S/P prop on a diagonal shaft

Discussion in 'Surface Drives' started by Sassriverrat, Sep 23, 2017.

  1. Sassriverrat
    Joined: Sep 2017
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    Sassriverrat Junior Member

    Good Evening All,

    Although not by any means new to boating, I've not dealt much with surface piercing and was wondering if anyone could tell me what a prop might do when half immersed but on a diagonal, saying between 12 and 20 degrees. This is a permanent shaft and at all speeds where the propeller is able to ventilate.

    Thanks!
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You mean on a diagonal in the horizontal plane or the vertical ?
     
  3. Sassriverrat
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    Sassriverrat Junior Member

    To the horizontal- essentially could a surface piercing propeller run (well) on an inboard shaft inclination, assuming it is half immersed as designed, or roughly so?
     
  4. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi Sassriverrat,

    Forgive me, but your question has me very curious as to what hull form or application would take advantage of a surface piercing prop, but require the use of a shaft angle used by inboards?

    Thanks
    Dave
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    With no ability to change the inclination ?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Is it for a catamaran ?
     
  7. baeckmo
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    baeckmo Hydrodynamics

    It has been shown that a slight "dogwalking" ie a horizontal shaft inclination will give a very slight increase in forward thrust. You may also find a similar setup in inboard racing "proprunners". A Surface-piercing propeller does not have its thrust vector in the same direction as the shaft, and the trust vector is varying with propeller loading (advance coefficient).
     
  8. Sassriverrat
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    Sassriverrat Junior Member

    So the question/concept stems from trying to build this hydrofoil conversion with minimal monies and maintenance add-ons.

    So this being a monohull, she has traditional inboard propellers and after the foils are on, I'd like to put surface piercing props on the shafts instead of the old wheels, but I didn't know how surface piercing handled being inclined at heavy angles, such as maybe 12 degrees below the horizontal.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like an enormously ambitious scheme to me. Any diagrams or plans of how this will look ?
     
  10. Sassriverrat
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    Sassriverrat Junior Member

    I do have some primitive drawings but my cad abilities/experience is quite limited. I guess I need to find someone that may be able to help clean up the cad drawing I have and show me how to add stuff like decks....

    But yes. Soon
     
  11. Sassriverrat
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    Sassriverrat Junior Member

    So then do we see issues mounting the propeller at a significant angle to the horizontal?
     
  12. baeckmo
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    baeckmo Hydrodynamics

    So, you are talking about a propeller with a vertical inclination then, I took it for a question on horizontal; my mistake (although the info I gave is correct per se). Provided the propeller rake is correct, a SP prop can work fine with vertical shaft angles up to 15 degrees (even above that).
     
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  13. Sassriverrat
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    Sassriverrat Junior Member

    Little confused by your vertical piece-

    I'm intending for a shaft 15 degrees below the horizontal, the same as mounting such a propeller on a traditional, fixed inboard shaft.
     
  14. baeckmo
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    baeckmo Hydrodynamics

    In your answer to Mr Efficiency (posts no 2 and 3) you state "horizontal", referring to Mr E's question. You must understand the difference between the planes we are using in marine engineering. A horizontal plane is a plane parallell to the sea surface and a vertical plane is parallell to a wall. The angles we talk about lie in the respective plane. So, when you are talking about a "diagonal" shaft with "15 degrees below the horizontal", you actually mean a shaft with a declination in the vertical plane, not a diagonal in the horizontal plane.
     

  15. Sassriverrat
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    Sassriverrat Junior Member

    Ah I understand what you're saying. Then yes, we are saying the same thing.
     
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