Choice of folding propeller for 29-footer

Discussion in 'Props' started by Runhammar, Mar 23, 2023.

  1. Runhammar
    Joined: Jul 2020
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    Location: Stockholm

    Runhammar Junior Member

    Hello
    I have a 3 metric tonne 29 ft sailboat with fin keel, quite easilly driven.
    I am installing a 10hp Volvo Penta in it (I know, weak, but I have it...) and can choose between two folding propellers (well, theoretically I can choose between more but I have these close).
    One is a 16'-11', one is a 13'-9', both right handed.
    The larger one is called flex-o-fold, the smaller is a Gori.
    Just off the top of your heads, which one is most likely to be the better for me?
    Please speculate freely, noone will be held responsible. Just place your bet. I am learning.
    Kindly
    Per
     
  2. Runhammar
    Joined: Jul 2020
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    Location: Stockholm

    Runhammar Junior Member

    Oh, thats right:
    Max rpm 3600, idling speed 925.
    /per
     
  3. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    None of those look like a good option according to the charts in the propellor handbook.

    10hp @ 3600 = 7in
    @3000 = 8in
    @2500 = 9in
    @2000 =10in

    I have over propped a boat to get hull speed at lower revs, but the engine will then not pull full revs. The reccomendation always seems to be for the engine to pull full revs and hence power.....you only have 10hp to begin with.
    I have a Bruntons auto prop of 13in, that was designed to run with a Yanmar 10hp. Pricey, but very effective.

    Are those props you refer to, two or three blades?
     
  4. Runhammar
    Joined: Jul 2020
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    Location: Stockholm

    Runhammar Junior Member

    They are two blades.
    But if I understand you correctly, the smaller one, 13' width, 9' pitch, should be nearer the mark...
     
  5. skaraborgcraft
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    skaraborgcraft Senior Member

    Yes. Not optimum, but will certainly work.
     
  6. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    The numbers i gave where for your engine rpm. You did not say what your gearbox ratio was. Its possible you can pull the larger prop given a low enough ratio to shaft speed.
     
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  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I agree. You can't calculate pitch without the gear ratio.
     
  8. Runhammar
    Joined: Jul 2020
    Posts: 33
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    Location: Stockholm

    Runhammar Junior Member

    Hi,
    sorry for being silent. I lost my password...
    The gear box ratio is 2,35, which would give me rpm 1500 roughly. But that sounds very little. Am I thinking the wrong way? I devided 3600 by 2,35. This rpm would put me in a whole other category and I should have a pitch of more than 10 inches...
    Skaraborgcraft: this propeller handbook that you refer to, where can I get it?
    Kindly
    Per Runhammar
     
  9. gonzo
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    gonzo Senior Member


  10. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    The 1500RPM curve at 10hp shows a 13in diameter.
    1000RPM shows a 16in diameter.
    These are shaft RPMs.
     
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