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In general:  Foam-sandwich construction in the following assumes the use of the pure polyvinyl-chloride rigid-elastic type of 5 to 6 lbs. per cubic foot density. Chemically blown cross-linked PVC foams as well as polyurethane foams do not have the same charateristics and will not necessarily function or react the same as pure PVC types. In foam-sandwich construction, the PVC foam is applied in sheet form over a male mold or form to make the hull of the boat. The FORM is a by-product and does not remain in the finished hull. The foam acts as a core material over which fiberglass material and resin are applied to both sides, thereby forming a rigid and strong sandwich-type of construction. See  Useful Information & Suppliers  for sources of foam core materials. Fig. 1  The form members are erected on a building form, accurately aligned, and braced to prevent movement. The traverse forms usually conform to the stations, and must be vertical, at right angles...