Bold claim for new propellors

Discussion in 'Outboards' started by Mr Efficiency, Nov 7, 2013.

  1. michael pierzga
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    michael pierzga Senior Member

    When i click the boat into forward the stern squats. The prop pull the stern down. This is with an 800 mm maxprop at conventional inboard shaft angle
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Mercury are claiming their proprietary stainless alloy as being a part of the improvement, presumably through thinner blade sections.
     
  3. TANSL
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    TANSL Senior Member

    Michael, I guess what you describe is what happens to all boats, only because the propeller is below the waterplane, regardless of its type.
    The theme of the propellers, let's call them "active" I find it amazing and very interesting. So I am asking someone explain to me that phenomenon that makes them "active." Several of you bear for granted that these propellers exist, and I believe it. Just ask an explanation.
    Regarding to thin or thick blades, also depends on the stiffness of the material. A blade very fine, if too soft, will be very inefficient.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    OB and SD propellers operate below a free surface, not under a hull. This will cause an asymmetry in the flow accelerated into the propeller. This is producing an asymmetrical pressure loading of the propeller disc. So, when the blades are inclined ("rake") forward or backward, there is a change in vertical force, since the lower half of the disc will carry a higher share of the load.

    This effect is less obvious under a planing hull, but there, the local static pressure is changed, leading to attitude variations, although to a lesser extent.
     
  5. TANSL
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    TANSL Senior Member

    I guess you mean that the force exerted by the blade has a vertical component, up or down, depending on the inclination of the blade ("raking"). Is it right?. If so, I understand now how these propellers work. In any case, the vertical component may not be very large, however, thrust is lost in the direction of advance. Is it worth these propellers?. Would not be more practical to place flaps at the stern?
     
  6. PAR
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    PAR Yacht Designer/Builder

    Wow and you've designed boats? Say you're bouncing along at 70 - 80 MPH, what amount of drag do you thing a 1/4" deflection of "flaps" at this speed might impart?
     
  7. TANSL
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    TANSL Senior Member

    PAR, the first thing to be clear, and you do not repeat it again, I've never said I have designed pleasure boats and yachts. Instead of doubting my technical ability (do not have any objective evidence to judge), why do not you explain, with facts and figures, what is the problem?. I beg you to support your opinions on facts.
    A ship sailing at 70-80 MPH has some problems, and solutions, that 99% of the ships do not have. (I guess at that speed the flaps are placed in neutral position. I donĀ“t know). Why put flaps in normal boats, is crazy?.
    I have been able to understand explanations of baeckmo. Why do not you try to explain what you say?. I have no obligation to believe what you say just because you say it.
    I'm here for fun and to learn and, may be, to help sometimes. Thanks for your patience and understanding.
     
  8. FMS
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    FMS Senior Member

    I'm not aware of any commercially available stern flap that lifts the bow.
    A prop change can be done --without adding new systems, maintenance, or labor -- for $300 to $700.
     
  9. TANSL
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    TANSL Senior Member

    You're right FMS, the flaps can not lift the bow. Where have I write that?
     
  10. daiquiri
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    daiquiri Engineering and Design

    In theory it could be possible, by using upward-curved flaps and the coanda effect. Don't know if there are actual models which work that way
     
  11. TANSL
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    TANSL Senior Member

    daiquiri, very, very interesting. I knew nothing of the Coanda effect. Now I understand why the Red Bull F1 had so much ahead of the other racers. I'll seek information and study the subject. Anyway, I'm afraid that I can not apply it to any of my boats designed for 86.7 mph, cruise speed.
     
  12. powerabout
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    powerabout Senior Member

    coanda effect works at very low speed as well
    its an issue for prop blast/thrust and your hull on a displacement boat
     
  13. TANSL
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    TANSL Senior Member

  14. powerabout
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    powerabout Senior Member

    sleds also can have prop coanda effect if the prop is under the hull
     

  15. TANSL
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    TANSL Senior Member

    I'm sorry, I meant, pleasure boats. I designed work boats, fishing boats, cargo or passengers, of lengths between 7 and 30 m. Just that.
    powerabout, interesting to know all calanda effect's applications, but are you going to relate them one by one?
     
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