A Cat Foils

Discussion in 'Multihulls' started by L. Hutchins, Aug 25, 2006.

  1. L. Hutchins
    Joined: Aug 2006
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    L. Hutchins Junior Member

    Hello All.

    I recently aquired an A Cat which is in need of new foils (all 4). I see that newer boats are using foils that are canted and have very large aspect ratios. Some are canted inboard to lift the boat up and some are canted out to make them vertical while the boat is flying a hull. Because I have holes in my hulls which are larger than the new boards I could make inserts to allow both of these conditions. What I realy need to know is what kind of NACA foil would be good on the rudders and dagger boards and what kind of chord/depth is a good choice.

    Leigh
     
  2. tspeer
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    tspeer Senior Member

    A NACA 4-digit section in the 8% - 12% thick range would be good. A laminar flow section like the 6-series doesn't have a wide enough drag bucket to be of use to you, and there's evidence that the laminar flow might not be there in water, either.

    I would make the foil as deep as you possibly can. It's depth, not aspect ratio, that minimizes the induced drag due to lift.

    For chord, that's a tradeoff between wetted area and stalling. I'd start with the same or maybe a little bigger area than the existing boat. The extra area will make it more forgiving in the tacks, which might be helpful as you learn to get the most out of the boat.

    Good idea swapping the inserts between the hulls to experiment with both outboard cant and inboard cant. You might taper the thickness of the top of the boards to get even more cant out of the same insert.
     
  3. tspeer
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    tspeer Senior Member

    Take a look at what Martin Schoon has to say about sizing daggerboards.

    A good indicator of the potential lift/drag ratio is wetted aspect ratio.

    ARwet = depth^2 / total wetted area

    Total wetted area includes the hull, boards, and rudders.
     
  4. Richard Hillsid
    Joined: Mar 2006
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    Richard Hillsid Senior Member

    Thanks for the link, now i have to build a new dagger board for my kidos optimist, lets see how extreme it evolves :)
     
  5. L. Hutchins
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    L. Hutchins Junior Member

    The Plan

    Thanks for the help.

    So far the plan is to build a NACA 0012 foil out of a laminated cedar blank which will be vacuum bagged in carbon with a built up carbon skin on the high point of both sides. I am planning on a 6" chord with 40" projecting below the hull.

    Any advice on the thickness of the lay-up on the sides? I was going to taper the amount of carbon with maximum thickness at the exit from the hull.

    How about the rudders. Is a 0012 good there as well or should I try something else? I was going to rake my rig back and load the rudders to try and increase efficiency and balance my helm by raking the rudders forward a little.

    Thoughts?

    Leigh
     
  6. Rolf Nilsen
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    Rolf Nilsen Junior Member

    Leigh,

    are asymmetric boards allowed in the A-class and did you consider using an asymmetrical section? The theorethical side looks advantageous, and the C-class use them, but there not many others.
     
  7. L. Hutchins
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    L. Hutchins Junior Member

    Yes I have considered an asymmetric section. I was even pondering a flap which would sweep the bottom of the hull when the boards were fully extended. Problem is with only one crew it will be difficult to raise and lower the boards effectively and the net result could be quite clumsy.

    I think I will start out symmetrical and move forward from there.

    Have you looked at Sailing Anarchy lately? A report from a new C class boat was just posted. I have met the owner. Tthey are very near by in Toronto. Looks quite fancy! I have heard cost is around $250,000.

    Leigh
     

  8. Rolf Nilsen
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    Rolf Nilsen Junior Member

    Handling is probably the worst issue on a singlehander, and especially so on a boat like the A which have only one sail and manouver quickly.
    I am considering putting asymmetric daggers on an F-16 platform. Plan to angle the off the centerline of the hulls by 3-4 degrees so the windward dagger is easy to raise/lower by a line available from the trapeeze. Theorethical gains are large, but there seems to be very little practical experience, except from the C-class that is, and I dont know how it would scale down..

    The "Alpha" is a beauty, and if it really is so much lighter than Cogito I guess Steve Clark is scratching his head.. (unless the Alpha break up catastrophically, that is)
     
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