gravity molding

Discussion in 'Multihulls' started by bill broome, Jul 20, 2008.

  1. bill broome
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    bill broome Senior Member

    has anyone tried this?

    lay a hull length fg cloth on a plastic film, on a flat table, with light wooden battens glued to the edges. wet down with epoxy and let dry to 'tacky'. lift the battens up to a frame which induces the cloth to drape in a 'bottom' shape.

    when dry, pull/sand off the plastic film, and you have the bottom of a cat hull. it will need bulking up and a finish layer, but the shape is the hard part, and gravity has done that for you.
     
  2. rwatson
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    rwatson Senior Member

    Thats virtually the idea behind the Kelsall Catamaran method

    http://www.kelsall.com/

    Only he puts foam on top of that and another layer of cloth
     
  3. bill broome
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    bill broome Senior Member

    nope.

    kss is a good method, but does require a rigid hull-shaped frame to develop the shape. i'm suggesting that 'hardest part' of hull shaping can be dodged by letting gravity produce a section, probably a 'tractrix' very suitable for cat hull bottoms, with no need for design or development beyond fixing a frame for attaching the battens on the edge of the cloth.

    step 2 on the development of this idea might be laying fiber bulking panels on the cloth, well-wetted, and then lifting into position. all that would remain would be a finish cloth on the inside.

    experiment with a strip of fg cloth and simple hanging frame suggests this might work, but a 7 meter hull would need commitment and money which i don't want to make, i've already got a boat, and am way too old to go into business again.

    but the woods are filled with people wanting a small cat, here's a way to get a slick shape, with comparatively little effort. go for it. or maybe a way to get a 7 meter mess, hmm. wonder which...
     
  4. tspeer
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    tspeer Senior Member

    Alex Strojnik used that technique for molding the wing skins on his motorized sailplane. It's described in one of his books, either "Laminar Flow Technologies" or "Laminar Flow Structures".
     
  5. bill broome
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    bill broome Senior Member

    that sounds useful, ts.

    i'm surprised someone would look at this for an ultralight plane, tho' - weight would be more than varnished cloth on battens, for a guess.
     
  6. rwatson
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    rwatson Senior Member

    I did part of a hull on a trimaran using a similar technique. I need to make a 'pod' wider than the hull, so I built a square frame, and stretched nylon fabric between the frame and the hull edge.
    It was pulled so tight i could lay fibreglass on it with no significant sag, and it turned out fine.
    If the mould wasnt really tight, I have found the curing glass deforms the supporting fabric enough to make it problematic.
     
  7. tspeer
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    tspeer Senior Member

    He used high-density foam to make a male mold for about a 4' section of the forward half of the wing section. Then he went into production, making a number of U-shaped molded skins, each of which was long enough to reach the trailing edge. The skin aft of the wing spar didn't need to be molded, because it was flexible enough to conform to the shape of the ribs.

    The wing spar was an aluminum channel. Half-ribs were bonded to the wing spar. Where two skin sections met, two half-ribs were used with a cut-down section of leading edge skin between them, forming a wide box rib. The wing skins were then butt-joined, with the box rib forming a doubler at the joint.

    The result was a light-weight fiberglass-skinned wing with minimal labor spend on fairing and finishing to a smooth, accurate shape.
     
  8. rattus
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    rattus Señor Member

    Bill, that's a very cool idea - but I think you meant catenary instead of tractrix - tractrix is an evolute of a catenary... kind of an inside-out version, but sideways ;-)

    Catenary is the curve formed by a rope when suspended from two horizontal points at the same height.

    Mike
     

  9. bill broome
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    bill broome Senior Member

    yep, catenary.

    been about 45 years since my last use of either word. seem to have collected some sludge on the synapse..
     
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