Plywood-foam-plywood composite: why not?...

Discussion in 'Fiberglass and Composite Boat Building' started by Laurent, Feb 23, 2012.

  1. Laurent
    Joined: Feb 2005
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    Laurent Junior Member

    Hi everybody,

    In his book "the nature of boats", Dave Gerr explains that for a given weight, wood is much better than conventional materials. For instance, he lists E-glass at 158 psi/(lbs/cuft) in tensile strength versus weight and douglas fir at 388. On stiffness vs weight, Douglas fir is at 60,938 psi/(lbs/cuft) where E glass is only at 14,737... (chapter 47, titled "believe it or not, wood is best").
    He uses the term "weight", but actually, he is talking about density...

    But I guess what he means is that for the same weight, a structure in douglass fir will be up to 4 times stiffer and more than twice stronger that e-glass.

    Did I get this right?

    If yes, then why don't we see more composite boats made of "thin plywood - foam - thin plywood" sandwich? All bonded with epoxy.

    Looking forward to getting some feedback from the experts.

    Laurent
     
  2. rwatson
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    rwatson Senior Member

    Dont forget the outside material has to be waterproof - you would have to protect the timber.

    But "for a given weight, wood is much better" - isn't logical for these materials at all sizes.

    250 gm fibreglass/epoxy is less than 2mm thick. The same <2mm thickness in wood isn't either practical or stronger as a raw plank due to grain, imperfections etc.

    If you could get plywood that thick ( which you just about can) it would be extremely expensive, and still not be as strong as E-glass.

    On the other hand, there are quite a few people on this forum who build plywood over foam at thicker sizes (using contact adhesive in some cases). I have a boat plan from Bateau that offers foam between ply for the cabin roof. There is a local builder who has substituted the internal panels for ply/foam/ply, with approval from the designer, for the internal frames of his 40ft cat - purely for economic reasons.

    So, ply/foam is very common practice for flat or develop-able panels - but would be problematic for compound panels due to the risk of voids in the joins of 2 stiff materials.
     
  3. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    ply foam ply

    This board is hoop pine 2mm ply/polystyrene/ply - also has 3 x stringers & laminated rails, the "inside" of the rails is glassed top to bottom before the ply is bagged on. Jeff.
     

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  4. johneck
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    johneck Senior Member

    I think Mr watson hit on a major limitation, plywood is flat can only be formed into developable shapes. Carving solid wood into the desired shape is very time consuming and wasteful. The wood also only has the strength if the grain is running in hte proper direction. The beauty of composite materials is that they can be formed in place to almost any shape and the fiber orientation is controllable, so the strength and stiffness of the part can be tailored.
     
  5. Laurent
    Joined: Feb 2005
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    Laurent Junior Member

    Thanks for all the input.

    I was not clear enough.

    YES, I am thinking of developpable surfaces only... There are 3 or 4 mm ply outhere. That's what I am talking about on Diniycell foam (for instance).

    To Mr. Watson. I hear you on 2mm thickness fiberglass. BUT, do not think thickness but weight. How much weight is that 2 mm thick fiberglass per square foot? Now, how thick can be a plywood sheet for the same weight per square foot? I bet at least 2 to 3 times...


    Whatdya think?
     
  6. johneck
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    johneck Senior Member

    There is a difference between strength (how much load can a structure hold before failure) and stiffness (how much deflection occurs due to a certain load). So it depends to some degree on what you are trying to achieve with the structure.
     
  7. PAR
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    PAR Yacht Designer/Builder

  8. Laurent
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    Laurent Junior Member

    Agreed, but according to the figures from Dave Gerr, see my original post, for the same weight, I should gain on both fronts, strength and stiffness.

    If you replace XX lbs of fiberglass sheathing by XX lbs of plywood sheathing, for developpable surfaces, you should end up with a stronger and much stiffer structure. Or the otherway around, one could achieve same strength and/or stiffness for a lighter sheathing (in the limits of what is available as thin plywood... of course).

    So I still don't understand why it is not more common.

    Is it a question of cost as suggested earlier? Too labor intensive? Limited to developpable surfaces only? Too conservative customer base that does not want to try something new? Or are there example of successful - or not so successfull - boats outhere that I do not know about?...

    In advance, thanks for sharing information on this topic; as you may have guessed, I am toying with this idea for my next boat...
     
  9. CatBuilder

    CatBuilder Previous Member

    It's actually pretty common, just possibly not so for curved panels.

    In the wooden plans my designer provides, he makes ample use of foam or balsa cored panels with 3mm skins. He uses these for bulkheads, cabin soles, decks, etc...
     
  10. DCockey
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    DCockey Senior Member

    Gougeon Brothers built wind turbine blades of wood, foam and honeycomb between 1979 and 1993:

    The manufacturing experience and research and development associated with the use of wood/epoxy composites in the construction of wind-turbine blades has also largely influenced our company. In 1979, the Gougeons' reputation for excellence and innovation in wood/epoxy composite construction captured the attention of NASA researchers who contacted the company to build experimental wind turbine blades for use on wind energy machines. The success of the wood/epoxy blades led to multi-million dollar contracts with General Electric, Westinghouse, and Bendix. Gougeon manufactured 4,300 blades, from 10 to 70 feet long between 1979 and 1993. The wind turbine business allowed the company to fund an extensive research program, the results of which have been instrumental to developing extremely light weight structures, both on and off the water. Data collected was also used to improve the performance of West System epoxy and to test new construction methods. http://www.westsystem.com/ss/history/


    NASA Technical Memo TM-83323 Fabrication of low-cost Mod-OA wood composite wind turbine blades is available for download
    http://ntrs.larc.nasa.gov/search.jsp?R=19830019076&qs=Ns=NASA-Center|0&N=4294850946

    Video from 1983 that includes segment showing the shop building the blades: http://www.youtube.com/watch?v=D6kV_uh1A-U

    http://ntrs.larc.nasa.gov/search.jsp?R=19830019076&qs=Ns=NASA-Center|0&N=4294850946

    Patent relating to the Gougeon blade design http://www.google.com/patents/US4474536
     
  11. Doug Lord
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    Doug Lord Flight Ready

  12. Wavewacker
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    Wavewacker Senior Member

    Holy cow and shipping on top of those prices! I was wondering about the R value as well but it wasn't stated. Doug, I think gold leaf is water proof!
     
  13. CatBuilder

    CatBuilder Previous Member

    You have to understand what you are buying from them, though.

    Boulter has some seriously good plywood.

    I bought a big stack of 3mm from him as well as a big stack of 12mm. I wasted the 3mm, but the 12mm is used for single hull bulkheads in my boat.

    His plywood is African Okoume, produced in a current and valid Lloyd's registered manufacturing plant in Europe and of course meets BS1088 specs. I have a copy of the Lloyd's Cert. It's also absolutely void free. Here is a picture of my 12mm Okume from Boulter.

    If you want the best, you do have to pay for it.

    I have scrap pieces of it that have been sitting out in Florida's tropical climate, on the ground, for about 18 months now. The wood is looking weathered, but the lamination is still strong as ever.

    [​IMG]
     
  14. rwatson
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    rwatson Senior Member

    I couldn't find any prices on cored ply. so I am not sure what you were looking at. The ply sheet prices were about what iIwould have expected.
     

  15. fcfc
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    fcfc Senior Member

    Limited to flat panels.

    Expensive. How do you ensure, and check good bonding between plywood and foam ? You are working blind.

    You have a structure lighter, but much more brittle : plywood elongation 0.81%. GRP UD 1.91% per ISO data.

    Also the minimum plywood skins seems to be 3mm.

    http://www.charles-acs.fr/produits/panneaux-derives/panneaux-legers/Poyolight/default.aspx

    You can go much thinner with GRP: http://www.finot.com/bateaux/bilanmini650/amateur.htm

    This mini has skins 480 gr glass per m². Built in infusion, this is 0.45mm thick weighting 800 gr/m². A 3mm light plywood will be at least 1.2 kg/m².

    I do not dare to do the computation with carbon.
     
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