Analysis of Classification Societies procedures for Structure Scantling of GRP Motoryachts

Since the first applications of composite material in ship construction the main classification societies tried to establish standard procedures for the mechanical characterization of laminates. While some of them, like Registro Italiano Navale and Lloyd’s Register, provide analytical formulas for determination of minimum values in the preliminary design phases, other classification societies, like Det Norske Veritas, American Bureau of Shipping, and Bureau Veritas, make reference to experimental data only. Existing formulas for the evaluation of ultimate stresses are both linear and polynomials, but all expressed in terms of Gc, the glass weight content of a laminate. New ISO standards draft for pleasure vessels constructions follow this philosophy as well; it appears then really important to evaluate the effectiveness of these approaches. The mechanical characteristic values coming out from classification societies rules have been compared with experimental data for some typical laminates used in the construction of motor yachts. In this paper the results of the comparison are presented: some substantial discrepancies have been found between theoretical and experimental values especially in the case of hand lay-up. This can lead to significant differences in the calculation of shell and beam geometric characteristics.