Keiper's Williwaw bow foils

Discussion in 'Hydrodynamics and Aerodynamics' started by revintage, Dec 2, 2020.

  1. revintage
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    revintage Senior Member

    Got a little curious when I read about Williwaw´s bullet fairings.

    tspeer: "As for foil joints, one might consider some sort of fairing to act as a structural fillet. Dave Keiper found bullet fairings to be very effective at eliminating ventilation on Williwaw's bow foil, which probably indicates they were eliminating flow separation there."

    Anyone who has pics or drawings of Keiper´s foil joints?
     
  2. upchurchmr
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    upchurchmr Senior Member

  3. revintage
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    revintage Senior Member

    I recently visited that site. Unfortunately no useful info there:(. Will have to use the old ”learning by doing”-method at the V-foil joint.
     
  4. tspeer
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    tspeer Senior Member

    There are pictures in his book, Hydrofoil Voyager. He called them torpedo fairings. They were basically circular cylinders with rounded noses and tapered tails. They had a constant cross section across the chord of the foils.

    A better example would be the fairings Paul Bieker put on the rudder foils of the AC72 that won the America's Cup.
    [​IMG] [​IMG]
    The basic idea is you don't want everything to get fat at the same place. So the fairing itself should have a dumbbell-like cross sectional area distribution. The concave region in the middle will have a low velocity/high pressure and the convex areas toward the ends will have a high velocity/low pressure. The foil and strut will have a high pressure at the leading edge due to the stagnation line, a low pressure in the middle due to the thickness, and the pressure will increase to near ambient pressure at the trailing edge. By superimposing the opposite trends from the fairing on the strut and foil, the maximum velocity at the junction is reduced and the pressure gradients are extended and smoothed out.

    Boeing used a similar approach on the fairings of the Jetfoil foils. See attached paper.

    You can also stagger the strut and foil to avoid directly superimposing the high velocity regions on each with the other.
     

    Attached Files:

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  5. revintage
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    revintage Senior Member

    Thanks Tom,

    This was what I was after: "They had a constant cross section across the chord of the foils."

    We discussed the other stuff deeply in my thread about "Coke bottle fairings" a while ago. Your remarks in the thread about Williwaw made me curious.

    Found the book on Amazon UK for a few pounds so I ordered it.
     
  6. revintage
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    revintage Senior Member

    Will begin with milling and then turning the 6082-T6 connecting piece to a constant cross section. Then bolt on the 52.5 dihedral V-foils, that are cut perpendicular in both ends. In the next stage there should not be a problem to mill the piece slimmer both forwards and rearwards. The rest of the fairing can then be built up with epoxy filler. Except for Biekers fairing the Basiliscus paper gives some clues.

    svarvtest.png basi.png bieker.png
     
  7. tspeer
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    tspeer Senior Member

    I think you're focusing on the wrong surface. It's the sharp V at the upper junction that is the problem.
     
  8. revintage
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    revintage Senior Member

    Tspeer, you probably did not read the whole post, it says "The rest of the fairing can then be built up with epoxy filler." (The lower part can be milled but not the upper.)

    Have realized this solution will be to labor intensive, also with unknown outcome. Instead looking at another of your suggestions, from another thread, cutting off the bottom of the V.

    Quoting you: " Perhaps the simplest option at this point would be to cut off the bottom of the V and add a flat piece connecting them together, changing the V to a trapezoidal U. You would lose a little of the depth, but it's probably not a part of the foil that is really working well for you at present. The junctions between the added piece and the foil panels would be an obtuse angle, with much less interference."

    The flat part has the same 20mm section as the foils. Have calculated I will only lose ca 2cm depth and I can live with that. Could the flat be cut shorter, it would be great?


    flatV.png
     
  9. tspeer
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    tspeer Senior Member

    I like the looks of that much better. To optimize the length of the flat portion you'd have to actually do some analysis with something like a panel code at a minimum.
     
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  10. revintage
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    revintage Senior Member

    Will have to have ca 50mm inner width for building reasons. The sketch shows a rather large inner radius at the joints and should maybe be kept to a minimum. The outside should probably be more radiused, but this will be hard to achieve due to the foils inner structure. What do you think?

    plattbotten.png
     
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