Feature 4 | COMPOSITE CONSTRUCTION


aramid/glass hybrid textile to meet the design strength and stiffness require- ments affordably. Te half E-glass/half Kevlar woven cloth material selected was made in France and sourced at the time from Composite Material Supplies Ltd. Aramid (Kevlar, Twaron) is widely favoured for modern powerboats because it is highly resilient. However, the subsequent V24 builder,


NP Marine, believed that, given judicious use of unidirectional and biaxial woven fabrics, ordinary fibreglass could do the job and would be more repairable aſter the inevitable knocks incurred during racing. What is more, a standard E-glass grade was selected, avoiding the more expensive S-glass or aramid. Isophthalic polyester was the chosen resin rather than pricier vinyl-ester or epoxy. The hull and deck are moulded


separately and joined by adhesively bonding their butted edges, which are about 13mm thick. Tis join area contrib- utes significantly to the strength and rigidity of the shell structure. Te overall result is an appropriate-technology powerboat that avoids the high expense and consequent exclusivity of its big Formula 1 sisters.


Lifesavers Lessons learned from the application of composites in performance sport and leisure craft have transferred to utility craſt sectors where they benefit high-performance vessels ranging from lifeboats to fast patrol boats to stealth warships. Among those that can expect to encounter the most extreme conditions are lifeboats. Significantly, composites are widely favoured by lifeboat designers today. In the UK, the Royal National Lifeboat


Institution (RNLI) has to provide boats that are supremely strong, fast and yet, given that the RNLI is a charitable organisation that relies on donations, affordable. A fine example of this combination is provided by the Atlantic 85 rigid inflatable boat. Tis RIB behaves superbly in rough seas and can dash to distress incidents at 35knots in extreme weather. Te RNLI worked closely with structural engineers SP Systems (now part


46


Wally Yachts are known for their black carbon structures and looks, along with their high performance (courtesy Wally Yachts).


of Gurit) to combine the latest materials technology, manufacturing methods, and engineering design so as to produce a light, durable craft of top quality. Carbon-epoxy composite was chosen as the base material for the rigid hull for its combination of strength, low weight, and stiffness. Corecell high-density structural foam was specified as the filling for the sandwich structure. Production was facilitated by specify-


ing for the carbon skins SP’s SPRINT, a ‘semipreg’ material in which plies of dry carbon fabric are interleaved with sheets of resin in its beta (non-liquid but uncured) state so that, when heat is applied, the resin permeates outwards through the carbon plies and then cures with further heating. Te material can be laid into moulds


quickly and, because heated resin has only to diffuse a short distance laterally rather than in a front that must advance from one side of a lay-up to the other, wet-out is rapid and complete. According to SP, this simple, one-shot manufactur- ing process resulted in a production time saving of about 70% when compared with an Atlantic 85 prototype that had been laminated by traditional means. To further reduce costs, SP provided


complete materials kits. Both the carbon SPRINT and the Corecell foam were templated and nested into a cutting pattern to ensure optimum production


rates with minimum material wastage. Each part was labelled and the fibre orientation mapped to ensure its correct positioning within the laminate stack (NB SP is not alone in its kitting concept. Under the Smart-Pac approach from competing materials and engineering organisation High Modulus, virtually an entire boat structure can be delivered in a box). In contrast to the stiff carbon structure


of the Atlantic 85, fast rescue RIBs built by Swede Ship Composite for the Swedish Sea Rescue Society (SSRS) incorporate materials selected for their ability to flex and absorb impact shocks imposed by slamming in a seaway. Designer Rolf Eliasson considered this property important for durability in craſt designed for repeatedly dashing through rough waters at 30knots plus, to reach casualties who might be succumbing to hypother- mia in the country’s cold waters. Te SSRS was clear it wanted compos-


ites for their ability to provide a light, strong, easily driven, and manoeuvra- ble structure that would have a low draught for operation in shallow waters. But it wanted to use male moulds rather than the more refined female moulding process, and wished to avoid vacuum bagging so that damaged laminate could easily be repaired on station. These considerations led Eliasson to specify a fully cored structure and


Ship & Boat International May/June 2008


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