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Rule of Thumb in converting one material to another in the construction of what ever is not just a guess , it is within the limits of experience of a given trade , and proven math , applied with common since . Evan used triax on his roof and saved weight -v- mat , and retained strength , that is the advantage of foam . But if it was below the water line of the hull I suspect he would factor in puncture resistance as well as strength in his conversion , that is where common since , experience , and math is applied . I think you need at least two of those to be applied for success . Rule of Thumb math for conversion from ply to foam is " A. ratio 1 cold mold ply ( CM ) to .66 single fiberglass skin ( SFGS ) , B. SFGS x 2 = core thickness . C. .7 of SFGS = Total FG laminate . Example of 1/2" ply with 1/16" glass = .562 , 1. A. .562 x .66 = .371" SFGS , 2. B. .371 x 2 = . 742 or 5/8 "or 3/4" core thickness , 3. C. .7 x .371 = .259 total lamination thickness . 4 . .259 divided by into 2 surfaces = .129" thickness of FG laminate per side . When replacing E glass and mat with other fabrics you also have to do the math , and use common since based on things like location and intent . You also have to factor in how much risk you want to take for the reward as these conversion formulas are not meant to supersede your designers specifications , when you convert you are on your own .
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