Fiberglass Sheet As Ply?

[Dave Brox]

Theoretical Question
Could you use fiberglass sheet (say a couple of biaxial laminates) as you would the ply in the stitch and glue.
Then Glass over these to retain the form.
I don't know how many layers of biaxial you would have to use to get the strength of the ply.
Then, the inside is already glassed.
Would flexing be a problem? Could you just use more internal compartmentalization (those ones that go across the boat).
Make the inside ones out of glass as well.
I'm assuming the glass laminate would be lighter than the ply as it was thinner.
I have no idea about the structural strength and properties of the glass to the ply.

If you had the extra cash to use it you could
Throw in a layer of Innegra-S fiber that doubles the strength of the layers each side of it.

[fallguy1000]

Your weight assumptions are wrong.

The reason the idea falls apart is the glass sheet is vert heavy.

Also, in very general terms, fiberglass sheets would require a very consistent resin ratio to avoid bending irregularities. Plywood has this feature as the glue ratios are very strictly maintained in the manufacturing process.

All that said, you can buy manufactured fiberglass sheet, but the qualities of plywood in s&g construction are hard to beat and glass sheet is generally used in special applications where wood is less desired for ingress or foam less desired as in loading, and generally not in bending.

I am certain it has been tried, but I have not ever seen it in all my reading.

I just built something solid glass yesterday. I will post pictures today.

[fallguy1000]

Further, solid glass boats are built over a form. There is no reason for the glass to not be formed to the desired shape at the onset versus bending and secondary bonding as in s&g. This way the bonds are all primary chemical bonds and as such stronger.

[jacquesmm]

All correct: the sheets would be heavier than plywood-epoxy-glass and very flexible, impossible to guarantee a fair shape.
If you want to avoid plywood, use a foam core.

[narfi]

I have been reading a thread on another forum where a guy has a large pane of glass he vacuums a single layer of carbon fiber to which makes a flexible, light weight sheet of carbon that is perfectly smooth on one side (no fairing needed). He can then lay it over/wrap it around and bond it to ribs and spars with the smooth side out and add areas of structure as needed.

This is for a very light weight airplane though and is not composite sandwitch, those skins are minimal for air loads and would not support water loads.

The way I think of sandwitch composite is like an I beam. Imagine a 2x4 steel I beam. You could walk out a long ways on it and it would support your weight just fine, there is no need for a solid 2x4 rectangle of steel, the light 1/4" web between the two caps is plenty strong to keep those caps separated which is where it gets its strength. Wood or foam sandwitch is the same way, you need the thickness for stiffness, but you dont need the weight, you just need to keep the two sides 1/4" or 3/8" or whatever the designer has calculated away from each other to create that stiffness.

I dont understand it all, and I certainly cant do the calculations, so I am happy to buy the plans from someone who can and follow their recommendations.

[jacquesmm]

What you describe looks like a prefabricated sandwich. We can do that, I am writing extensive building notes for it for a few months, almost finished.

Flat panels made from fiberglass will work over a very tight slat mold. It has been done but it is different from my plywood method. In my method, the span between molds and longitudinals is 3' or more. In the slats molds, the slats are spaced 2 or 3" because those panels are flimsy. That is like building the boat two times: the slat mold first then the planking.

The slat mold is described in my foam sandwich tutorial:
http://bateau2.com/howto/foam1.php

[RobCiminieri]

Would Coosa be an option?

[Fuzz]

Would Coosa be an option?

I think CoreCell or Divinacell would be far better for a hull. Coosa is best used for transoms and stringers.

[Evan_Gatehouse]

For a hull panel you need both strength and stiffness. Strength can be found in plywood, fiberglass or foam cores or some combination of these.

Stiffness is based on panel spans (distance between supports) and panel thickness. This is generalizing a bit but we'll get there.

One way to have adequate thickness with thin skins is to have lots of closely spaced supports (stringers/ribs/internal beams). This is way conventional aircraft are built. Here's a cross section of a 737. Longitudinal stringers are 6-7" apart and circular ribs every 20". This means the panels are very small and you get enough stiffness with thin aluminum.



The other way to gain adequate stiffness is by making the panel have some thickness. This is the way most plywood/fiberglass composite or foam core boats are built. Because the panel is so much stiffer than say thin aluminum you can have panels that are a few feet wide.

Solid fiberglass sheet is very STRONG but is not very stiff until the panel gains adequate thickness. And when it does get that thick, it gets very heavy. Builders can get around this with lots of little moulded in stiffeners (chines/rubrails/strakes/fake caravel planking) but it's still a relatively heavy way to build a boat. And you need a complex mold for built in stiffeners.

So a lighter and more efficient way to build a boat is with a cored thicker hull panel, with a foam or plywood core. Plywood has more strength than foam so lighter amounts of glass can be used. Foam has very little bending strength, only shear strength so the outside layers of glass get thicker.

Additionally Coosa board: It is a very material, heavier than plywood in many types. It is designed to resist crushing and have good compressive strength (outboard motors bolts/clamps) and is ideal for structures highly loaded in compression (transom, mast steps, bolted on hardware). But it's very expensive and not that strong in bending for it's weight. There is a reason that no boat builder uses it for building a whole boat. It's just not a good idea.