3 bladed propeller vs 4 bladed propeller

Discussion in 'Props' started by alby joy, Apr 18, 2019.

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  1. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Maybe a video with sound to hear both the motor and the vibrations of the hull could help diagnose? Over a range of RPM?
     
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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    It should be possible to attach a photo on your phone if you log in to the Forum on your phone?

    The only way you can change / reduce the natural frequency of hull vibration (if this is the cause - a big IF) is by changing the properties of the hull, perhaps by adding extra stiffening.
    What is the construction material of the vessel?
     
  3. bajansailor
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    bajansailor Marine Surveyor

    Alby sent me a photo - but it is only 5 kb in size, and it doesn't help us very much.

    Alby, can you take some more photos please, at higher resolution?

    Alby's boat 1.jpeg
     
  4. alby joy
    Joined: Jul 2017
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    alby joy Junior Member

    Boat hull is made of fibre reinforced plastic material
     
  5. kapnD
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    kapnD Senior Member

    Are you certain that the propellers and gear ratios are engineered correctly for the boat? (Cavitation)
    Is there a chance that propellers/shafts could have sustained damage that pushes them out of spec?
    Is the clearance between props and hull surfaces adequate?
    Is the hull strong enough above the props and struts to resist flexing?
    Is the hull faired well in front of the props?
    You really need to post up a LOT more information in order to get anything other than guesses as response!
     
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  6. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    The other, identical hull, is fine so it's not the design.
     
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  7. kapnD
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    kapnD Senior Member

    Blue Bell,
    The hulls May be identical, possibly even produced from the same mold, but there could still be anomalies due to layup schedule, materials, or even fairing problems in the mold that only show up on one side due to counter rotation of props.
    The fact that strut bolts are working loose points more towards mechanical type problems, but it could even be a motor running rough .
     
  8. alby joy
    Joined: Jul 2017
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    alby joy Junior Member

    Hi,

    I am adding more pictures to get a clearer idea of the system

    upload_2019-4-27_15-1-48.png
     
  9. alby joy
    Joined: Jul 2017
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    alby joy Junior Member

    The a-bracket inside boss we have a flanged bush which is tightened to a-bracket which is getting loosened. Hence request you to check the issue why these bolts are getting loosened.
     
  10. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I am not qualifed to answer, but qualified to ask dumb questions!

    Are the A brackets mounted on vibration dampening mounts?

    If the A brackets are loosening; your shaft is running out. Test it.
     
  11. alby joy
    Joined: Jul 2017
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    Location: india

    alby joy Junior Member

     
  12. alby joy
    Joined: Jul 2017
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    alby joy Junior Member

    I am telling it again it's not the a bracket mounting bolts but the flanged cutless bush is loosening from the boss of a bracket.
     
  13. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    This is what I'm hearing :p
     
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  14. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    It sounds to me like you have a failed cutlass bearing in the A type bracket, but like I said, I am not an expert. I am qualified to ask questions.

    Curious why you would have an isolated problem in the star hull and then assume both hulls have a problem and ask about a prop change? I think that led a bunch astray here.

    Have you rechecked engine alignment? Uncouple and recheck by someone who did not do the install. The engine may have dropped since install.

    Then, have you checked the bearings? I am assuming we are talking about a bearing in the skeg/A styled bracket. If so, what are the clearances?

    Is the boat eating cutlass bearings? How old is the vessel and hours since prior cutlass install?

    If the install is new, has anyone checked for a bent prop shaft?

    Sorry, but it sounds like a routine problem that you have driven here as a design issue. Kind regards.
     
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  15. johneck
    Joined: Nov 2011
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    johneck Senior Member

    This arrangement looks a bit questionable. How was the lateral vibration analysis done, particularly the stiffness of the propeller bearing? There may be a very basic vibration issue with the shaft that will only be cured by making a significant change. It is impossible to say for sure what change may be necessary without this analysis.
     
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