Why not *very* thick core in hull & deck?

Discussion in 'Boat Design' started by u4ea32, Jun 10, 2013.

  1. u4ea32
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    u4ea32 Senior Member

    What is the limit for core thickness?

    It seems that making a core 2x or 10x thicker than typical only improves stiffness and strength while reducing in-core stress.

    So why not use, say, 6" think foam core instead of 1" thick core?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    What type of core material do you have in mind ? Costs will blow out for a start, add to that loss of internal space, and it just sounds like over-kill.
     
  3. u4ea32
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    u4ea32 Senior Member

    The cost to triple the core thickness is something, but for certain, material prices are a tiny percentage of a total boat. True, the core consumes internal volume, but so do frames, bulkheads, floors, etc.

    And remember, wood or metal boats consume much, much more than core thickness, and that price seems to be commonly made.

    In the equations for sandwich composite strength and stiffness, it seems there is no limit to the advantage gained by increasing sandwich core thickness: strength increases linearly, stress within the core decreases linearly, and stiffness increases by the square of the thickness. All very good. Yet there is a clear practice for choosing seemingly insignificant core thicknesses.

    I am curious why cores of 1" instead of say 6" are so commonly used.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You have a number of problems to consider, for example having your super-thick core conform to the hull shape, whether in a female or male mould situation.
     
  5. alan white
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    alan white Senior Member

    The stiffness you get with the usual core thickness is pretty adequate. Make it thicker and it's overkill and heavy too. It also raises the cabin/deck height which affects windage, cost, and aesthetics.
     
  6. MoeJoe
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    MoeJoe Junior Member

  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Unless all surfaces are flat, I see a doubtful future for this proposition. Make a great icebox though.
     
  8. tunnels

    tunnels Previous Member

    what about the Boston whaler boats that sort of what they are all about with Foam injected and fills the cavity between there hulls and decks ! it also helps to stiffen the hull laminates' to a certain degree .

    Amazing when I worked in south Korea we had sheets and blocks of polyurethane foam that had long strands of chopper gun glass in it . out in the weather for months nothing happened it just changed colour sheet stayed dead straight and not a hint of a bend
    laying flat on the concrete with water and snow and burning sun it never absorbed water and again laid absolutely flat as could be !!
    Was really stable to make plugs from that could be skinned with a couple of layers of glass and never moved ever !! . :D
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    That Whaler "double hull" idea with the PU foam involved wasn't what the OP had in mind, I expect. Certainly not a method for one-off builders, at least.
     
  10. tunnels

    tunnels Previous Member

    But it is still a core tying the two together !! what about making a boat from a block of foam !! what the difference
    What about a surf board it uses a thick core and really thin glass each side !! same thing !! you cant get any more one off than a surf board !! :p:D
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You can't bend 6" thick foam to follow the curves of a boat.
     
  12. rwatson
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    rwatson Senior Member

    There was a guy who built a couple of boats with very thick spray on foam.

    http://www.boatdesign.net/forums/boat-design/bourne-method-boat-building-17840.html


    His website is no longer working, but the people who sell the foam have him on their web site.

    http://www.spray-insulation.co.uk/bourne_method.htm

    He would make a steel cage and spray foam over it, then cover it in fibreglass.

    The method always appealed to me, but the longevity and material performance was always a point of debate. for example, you would never get insurance for a boat built that way.

    One benefit of much foam is terrific insulation, which would be a great bonus in cold climates.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I can't imagine standard ( 2lbs/cu.ft.) PU foam being a success, and heavier densities with greater integrity would get heavy if very thick.
     
  14. tunnels

    tunnels Previous Member

    So true but of it was wood what would you do to make it bend ??don't think of it as foam just think of the thickness !!
    :idea:
     

  15. rwatson
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    rwatson Senior Member

    I was just looking up popular spray on foams here in Australia.

    The one without any CFC, other bad gasses and with proper fire rating
    http://www.demilecusa.com/wp-content/uploads/2013/05/Sealection-500-TDS.pdf

    SEALECTION 500

    is only .5lb per ft3, or in proper talk - 8 kilos per cubic metre

    Corecell, a popular boat building foam is 13-19 lb/ ft3
    or 210 - 315 kg per m3

    http://www.gurit.com/files/documents/corecell-s-foamv8pdf.pdf

    Compression strength and shear strength are no comparable of course
     
    Last edited: Jun 10, 2013
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