: . . .(2 * 175 grams) per square meter * (2*3 meters squared) + ((20
: pounds per cubic foot in kilograms per cubic meter) * 3 square
: meters * 5 millimeters)
: = 6906 grams (and plainly my epoxy/fibreglass ratio must be
: appalling :)
Either that or you are building a very small boat, and only putting your epoxy on one side.
Since you stated that you would be glassing both inside and outside of the boat, then your glassing surfavce area is going to be twice that of the surface area of your wood. If your example was for 3 square meters of fiberglass cloth, then you would want to figure th density of 1.5 square meters of wood. As it is, you are figuring 3 square meters of both--so either you are over estimating the amount of wood used, or underestimating your glass and resin.
While the "best" mix of glass and resin is roughly an equal weight of each, you won't see this on any homebuilt boat. Such a mix would leave the weave visible, so we routinely slap on one or more fill coats, and just as routinely, we minimize their existence in our memories. We also tend to (conveniently?) forget that the 3 to 10 layers of varnish we so lovingly apply also has a certain weight, so the boat gets lighter everytime we talk about it :)
For a better "quick" estimate of what your boat will weigh, go with the measurement of the cloth you bought to cover it, and figure half of that as the measure of the surface area of your wood. The waste from the cloth should be about the same weight as the extra resin and varnish. Going with a slightly large figure for the wood's surface area, and consequent volume, helps to account for the coaming, back rest, hatch fittings, and other such parts. Easiest way to mdify your formula would be to insert a "/2" after your wood density, effectively making the wood's surface area half of the glass length.
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- Other: Google/wolfram unit conversion/calculation tricks