molds for the production of fiber reinforced plastics material boat hulls filled with hard foam plastics material are constructed of plastics material and reinforced against deformation by rigid cores and tension bands to accurately shape the hulls even when subjected to high stresses during molding and foam filling . the molds have separable top and bottom halves connected around their peripheries by interfitting flanges forming a seal and also provided with ventilating channels . the outer shell of the bottom mold half between the flanges has a free catenary curve configuration supporting the reinforcing cores for the molding surface or inner shell of the mold .