lateral resistance in amas and vakas

Discussion in 'Multihulls' started by Anatol, May 28, 2015.

  1. Anatol
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    Anatol Senior Member

    Assuming an atlantic proa - I'm trying to understand the design issues regarding which hull should have low lateral resistance and which should have high.

    I think a 'conventional' approach would be to have major lateral resistance in the main hull, in the form of deep foils, and a skidding round bottomed ama, to minimise lateral resistance, so the ama would turn around an axis in the vaka.

    What if the ama was a deep V foilish hull, and the vaka was flat or curved bottomed?

    What if the (Bruceish?) foils were on the ama? The Vaka would turn around an axis in or near the ama.

    What if both hull are deep V-ish? People say Wharram cats are a bear to steer, but a proa doesn't go about, thus straight tracking would be more of a virtue.

    Just trying to undertstand the hydrodynamic design issues :)
     
  2. catsketcher
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    catsketcher Senior Member

    Gday Anatol

    If you read Project Cheers you will see that Dick Newick originally thought of putting an assymetric shape into her hull lines. If you look at some historical proas again you will see assymetry.

    I think assymetry does not make sense unless you really need to not have centreboards - a la Hobie 16/14. In other circumstances you get more lift/drag with a symmetrical hull and efficient boards.

    Unless you want to play in the shallows I would go boards and symmetrical.

    cheers

    Phil
     
  3. Petros
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    Petros Senior Member

    I have built a number of small trimarans that had outriggers specifically designed for lateral resistance. they were very narrow deep V hulls with fairly sharp leading and trailing edges. It worked surprisingly well, but a foil shaped dagger board would have been more efficient (less drag). The amas provided both floatation/stability and acted like a center board ("off-center" board?). it was simple, required no additional parts or a dagger board box (simplifying the hull construction considerably), and worked reasonably well. the amas had a rounded profile so it could be beached.

    If you are looking for best performance, than a high aspect ratio dagger board is required. If you just want to build a fun day sailor than the simplicity of using the hull or amas instead of a keel or center board is viable.
     
  4. Anatol
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    Anatol Senior Member

    Phil and Petros,
    thankyou for your replies.

    Phil - I know about asymmetrical hulls and have abandoned the idea (for now :)

    Petros - My concern about two lateral resisting hulls was, in part, the forces exerted on the cross beams.

    > a foil shaped dagger board would have been more efficient (less drag)

    clearly, with added complexity, need for kick-up mechanisms, etc.

    >If you are looking for best performance, than a high aspect ratio dagger board is required.

    Again, I'm looking for some sense of scale here. Guestimates - are we talking 2%, 5%. 10% or more difference. And where is the difference - max speed, lateral resistance?

    > The amas provided both floatation/stability and acted like a center board ("off-center" board?). it was simple, required no additional parts or a dagger board box (simplifying the hull construction considerably), and worked reasonably well.

    right, this was my gut feeling.

    >the amas had a rounded profile so it could be beached.

    wait! I thought they were deep V? Small radius I assume?

    One Ama design I've been kicking about is a square section, rotated 45 deg. Built of ply, with all panels tapering towards bows - a kind of squared off torpedo shape. This is easy to build, gives bouyancy at bows, is automatically water shedding, has less wetted surface that a deep V and offers some lateral resistance. Any thoughts?

    And of course, the 45 deg lee side would permit easy external Bruce foils if required. (Pivoted like leeboards, with a dowel fuse?) But this would give the ama more lateral resistance than the Vaka. So back to my original question - how would this effect steering? - Assuming rudders were on vaka.

    I'd also considered putting the rudders on the ama. Any thoughts on dedicating the ama to steering and lateral stability, with, say a skiddy flat bottomed vaka with the sails on it?

    thanks again!
     

  5. Petros
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    Petros Senior Member

    cross section is deep V, about 6" at top, and about 2 ft deep coming almost to a point at the bottom (about 1/2" radius). the profile or side view is flat on top, and the lower part is a half of an ellipse. Kind of kidney bean shape but with a flat top. it forms a low aspect ratio elliptical lifting surface. all lateral loads were taken by the amas out on the end of a single 2x6 beam about 10 ft long (5 ft either side of the center hull center line), center hull was about 18" wide and 18' long. it made a very stable little tri, two person, very controllable and simple to use.

    it was built for a boat building contest, won the race part of the contest by a long way, came in 2nd in total score that included time to build (that took longer than most other entries because we built a better boat).
     
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