propellers offer this!

    If you were to run an F5 (in either an AFT or FWD prop) paired with an F7 (in either an AFT or FWD prop), you'll not only upset the propulsion balance, you will upset the gear load.

    Last edited by RicardoMarine; 10-18-2020 at 09:54 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  5. #5
    Join Date
    Sep 2010
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    20,493

    Default Re: Run DuoProp Rig with Only 1 Propeller

    Did it run to 4800 rpm at one time ?----Now only reaches 3700 ?

  6. #6
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    Mar 2009
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    Default Re: Run DuoProp Rig with Only 1 Propeller

    Another suggestion would be to verify the accuracy of your tachometer!
    If the tach is off, it may explain why your WOT RPM appears to be so low, when it may not be.




    .
    Last edited by RicardoMarine; 10-18-2020 at 10:48 AM.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  7. #7
    Join Date
    Jun 2008
    Location
    Island Heights, NJ,
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    3,490

    Default Re: Run DuoProp Rig with Only 1 Propeller

    .... and if you try to run with only one prop, it likely won't move the boat at all.... Think a car's non posi differential/tires with one wheel on the ice.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  8. #8
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    Sep 2010
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    Default Re: Run DuoProp Rig with Only 1 Propeller

    It will move the boat !----Just not very well.

  9. #9
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    Default Re: Run DuoProp Rig with Only 1 Propeller

    Quote Originally Posted by sandkicker View Post
    ........ Think a car's non posi differential/tires with one wheel on the ice.
    Bob, please give me a few minutes to ponder your thought!


    Attachment 25279





    OK..... I now get it!



    .
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

  10. #10

    Default Re: Run DuoProp Rig with Only 1 Propeller

    Did it [ever] run to 4800 rpm? YES (kind-of: 4500), in fresh water.
    I am currently running in salt water, and its higher density may be the problem.
    Before switching to a smaller prop, I plan to test-out the current rig in a local lake.
    Last edited by Solarity; Yesterday at 01:35 AM.

  11. #11
    Join Date
    Mar 2009
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    Default Re: Run DuoProp Rig with Only 1 Propeller

    Quote Originally Posted by Solarity View Post
    Did it [ever] run to 4800 rpm? YES (kind-of: 4500), in fresh water.
    I am currently running in salt water, and its higher density may be the problem.
    Before switching to a smaller prop, I plan to test-out the current rig in a local lake.
    FYI:
    If your engine cannot reach the OEM specified WOT RPM, it will be over-burdened throughout the entire RPM range.
    This may damage your engine!

    I would encourage you to:

    .... look for marine growth on the hull bottom.
    .... look for propeller damage.
    .... look for anything else that may be holding it back.
    .... drop down in prop size and test.
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

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