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[bolger] Re: Door skins & glass

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  • berniew@nwlink.com
    ... This idea seems to stem from the mistaken notion that glass is stronger and lighter than wood, especially plywood. It is not, particularly in the stiffness
    Message 1 of 8 , Nov 1, 1999
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      David Ryan wrote:

      > I've been wondering about using very thin plywood in combination with
      > a heavier glass job to make very light boat (kayaks, hollow paddle
      > boards, etc.)

      This idea seems to stem from the mistaken notion that glass is stronger
      and lighter than wood, especially plywood. It is not, particularly in
      the stiffness modulus, which is the most important in boat building. In
      fact fiberglass is a relatively limp material. It is also relatively
      heavy for its strength. Plywood is still one the strongest material
      available on a pound of pound basis.

      Fiberglass is used on plywood boat construction not for strength, but
      for waterproofing and abrasion resistance. On a plywood boat it adds
      almost nothing to the overall strength of the boat.

      To make a very light boat, use the thinnest plywood possible for the
      boat being built and omit using any glass except perhaps on the seams.

      The best way to make something stiff yet light is to separate two
      materials that are very strong in both shear, tension and compresion
      with a very light core. Cores are often some type of foam of balsa
      wood. Strength of the core doesn’t matter, what’s important is how
      friable it is (meaning how well the surface is attached. If you rub two
      pieces of foam together and the surface rubs off the material is
      friable and no good for composite construction. The reason is the skins
      will separate form the core under stress making the structure no longer
      a composite and very weak.) What makes a composite strong is that the
      structure transmits all loads to its strongest part, the skin. A
      composites strength is directly related to how far the skins are held
      apart, the farther apart the stronger. Obviously, if you bend a
      composite structure the skin on the outside of the bend is loaded in
      tension, the inside skin in compression. Here again, fiberglass is very
      weak in compression, especially compared to plywood. Plywood is very
      strong in compression. In short, one of the lightest and strongest
      composites you can make is by gluing very thin plywood to the inside
      and outside of a foam sheet. Blue polyfoam is used in some airplane
      construction because it is light, cheep and relatively unfriable.

      Bernie
      Change is a proccess not an event.
    • GHC
      Actually, glass is very, very, very strong. A good fir might be 14,000 psi in tensile strength (at best with no flaws), but E glass is good for 450,000 psi!
      Message 2 of 8 , Nov 1, 1999
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        Actually, glass is very, very, very strong. A good fir might be 14,000 psi
        in tensile strength (at best with no flaws), but E glass is good for
        450,000 psi! It's relatively dense, though, and a thin sheet of plywood
        doesn't spread it apart very far for your composite sandwich. Wood is
        light, and relatively strong, so it's easier to get a lot more wood in a
        structure than glass (with minimal resin).

        In bending of a panel, the tensile and compressive stresses do lie on the
        surface, but the shear flows through the middle, trying to separate your
        sandwich. Your core needs to have good material properties, like balsa or
        klegecell, not like blue styrofoam.

        Gregg Carlson





        At 02:24 PM 11/1/1999 -0800, you wrote:
        >David Ryan wrote:
        >
        >> I've been wondering about using very thin plywood in combination with
        >> a heavier glass job to make very light boat (kayaks, hollow paddle
        >> boards, etc.)
        >
        >This idea seems to stem from the mistaken notion that glass is stronger
        >and lighter than wood, especially plywood. It is not, particularly in
        >the stiffness modulus, which is the most important in boat building. In
        >fact fiberglass is a relatively limp material. It is also relatively
        >heavy for its strength. Plywood is still one the strongest material
        >available on a pound of pound basis.
        >
        >Fiberglass is used on plywood boat construction not for strength, but
        >for waterproofing and abrasion resistance. On a plywood boat it adds
        >almost nothing to the overall strength of the boat.
        >
        >To make a very light boat, use the thinnest plywood possible for the
        >boat being built and omit using any glass except perhaps on the seams.
        >
        >The best way to make something stiff yet light is to separate two
        >materials that are very strong in both shear, tension and compresion
        >with a very light core. Cores are often some type of foam of balsa
        >wood. Strength of the core doesn�t matter, what�s important is how
        >friable it is (meaning how well the surface is attached. If you rub two
        >pieces of foam together and the surface rubs off the material is
        >friable and no good for composite construction. The reason is the skins
        >will separate form the core under stress making the structure no longer
        >a composite and very weak.) What makes a composite strong is that the
        >structure transmits all loads to its strongest part, the skin. A
        >composites strength is directly related to how far the skins are held
        >apart, the farther apart the stronger. Obviously, if you bend a
        >composite structure the skin on the outside of the bend is loaded in
        >tension, the inside skin in compression. Here again, fiberglass is very
        >weak in compression, especially compared to plywood. Plywood is very
        >strong in compression. In short, one of the lightest and strongest
        >composites you can make is by gluing very thin plywood to the inside
        >and outside of a foam sheet. Blue polyfoam is used in some airplane
        >construction because it is light, cheep and relatively unfriable.
        >
        >Bernie
        >Change is a proccess not an event.
        >
        >
        >------------------------------------------------------------------------
        >-- Check out your eGroup's private Chat room
        >-- http://www.egroups.com/ChatPage?listName=bolger&m=1
        >
        >
        >
      • bianco@asi.it
        Just as an added info, Jacques Mertens of bateau.com is reengineering his larger boats (Vagabond and Serpentaire) using thin plywood (6 mm) sandwiched between
        Message 3 of 8 , Nov 2, 1999
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          Just as an added info, Jacques Mertens of bateau.com is reengineering
          his larger boats (Vagabond and Serpentaire) using thin plywood (6 mm)
          sandwiched between two layers of biaxial fiberglass (9 oz) in epoxy
          resin. The advantages are not only structural: the 6mm plywood is much
          easier to bend and wrap around bulkheads than thicker stuff. Jacques
          says that such sandwich is significantly stronger than 9 mm plywood.
          According to my computations, though, the gain in weight is very
          limited if any, and the overall cost is very significantly increased.
          And, folks, just think about crawling into a 6 meter hull and laminated
          10 square meters of biaxial cloth...
          Best, Pippo

          ghc <ghart-@...> wrote:
          original article:http://www.egroups.com/group/bolger/?start=643
          > Actually, glass is very, very, very strong. A good fir might be
          14,000 psi
          > in tensile strength (at best with no flaws), but E glass is good for
          > 450,000 psi! It's relatively dense, though, and a thin sheet of
          plywood
          > doesn't spread it apart very far for your composite sandwich. Wood is
          > light, and relatively strong, so it's easier to get a lot more wood
          in a
          > structure than glass (with minimal resin).
          >
          > In bending of a panel, the tensile and compressive stresses do lie on
          the
          > surface, but the shear flows through the middle, trying to separate
          your
          > sandwich. Your core needs to have good material properties, like
          balsa or
          > klegecell, not like blue styrofoam.
          >
          > Gregg Carlson
          >
          >
          >
          >
          >
          > At 02:24 PM 11/1/1999 -0800, you wrote:
          > >David Ryan wrote:
          > >
          > >> I've been wondering about using very thin plywood in combination
          with
          > >> a heavier glass job to make very light boat (kayaks, hollow paddle
          > >> boards, etc.)
          > >
          > >This idea seems to stem from the mistaken notion that glass is
          stronger
          > >and lighter than wood, especially plywood. It is not, particularly in
          > >the stiffness modulus, which is the most important in boat building.
          In
          > >fact fiberglass is a relatively limp material. It is also relatively
          > >heavy for its strength. Plywood is still one the strongest material
          > >available on a pound of pound basis.
          > >
          > >Fiberglass is used on plywood boat construction not for strength, but
          > >for waterproofing and abrasion resistance. On a plywood boat it adds
          > >almost nothing to the overall strength of the boat.
          > >
          > >To make a very light boat, use the thinnest plywood possible for the
          > >boat being built and omit using any glass except perhaps on the
          seams.
          > >
          > >The best way to make something stiff yet light is to separate two
          > >materials that are very strong in both shear, tension and compresion
          > >with a very light core. Cores are often some type of foam of balsa
          > >wood. Strength of the core doesn’t matter, what’s important is how
          > >friable it is (meaning how well the surface is attached. If you rub
          two
          > >pieces of foam together and the surface rubs off the material is
          > >friable and no good for composite construction. The reason is the
          skins
          > >will separate form the core under stress making the structure no
          longer
          > >a composite and very weak.) What makes a composite strong is that the
          > >structure transmits all loads to its strongest part, the skin. A
          > >composites strength is directly related to how far the skins are held
          > >apart, the farther apart the stronger. Obviously, if you bend a
          > >composite structure the skin on the outside of the bend is loaded in
          > >tension, the inside skin in compression. Here again, fiberglass is
          very
          > >weak in compression, especially compared to plywood. Plywood is very
          > >strong in compression. In short, one of the lightest and strongest
          > >composites you can make is by gluing very thin plywood to the inside
          > >and outside of a foam sheet. Blue polyfoam is used in some airplane
          > >construction because it is light, cheep and relatively unfriable.
          > >
          > >Bernie
          > >Change is a proccess not an event.
          > >
          > >
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