: Sorry, I know this has probobly been discussed to death but I can't seem to
: find my notes. I'm in the process of designing a 14'x28"
: recreational/fishing kayak to be built hybred style. I'd like to calculate
: the weight of the ( 40 sq ft) hull but need to get an approximation of how
: much a coat of 6 oz cloth and 3 layers of epoxy add. I'm thinking about
: 2.5oz /sf. Am I off by much?
: Thanks
: Dave

At a minimum you should have about a 50/50 mix of glass cloth and resin. Most people add more resin to the outside to fill in the weave pattern and provide a smoother finish. Your cloth is going to be 6 ounces for a squuare yard, so it should take at least another 6 ounces of resin for the square yard. that is a minimum of 12 ounces for 9 square feet, or 1 1/3 ounce per square foot. If you figure that the glass on the outside gets two or three more coats of resin, then you might get up to 12 ounces of resin to a yard. That would be a total of 18 ounces (resin + fabric) per square yard, or 2 ounces per square foot. That is jsut a bit under your 2.5 ounces.

If you use glass cloth and resin on the inside and outside of the boat, figure a bit less on the weight for the inside fabric if you won't put on extra coats of epoxy.

If you use a double layer of glass cloth on the bottom, figure at least 12 ounces of resin per square yard, as you'll have 12 ounces of glass cloth, and then add on another 6 ounces for the resin to fill the weave.

Or, to make the calcualtions a bit faster. Figure a gallon of resin weighs about 8 pounds and your glass cloth will weigh about 7 pounds, so you'll have about 15 pounds of potential weight if you use all the resin and cloth.
Then add up the inside and outside glass and resin.

How critical is weight to you? Building the hull from 3mm plywood instead of 4 mm will cut the weight of the wood by 25%. Using 4 ounce cloth with some local reinforcement of two layers of cloth, instead of 6 ounce cloth will cut the weight of both cloth and resin by about 1/3, or at least 5 pounds.

Build the boat over a frame, and you can just seal the hull seams with epoxy and a strip of glass cloth. No need to reinforce the plywood as the frame will carry most of the stresses, just as it does on a skin-on-frame kayak. No glass cloth sheathing needed, and you finish the wood work by sealing with a light coat of epoxy resin, followed by clear urethane, varnish, or marine paints. That would take about 2 quarts of resin (4 pounds) and the paint or varnish -- which you would probably put over a glassed hull anyhow.

hope this helps

PGJ