I have a couple of friends that are boat builders. One creates his own designs & molds. The other "pops" a mold from modified existing hulls. "Popping molds" is generally frowned upon amongst most builders, but this is how it's done: Cut boat into topside and hull sections. Clean, polish, fix any imperfections on hull surface. Wax hull. Spray waxed hull with gelcoat. Coat with many layers of fiberglass. Cut plywood braces (They look like stringers) to fit outside of mold and fiberglass outside "stringers" for rigidity & support. Let cure, then "pop" mold off of hull. Repeat for topside. Build boats, hoping the original mold owner doesn't sue you
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It costs a substantial amount to do this, and it's labor intensive. It's much cheaper than starting from a non-proven design, however.