ply tp alu

Discussion in 'Boat Design' started by wardd, Sep 30, 2009.

  1. wardd
    Joined: Apr 2009
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    wardd Senior Member

    i,ve been looking into the bolger advanced sharpie -29 as a candidate

    what would the weight diff between ply and alu be

    and cost diff because i wouldnt need so mush epoxy and glass

    and i assume building would be much faster

    any gustimates would be appreciated
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There wouldn't be any reasonable "guestimates" as you haven't a reasonable scantling list to work from. Ideally the weights would be nearly the same, other wise the boat will not trim properly or sail on the displacement intended.

    You're correct in that light is good, but you have to be designed for the weight. In other words, taking a 5,000 pound design and building it at 3,000 pounds will not do it's performance any good. It's just not as simple as it seems, both in scantlings conversion and performance. I do think you're onto something though, as this is a good design for this type of conversion. It wouldn't cost much to preform the conversion, but it should be done professionally.
     
  3. wardd
    Joined: Apr 2009
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    wardd Senior Member

    i was thinking of keeping the weight the same
    but that could be adjusted with the ballast

    ive worked on aircraft structures and the are amazingly light

    it seems to me that the as 29 could be made heavier out of alu than needed and still stay within weight

    also it seems a good candidate for riviting
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I'd weld the boat and I also wouldn't attempt to make a bullet proof hull with thick plates. I'd spec it for expected service and let the chips fall where they may, making up the difference with ballast/displacement ratio and rigging choices.
     

  5. messabout
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    messabout Senior Member

    If you are thinking in terms of using aluminum that is thinner than the ply that Bolger specified, then the problems begin. These boats have a bunch of flat panels, Bolger liked to use really thick bottoms. In doing so he could eliminate much of the interior framing. Let us say that you might use 0.125 plate for the skin....... where Bolger will have used one inch thick ply. There is a collossal difference in stiffness between the two materials. Roughly, the stiffness of a plate will vary with the third power of its' thickness. That means that the ply will be hundreds of times stiffer than the thinner alloy plate. Aluminum has a better elastic modulus than ply but not nearly enough better to make up the difference. So scantlings are the main problem here. You can use the aluminum but you will need to add a whole mess of stiffeners, which will increase cost, labor and aggrevation. All that honeycomb of stiffeners also contribute to problematic bilge sanitation. If you build a different sort of boat with a lot of curvature in the panels, then the sheet metal becomes more practical.
     
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