Bridgedeck centreboard why don't they work???

Discussion in 'Multihulls' started by valery gaulin, Jan 10, 2017.

  1. UpOnStands
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    UpOnStands Senior Member

    :idea: excellent image. here is one "possible" explanation using two lateen sails
    sorry if the proportions are off somewhat. The hull is upside down and we run a water line at 30 degrees to the boats keel (water flow is left to right)
    the final image shows the aerofoil as the water sees the hull
     

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  2. brian eiland
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    brian eiland Senior Member

  3. brian eiland
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    brian eiland Senior Member

    No, I think our aircraft guy Pogo got it correct when he made this reference:
    https://en.m.wikipedia.org/wiki/Lifting_body

    ...from that same source
    OrbitalSpacePlaneInOrbit600x597-cropped.jpg
     
  4. redreuben
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    redreuben redreuben

    FFS.
    Now I'm totally confused.
    So is said delta/dart foil shaped or a flat plate ?
    Or is Delta foil shaped and dart not ?
     
  5. UpOnStands
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    UpOnStands Senior Member

    popcorn time.
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    "The canard board of the big Dazcat seems to have additional tasks, especially together with a delta keel and a dart keel.
    I'll try to figure them out.

    pogo"

    The answer is evident. Like on the old Cheers, to put the center of "leeway" forward for upwind sailing, specially at low speeds. And to get a better angle upwind. Without the front board, the bow, at each wave, will "slide" laterally, as the boat will pivot around the dart keel, thus opening the angle. It permits also to reduce the drag of the rudder blade, which won't have to correct the "lateral" sliding.
    The other task is to keep the desired repartition of volumes which are for speed "downwind", so the front board is most used for a close upwind although it could be used to equilibrate the cata for long runs. It's a bit equivalent of the canard plan on a Piaggo, or a Rutan EZ which used a hyperlaminar canard, an apex and a part lifting fuselage in conjunction with the main wing. The rear propeller vortex on the EZ acts as the aft rudder on the Piaggo or on the Dazcat.
    As the levers are pretty big you need only small surfaces with only a small incidence (angle of attack) on the rudder, so the final sum of the induced drag is lower. You can even use a front daggerboard with a auto correction incidence feature, fixed or variable, like on several dinghies. It can be used also as a turbulator for the dart keel.
    Many times a turbulent flow, when the turbulences are well organized, gets better results than a laminar flow. For example the shark teeth or dog teeth shaped turbulators user on some laminar wings. Often a simple string glued at the good place on the extrados, or a zigzag tape can make miracles on a badly behaving laminar profile.

    The picture by Dazcat I've joined is very clear on the subject as it's a very detailed drawing with tons of information.

    Briand. Yes a catamaran hull can be designed as a vertical lifting body, the density of the water permits that, as the behavior of the water flow is similar in some way (but not identical) to a the flow of a high speed lifting body. There are very hard constraints, the prismatic coefficient, its place, and damping done by the stern and bows, and the fact that a hull is always moving up and down, add the orbital motion of the water on the waves and you need tons of aspirin for the headache. That explain why so few NA approach this kind of design. The fast racers use a different approach as the goals are different, the comfort of the crew is the last concern so simplicity, slamming, spray, yawn and short period rolling at the mooring are not in the racer SOR.
     

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  7. pogo
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    pogo ingenious dilletante

    Yepp, it' s evident and we knew already about these features for upwind courses and forward apparent wind.

    The other feature ,
    in my words
    i was examining the additional feature during a surf and downwind courses for avoiding broaching.
    Under this condition the canard board seems to act like a canard rudder, ideally with 0 degrees for holding the boats' course.
    So , downwind with an angle of attack of zero degrees the canard board is a front stabilizer, holding the lifting body at zero degrees , even when the stern is hit by a breaking wave ( yaw effect).

    What happens when a dart keel is surfing down a wave --say 70* --and the stern is hit by a wavecrest ?
    Yes , the stern will be pushed leewards and boat , still surfing, begins to decribe a curve.
    In a curve the angle of attack for the dart keel is defined via the angle of the tangente and the ( turning)circle. With other words , the curve generates an angle of attack, it generates lift , unwanted windward lift.
    This lift makes the turning circle smaller and smaller, the angle of attack bigger. The boat begins to descibe a spiral. A curve becoming narower and narrower ---towards the wavecrest ! This is exatly what don't want at all.
    Of course one can avoid the beginning curve with the normal rudder , but , often the rudder is in the wavecrest with all it's bubbles, it couldbventilate till being useless.
    So, a canard board ( stationary rudder) , this "front stabilizer" makes sense-- it avoids the boat describing a curve.




    from Dazcat website page on the 1495
    WA combination of optional dagger boards and dart shaped keels are optimised to create exceptional upwind performance and highly controllable downwind performance, as the keel shapes are designed to reduce the yaw effect that can be experienced in larger seas sailing downwind."http://www.dazcat.com/d1495.html


    pogo
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    No. There is the interaction with the hull, a primordial fact. The approach must be global, a dart or delta keel is not a simple adjonction like a daggerboard, or you'll get a simple skeg like on a motor boat.
     
  9. brian eiland
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    brian eiland Senior Member


    I've confused myself,...and may have confused this whole discussion on the Dazcat when I posted this image of a Dazcat with some sort of canard foil up front. I don't think this ever existed in reality, and I'm not sure what it is suppose to represent. I seriously doubt anyone would be considering such a thing for a production catamaran.

    We really need to look more intently as to what this dwg is suppose to represent. :?:
    [​IMG]
     
  10. Ilan Voyager
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    Ilan Voyager Senior Member

    You're right. I did not mentioned it on my post.
    Pompously worded for a symposium paper that would be "Three surfaces stabilization, phugoid and damping optimization on sail boats" After one page of variables, Aa, Aab, Aa_im and so on, you throw fifteen pages of mysterious, even for a mathematician, differential equations. You conclude that the phugoid varies by the size of the waves, the speed of the wind, the speed of the boat, the age of the captain and that the damping of the perturbation is about 3 cycles with the help of God and the Holy Ghost. For sure you forget to give the units. You got a publication on your records.

    The advantage of a daggerboard in that case is that you can tune the surface size to get the best result. So you have a fixed surface (the dart), a variable size surface incidence 0 ((the front daggerboard) and a variable incidence surface (the rudder).
     
  11. brian eiland
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    brian eiland Senior Member

    Hissy Fit does NOT aapear to have Dart or delta shaped keels?

    Okay I looking at this video,
    https://www.youtube.com/watch?v=G6ZRoMj3iEc

    and at about minute 2:30 the narrator is saying we are looking at "Hissy Fit", the original Dazcat 1495. I'm sorry I don't see any Dart or delta shaped keels??



    Well looks like I stand corrected when I look at these photos from "Spring News Letter: Dazcat 1495 Launch 2015"
    But they are NOT Dart catamaran style keels.

    Doesn't appear as though they want to talk about their hull and skeg/board arrangements much??
    http://www.dazcat.com/news.html
    http://www.dazcat.com/assets/portsmouth-multihull-show---special-edition-newsletter-2---------------------june-2016.pdf
     
  12. lucdekeyser
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    lucdekeyser Senior Member

    natural for some proa's

    "The advantage of a daggerboard in that case is that you can tune the surface size to get the best result. So you have a fixed surface (the dart), a variable size surface incidence 0 ((the front daggerboard) and a variable incidence surface (the rudder)."

    hmm ... this setup reminds me of some proa's with retractable rudders at each bow and a centrally located board ...
     
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    Brian, I think thatt the front daggerboard exists or it won't be published.
    The drawing is a diagram of the static displacement of the center of lateral surface when using the canard daggerboard.
     
  14. Ilan Voyager
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    Ilan Voyager Senior Member

    That reminds but the purpose is totally different, as the proa was an amphidromic boat. But the basis is a 3 surfaces stabilization.
     

  15. lucdekeyser
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    lucdekeyser Senior Member

    agreed that such a proa has no choice but it realizes in each direction the potential of a three surfaces stabilization much like with " a fixed surface (the dart), a variable size surface incidence 0 ((the front daggerboard) and a variable incidence surface (the rudder)"
     
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