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

    thanks
    been trying to work up a plan for fabricating/laying up the central support and it seems really really difficult to do accurately.
    are you a hundred percent sure that split asymmetrical boards are fantastically better than a single symmetrical board?
    Single is far easier to build and maintain and operate. Its plain cheaper.
    occasional 1/2 knot loss? I'd take the symmetrical
    2 knots down? maybe.
     
  2. valery gaulin
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    valery gaulin Junior Member

    @uponstands: This seams to be a good implementation suggestion. The attachement is similar to what they use for dutch tjalk leeboard.
     
  3. UpOnStands
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    UpOnStands Senior Member

     
  4. DennisRB
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    DennisRB Senior Member

    A good friend of mine is a mechanical contractor for the AU airforce who works on helicopters. I asked him and he pretty much said no chance of getting old blades.
     
  5. UpOnStands
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    UpOnStands Senior Member

    large helo blades are likely to have some weird camber built in so most likely not useable. Break out the carbon tow and the resins.
     
  6. UpOnStands
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    UpOnStands Senior Member

    Attached Files:

  7. ImaginaryNumber
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    ImaginaryNumber An Imaginary Member

    Thinking about how helicopter blades are attached to their central hub...

    If the centerboard foil was completely submerged, but was connected to the bridgedeck by an aerodynamic shaft, would that reduce the problem of ventilation?
     
  8. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante

    The mostest bridgedeck- centerboard, max. complicated to build but with ALL (all !) possible features.
    Vorsprung durch Technik ?
    According the designer/builder it works very well.

    http://www.katamaran-shambala.de/apps/photoalbum?aid=226947

    Description:
    "

    Mittelschwert

    *

    Um gute Kreuzeigenschaften zu erzielen wurde von mir ein Mittelschwert entwickelt, wie es (meines Wissens) so noch nie auf einem Katamaran verwirklicht wurde.

    Das Schwert ist verstagt auf einer Querachse unterhalb des Mastschotts drehbar gelagert, so dass es sich entlang der Schiffslängsachse vor und zurück bewegen sowie nach vorne ganz aus dem Wasser holen lässt. Darüber hinaus ist es entgegen der Fahrtrichtung wie ein Ruder nach beiden Seiten hydraulisch bis zu 10° anstellbar. Das Schwert wird über 3 Leinen bedient. Über Vor- und Achterholer wird es in seiner Position in Schiffslängsachse fixiert, der Aufholer hebt es bei Nichtgebrauch aus dem Wasser. Über zwei Handpumpen lässt sich hydraulisch die Anstellung einstellen. Die Konstruktion ist robust und hat sich bestens bewährt, über Grund sind nach GPS Wendewinkel von 90 Grad erzielbar, für einen Fahrtenkatamaran ein Traumwert. Die Vorteile dieses Schwertsystems sind bestechend: Nur ein Schwert, einfachere Bedienung, wesentlich weniger Gewicht Bei jeder Beseglung lässt sich ein ausgeglichener Trimm erzielen, selbst nur mit Genua, Fock oder Stumfock lässt sich kreuzen, indem das Schwert entsprechend nach vorn getrimmt wird. Die Anstellbarkeit lässt die Rümpfe gerade durchs Wasser fahren, was die Kreuzeigenschaften wesentlich verbessert. Da keine Reibung im Schwertkasten besteht, lässt sich das Schwert auch unter vollem seitlichen Druck bedienen. Keine Schwertkästen (Gewicht!) in den Rümpfen, keine Wirbel an den Schwertkästen bei auf geholtem Schwert. Kein Klappern oder Verklemmen der Schwerter, kein Bewuchsproblem. Bei Berührung mit Hindernissen kann das Schwert nach hinten wegklappen."



    Translation by PONS :

    "To achieve good beating qualities a single centerboard under the bridgedeck was developed by me as it was never realised (my knowledge) thus on a catamaran.
    The centerboard is stored supported on a transverse axis below the mast's bulkhead swivelling, so that it to itself along the Centerline before and retrogress as well as forwards completely from the water sends. In addition, it is employable against the direction of the traffic like an oar after both sides hydraulically up to 10 °. The sword is served more than 3 linen. About pre and afterguy it is fixed in his position in Schiffslängsachse, the Uphaul lifts it when not in use from the water. More than two hand pumps the employment is regulated hydraulically. The construction is robust and has proved itself very well, about reason turn corners of 90 degrees are attainable after GPS, for a journey catamaran a dream value. The advantages of this sword system are fascinating: Only one sword, an easier service, substantially less weight with every Beseglung a well-balanced trimming is achievable, even only with Genoa, foresail or Stormjib can be sailed upwind , while the centerboard is trimmed accordingly forwards. The Angle of attack means that the hulls have no drifting angle gives which improvesimproves the beating qualities substantially. Because no friction exists in the trunk, the centerboard can be also served under full lateral pressure. No trunks (weight!) in the bodies, no turbulences in the ( non existing) trunks. No rattles or becoming stuck the centerboard , no growth problem. colliding with obstacles the centerboard swings back."








    pogo

    P.S.
    frenglish "centreboard;
    american english "centerboard";
    german "Schwert"
    daggerboard in german "Steckschwert"
    Centerboard in german " Klapp-, Senkschwert" , either more or less semicircular " Kreissegmentschwert" , or rectangular "Spatenschwert".
    I write this ' cause a circular-segmented- centerboard under a bridgedeck has advantage thru it's trunk ( more supported area), less depth and less shifting of CLE center of lateral effort). In this case a 25 to 45 degrees angled kreissegnentschwert would be the optimum , it comes close to an ellipse. See centerboards of Dragonfly trimarans. The trunk under a cat's bridgeck is of course above WL.

    On the other hand a Spatenschwert ( a more or less rectangular centerboard) without trunk normally needs additional lateral support thru svivelling dolphinstrikers ( lateral struts; complicated), see link above. A rectangular centerboard also is much longer ( depth).of course one can integrate the trunk of a rectangular centerboard into a nacelle , but the disadvantage of it's geometrie remains.-- the center of lateral resistance goes backwards when the centerboard is half lifted. the longer the board ....
    Not so with a properly designed geometrie of a " Kreissegment".
     
  9. brian eiland
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    brian eiland Senior Member

    That might be true, I've never really tried finding them before. But I suspect you could find one blade (one is all you would need) that you could convince the 'authorities' that you intend to cut it up into several lengths (maybe in front of their eyes). And I'll bet there are some really nice carbon fiber ones around.

    No sir I am not 100% sure that the two asymmetrical boards are that much superior, .....probably are superior, but to what degree, question mark. I likely chose that arrangement for its 'symmetry', and the fact I was replacing two boards (dagger or centerboards) of the conventional approach,...so two asymmetric boards became my choice.

    BTW, I began formulating this idea back in the early 1990's, when I drew up my sportfishing-under-sail catamaran. I didn't become computer literate till 2000, and then posted this idea on the forums in 2003
    http://www.boatdesign.net/forums/boat-design/big-cat-alt-cbs-sail-rigs-2225.html
    You can see it didn't get much of a response (none) :eek:
     
  10. brian eiland
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    brian eiland Senior Member

    Surtees Catamaran Experiment

    That Surtees boat is quite an interesting little experiment. It desires more exposure on this forum. His website is indicating that it may be taken down soon. It would be a shame to lose this info.

    http://www.s-boat.com.au/

    Video of his first launching,..
    https://www.youtube.com/watch?v=4TSMPVlCDzk
    https://www.youtube.com/watch?v=btGxauYS8XE

    moreton_bay_docked, 900pix.jpg

    Clean looking board/nacelle arrangement
     
  11. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante


    Yepp !


    What about the so called " canards".
    Normally canards are actuating , either with flaps or the whole balanced wing moves. It means that " real canards" are able to generate positive AND nenegative lift. Real canards are eithe stabilizers for most efficient angleog attack for the mainfoil , or for very agile manouverability.
    So, what profile do those permanent canards, , those non- articulating foils have, asymmetric, symmetric ?
    The first Division B and C cats also had canards above the waterline with very , very poor aspect ratios. They were angled abpou 10 degrees to WL . Simple sprayrails. they disappeared. Those sprayrails , or integrated sprayrails as on Iroquois , Comanche, apache generate more drag than lift.
    The famous , still active Shark is the best example. First boats came in 1963 with sprayrails , about 1m long. In lateral direction a sort of positive angled foils with poor aspect ratio . Nowadays the still competitive Sharks have no rails.

    Old Shark w. rail
    http://setsail.com/wp-content/uploads/2010/01/shark-nationals0001-edit-2.jpg

    Newer Shark
    http://www.catsailor.com/images/StoryPix/Sharks/DSC_0106c.jpg

    Integrated sprayrail ; Iroquois Mk II
    http://www.cruisersforum.com/forums/attachment.php?attachmentid=58917&d=1365605234

    Shark, Aztec, Iroquois, Comanche, Cherokee, Apache, Navaho are designed by multihull pioneer Rod Macalpine-Downie, built by Sailcraft Ltd in England --Reginald White. If interested, see also his early successful C-cats , Cowes Speed Week racers and Offshore Racers.



    I propose an own thread for this interesting boat.


    pogo
     
  12. valery gaulin
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    valery gaulin Junior Member

    The canard is a really good idea.The profile must be asymetric to create lift to prevent bow burrying. I don't think the canard needs to be control with flap just a good assymetric shape for the expected average speed and it is probably good enought.

    You never want negative lift at the bow!!! So why controlling it?
     
  13. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante


    Because of a too high negative angle of the whole boat/hull.

    With massive negative angle of the boat even an asymmetric foil ( canard) can generate negative lift.
    Why can planes fly on their back ?


    If you are interested in this topic, Please open a new thread for this boat, we are off topic now.

    pogo
     
  14. pogo
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    Location: Germany Northsea

    pogo ingenious dilletante


  15. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Bridgedeck Centerboard

    Thanks, Pogo. I was wondering if it had been tried as a bridgedeck board on a cat. It's been used on many monohulls.
     
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