Asymetric Daggerboard or Symetric ????

Discussion in 'Hydrodynamics and Aerodynamics' started by Catsailor65, Nov 5, 2011.

  1. Catsailor65
    Joined: Nov 2011
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    Location: Germany

    Catsailor65 New Member

    Dear Boatdesign Members ,
    I just building a 65 feet Sailing Catamaran as a full Carbon construction .
    The designer suggest to use a Asymetric Daggerboard , which means a straight Daggerboard with a Profile to produce a force against abdrift from the wind when using it . The risk is the high load when used and bending itself and perhaps not known before vibrations ?? The advantage is the high wind angle in light winds and automatically force against drift .
    With symetric Daggerboards there will be no risk of vibrations and too much drag and the bending it self . Are there any experience or recomendations .
    Thank you in advance .
    Catsailor65
     
  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    As best I can tell both boards would be equally loaded-leeway is leeway.
    I'd go with the designers recommendations.

    Antrim: http://www.antrimdesign.com/trimarans/erin/
    Canted Asymmetrical Daggerboards

    The A30+ has a daggerboard mounted in each ama. Aside from opening the main hull interior to a phenomenal degree, this arrangement has several hydrodynamic benefits:
    ·Each board now only need operate on one tack; so an asymmetric foil can be used, optimized like an airplane wing for normal operating conditions. This makes a tremendous improvement in upwind efficiency - the boat points several degrees higher than it would with symmetric foils.
    ·
     
  3. Catsailor65
    Joined: Nov 2011
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    Catsailor65 New Member

    Hi Doug ,
    thank you for your kind answer .
    I am short before building them in Prepreg technology as we will have up to 4,5 tons bending force max and a lifting hydraulique cylinder in . All this is calculated also by my engineers . But still nobody guarantee for the result . So the risk is at my side , and if I will build symetric ones I am free of any problem , but also perhaps without the advantage to work against the drift when build the asymetric one in the right angle of attack . For this I ask for real experience form members out of this Board with High Performance Cats up to 70 feet .
    Thank you
    catsailor65
     
  4. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ==========================
    Why don't you contact Eric Sponberg? He is a naval architect and marine engineer and a member here. You can google Sponberg Yacht Design.
    You seem to think there will be a different load on the asymetric foils than on the symetric foils-can you explain why? Why do you think you are "free of any problem" with symetric foils?
    Good Luck!

    http://www.sponbergyachtdesign.com/
     
  5. sorenfdk
    Joined: Feb 2002
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    sorenfdk Yacht Designer

    Are you sure that he suggests? A design is not just a suggestion - it is based on the designers experience and knowledge about what will work and what will not. If the design comes from a reputable designer, then I suggest that you stick to his design.

    Where do you get this from? The load is ultimately depending on the stability of the cat, so there is no difference here. The stresses due to bending will also be more or less the same. And as far as I know, asymmetrical daggerboards are not more prone to vibrations than symmetrical ones (and symmetrical daggerboards are not free of any risk regarding vibration!)
     

  6. HJS
    Joined: Nov 2008
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    HJS Member

    Asymmetric centerboard?

    Catsailor

    How big is the benefit of asymmetric centerboard, really?. With my experience it is just theoretical, not possible to measure in the real world.

    It is in relation to how much you loose that the hulls are going straight or diagonal in the water. There are so many other things that come into play.

    Each centerboard must be dimensioned so that it can take all the load alone. If the boat lifts the windward hull so high that the centerboard comes out of the water, all load will be on the leeward centerboard.

    Moreover, in hard weather it may be appropriate to raise the leeward centerboard and just run with the windward centerboard to avoid that the boat will capsize over the leeward hull / centerboard. The boat will slipping away laterally instead when windward hull comes up too high.

    js
     
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