Hollow aft section for a daggerboard?

Discussion in 'Hydrodynamics and Aerodynamics' started by Luk, Mar 4, 2018.

  1. Luk
    Joined: Jan 2018
    Posts: 3
    Likes: 0, Points: 1
    Location: Germany

    Luk New Member

    Hi guys,

    I design a new Daggerboard for an International 14 skiff and i am not sure about the section i should use. Our current Daggerboard have a section which is close to the NACA 0012-34 (should be a copy of the 49er section) and make a pretty good job.

    But the most popular Section in the class is the E836 with a hollow aft. Xfoil (NCrit=1) also say that the E836 is the better choice. In contrast to that, Julian Bethwaite says he don t like hollow aft sections for Daggerboards and justified that with his 49er tests. #22 49 er C board section http://forums.sailinganarchy.com/index.php?/topic/144666-49-er-c-board-section/

    What is the philosophy behind the hollow aft and where are the problems? What are your opinions?
     
  2. tspeer
    Joined: Feb 2002
    Posts: 2,240
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    The hollow is a result of a a somewhat concave shape to the pressure distribution. The E836 pressure distribution has a flat "rooftop" followed by an increase in pressure to the trailing edge - the recovery region. The concave shape to the E836 pressure recovery is not very extreme, and the hollow in the contour not very deep.

    If you were to make the contour flat from, say, 70% chord aft, the effect will be to put dent in the pressure distribution at 70% chord because of the discontinuity in the curvature, and give a slightly convex shape to the pressure recovery, especially around 90% chord. The result will be a little earlier onset of stall, due to the more adverse pressures at the trailing edge. The effect on drag at low angles of attack will be negligible. On the whole, I doubt you'd be able to tell the difference, with or without the hollow. I wouldn't make the aft end convex, however.
     

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  3. Luk
    Joined: Jan 2018
    Posts: 3
    Likes: 0, Points: 1
    Location: Germany

    Luk New Member

    Thank you for the input.

    What do you think about this section? I maintain the aft end of the e836 but use a more convex rooftop. The numbers are pretty good.
     

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  4. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    If the numbers meet your requirements, then it's good. It has a pretty narrow range of angles of attack, though. You might want to soften the leading edge suction peak.
     
  5. Luk
    Joined: Jan 2018
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    Location: Germany

    Luk New Member

    If my calculations and experience do not fool me, the angle of attack will be around 2 degrees (upwind). But to be honest, i am not sure....

    Do you know a way to measure the yaw accurate?
     

  6. tspeer
    Joined: Feb 2002
    Posts: 2,240
    Likes: 212, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    No, I don't. A wise man once told me, "Analyze the loads you know, and design for the loads you don't know." The angle of attack may be 2 degrees when the boat is at speed upwind, but there will be times when the angle of attack will be greater than that, such as when the boat is in a roll tack or trying to accelerate. You don't want the board to turn into a pumpkin if the conditions are a little outside of what you estimated, so design for lower Reynolds numbers and higher angles of attack than the equilibrium conditions.
     
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