Feature 4 | COMPOSITE CONSTRUCTION
Pushing the performance envelope with composites
Engineered composite materials continue to enable naval architects, structural engineers, and constructors to push the boundaries of vessel performance, writes George Marsh.
craſt sector, since this is where regula- tory constraints are least and engineered structures can be tested, literally to destruction in some cases, in gruelling competitive and record-breaking events. An area where the transformational
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capacity of composites has been most marked is that of performance sail craſt. Where 10knots to 15knots used to be
considered fast, speeds of up to 40knots are now being achieved at the leading edge. As an example, consider the Volvo Extreme 40 catamaran, the latest interest of yachting diva Ellen MacArthur. Te Extreme 40 was designed to entertain sponsors and shoreside spectators at all the stopover ports of the round-the- world Volvo Ocean Race. Having clocked speeds approaching 40knots, it is one of the fastest engine-less objects afloat. Swedish constructor Marstrom
Composites AB uses high-end aerospace methods to produce these craf t, fabricating them in carbon fibre fabrics pre-impregnated with epoxy resin (prepreg). Moulded laminates are cured to a high temperature and Marstrom claims to be the only boat builder regularly using an autoclave (as distinct from an oven, which can heat items but not exert compacting pressure) to cure large series production structures - in this case the twin hulls and other components of a fleet of identical Extreme 40s that is currently nine-strong. The entire Extreme 40 structure,
including cross beams, mast, and appendages, is of carbon compos- ite. Carbonfibre/epoxy prepreg was selected as the primary build material for its combination of tensile strength,
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t the forefront of this materials and performance revolution is the advanced sport and leisure
Extreme 40 Team Basilica, winners of the iShares Cup Series 2007 (courtesy Basilica).
stiffness, fatigue resistance, and high finish quality when cured. Te two slim hulls are of sandwich construction, the outer carbon layers sandwiching Nomex aramid core. To keep costs within reason,
high-tensile carbon fibres such as Torayca T-600 with tensile modulus of approximately 240GPa are specified (Marstrom uses ultra-modulus fibres in other craſt, such as the Imoca 60 catama- ran, for the mast of which it uses fibres with tensile modulus of 440GPa). Te hull base laminate comprises, for
the outside skin, three layers of 200g/m2 unidirectional (UD) orientated at 0deg and 45deg; a core of 15mm thick 64kg/ kbm Nomex and, for the inside skin, two layers of 200g/m2
orientated at
+/-45deg. Most of the carbon is sourced from Hexcel, Epo, and the Advanced Composites Group. The Nomex (DuPont) core is supplied by Hexcel and Eurocomposites. Marstom lays up the structure in
female moulds, a process that results in a good, hydrodynamically fair surface finish with minimal need for filler. Tornadosport, the organisation that supplies Extreme 40s to customers, has provided the hull, crossbeam, rudder, and dagger board moulds. The hull mould is in two halves,
port and starboard, although the hull, including the daggerboard housing, is laid up by hand in one piece. Accord- ing to Marstrom’s head of engineering Kare Ljung, the thicker laminate in the
Ship & Boat International May/June 2008
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