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→Sandwich Construction
Sandwich construction was introduced to reduce material cost. It uses a foam-core material, plywood or balsa wood between two layers of fibreglass. Wood of any kind is not recommended because it readily absorbs water, which will delaminate the sandwich.
Although introduced for cost reasons, a sandwich is stiffer than a single skin, and lighter for the same stiffness. Interestingly, the stiffness is not imparted by the foam-core but by the equal separation of the two skins, and the effect this has on bending forces. Another advantage of a foam sandwich is no extra insulation is required inside the hull. <ref>http://triloboats.blogspot.ca/2012/01/sea-going-sips-toward-creating-market.html</ref> <ref>http://www.nauticexpo.com/boat-manufacturer/thermal-insulation-panel-22312.html</ref> <ref>http://duflex.com.au/duflex2/</ref> Usually the final paint is applied in the gel coat.
A sandwich is not as durable as solid 'plastic', partly because more skill is required to ensure the skins bond solidly to the core without voids. Incomplete bonding and voids cause delaminating and failure of the component. These risks can be overcome with vacuum infusion. Biaxial fibreglass material is recommended with sandwich construction. Some experts advise against any kind of core below the waterline; although reasons have not been well articulated. Perhaps this was based on experience with early foam-cores.