Foils for sailing boats

Discussion in 'Hydrodynamics and Aerodynamics' started by TANSL, Oct 22, 2017.

  1. TANSL
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    TANSL Senior Member

  2. CT249
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    CT249 Senior Member

    It's not the biker, but the query (and that's all it was, not a statement) about the hardware. If a TT bike can generate lift at an angle of attack, could a cat hull? The aspect ratio is different, but is it known that the cat hull is just windage??
     
  3. sigurd
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    sigurd Pompuous Pangolin

    No indeed CT, there will be lift. But won't it be difficult to create a hull shape that is a good low drag (sideways) lifting shape at 18-30 degrees angle of attack?
     
  4. sigurd
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    sigurd Pompuous Pangolin

    Maybe this boat is helping your argument... It's made to crab to leeward...
    2548Crab_Foiler_ThreeQuarter.jpg
     
  5. markdrela
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    markdrela Senior Member

    Here you need to distinguish between drag (defined along the relative wind), and resistance (defined along the path of travel). When a crosswind is added, a TT bike will surely have increased drag, but lowered resistance. The reason is that the crosswind generates lift, which is normal to the relative wind and therefore points partly forward and provides some negative resistance. The cyclist/bike is a sail and the wheels are the keel.
     
  6. TANSL
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    TANSL Senior Member

    When the ship has taken off, would a scenario like the one described in the figure be possible: the hull oriented in the direction of the apparent wind to offer less resistance to the wind?
    Snap36.jpg
    Another question, why do America's Cup boats in 2017 adopt negative trim when they fly?
    Thank you very much for your answers.
     
  7. sigurd
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    sigurd Pompuous Pangolin

    Tansl, yes that diagram shows what I ment.

    "Another question, why do America's Cup boats in 2017 adopt negative trim when they fly?"

    They can't adjust incidence or flaps on the main foils (?), so they have to set incidence so that they are able to take off (positive incidence, relative to hulls in displacement mode). So when they fly, they have to trim nose down to reduce lift at these higher speeds. That's my theory anyway, haven't studied the boats.
     
  8. TANSL
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    TANSL Senior Member

    But how can they get it?, rotating the foils slightly, I suppose.
     
  9. tspeer
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    tspeer Senior Member

    Cavitation is avoided by designing foil sections whose minimum pressure/maximum velocity does not exceed the cavitation threshold. Sections designed this way have "rooftop" pressure distributions that are flat at just under the cavitation threshold at the maximum speed condition. This also puts a limit on the thickness ratio.

    The structural designers typically have a different idea of what they would like the thickness to be, so the section shape, foil area, physical thickness, and intended cavitation speed need to be negotiated between the Design Team and the sailors.
     
  10. tspeer
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    tspeer Senior Member

    The bow-down attitude was largely a result of the Design Rule. The Rule allowed changing the incidence of both the dagger foils and the rudder foils, so you'd think the boats could be trimmed to fly level. And they could. However, the trim range of the rudder foils was quite restricted in order to limit the teams from doing what, of course, they did - which was to use differential rudder incidence to increase the righting moment of the boat. When the rudder foils were at the maximum differential deflection allowed by the Rule, they were essentially fixed. The collective rudder incidence was set so the rudder foils produced the right amount of lift at takeoff, which meant the boat had to adopt a bow-down attitude in order for the rudder foils to have the right angle of attack at high speed.
     

  11. TANSL
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    TANSL Senior Member

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