: Extra glass? Why use any glass at all?

: The plywood itself is far stronger than any fabric covering on a
: skin-on-frame design. On fabric covered boats the only thing keeping out
: water is a thin layer of paint, or something similar. Plywood covered with
: paint, or perhaps two coats of epoxy would be better, is going to be able
: to keep out the ater everywhere but the seams--and those are filled with a
: fillet made from reinforced epoxy.

: Consider that the displacement of a single place kayak is going to be about
: 360 pounds at most. That means about 6 cubic feet of water have to be
: shoved aside (displaced). On a 16 foot kayak with a 24 inch beam the
: surface area of the boat in contact with the water is about 16 square
: feet. Water pressure against that area is going to be 360/16 or under 24
: pounds per square foot. At 144 square inches to a square foot, we are
: talking about needing a material with the ability to resist penetration by
: only 1/6 th of a pound (about 2.5 ounces) per square inch.

: You would need hundreds, if not thousnads of pounds of pressure per square
: inch to get water through even the thinnest sandwich of epoxy saturated
: glass cloth and 3mm plywood.

: As long as the wood panels on a S&G design meet at angles which help to
: reinforce the strength of the structure as a whole (the difference between
: a whole egg and a portion of an eggshell) and the glue which holds them in
: place is adequate, a layer of glass is not going to add much, if anything,
: to the strength of the structure.

: So if the glass isn't adding much to the structural strength, and it is not
: needed as a waterproofing layer, then why do we need glass?

: Cedar strip boats built over an array of closely spaced ribs were built by
: Rushton over 100 years ago. They were light, and worked well.

: We have better adhesives and sealants now than Rushton did. We have computers
: and CAD-CAM equipment to precisely cut forms, and strips. So why isn't
: there much activity in building more natural products which use less
: synthetic materials?

: Perhaps I ponder too much. Some of these questions keep me up at night.

: PGJ

Paul,

If all we had to consider was loading from water, we wouldn't see much use for glass in "wood" boats, particularly plywood. But glass/epoxy does add good resistance to localized loading, eliminates the need for framing in strip boats, and provides better moisture protection than epoxy alone.

It seems to me that the popularity of SOF boats is testament to the ability to build in a style other than wood/glass/epoxy. My guess is that the level of activity outside of strip, S&G, and SOF is more an indicator that most people like to build/innovate with proven methods, rather than experiment with new stuff. It also seems that having an active community is helpful.

Sam