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Advanced composites for high performance marine craft
Marine Structures 3 (1990) 111-131
Advanced Composites for High Performance Marine Craft D. S. R. N e s s & D. A. W h i l e y aStructural Polymer Systems Ltd, Love Lane, Cowes, Isle of Wight PO31 7EU, UK
ABSTRACT Advanced composite materials, as distinctfrom conventional GRP, have been used to construct high performance raceboats and marine craftfor a number of years, and these materials have come to dominate the market for racing sailboats, to the virtual exclusion of all otherforms of construction. Also, their use in advanced composite powerboat projects has shown that advanced composite construction compares very favourably with conventional structures for weight, cost, impact strength and stiffness. This paper reviews developments in the industry which have led to the building of numbers of large, high performance, advanced composite sail and powerboat hulls. In particular, developments in materials technology, manufacturing methods, structural configurations, design methods and quality assurance are discussed in detail. Advanced composites have the potential to improve most large high performance structures, and the necessary technology is proven and available as a basis for developing a wide range of very large, advanced composite structures. Key words: advanced composites, construction methods, materials, design, raceboats, quality assurance, costs.
1 INTRODUCTION A d v a n c e d composites, as distinct from c o n v e n t i o n a l G R P , have b e e n used to construct h i g h p e r f o r m a n c e raceboats a n d m a r i n e craft for a n u m b e r o f years. Starting in 1979 with the first high p e r f o r m a n c e 111 Marine Structures 0951-8339/90/$03.50©1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
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multihull built with carbon/aramid reinforcements, epoxy resins and Nomex Aramid honeycomb cores, advanced composite structures have come to dominate the market for racing sailboats, to the virtual eS~clusion of all other forms of construction. Racing and high performance powerboats still favour a conventional GRP or a welded aluminium structure, but a number of advanced composite powerboat projects have shown that advanced composite construction compares very favourably with conventional structures for weight, cost, impact strength and stiffness. This paper reviews development in the industry which have led to the building of numbers of large, high performance, advanced composite sail and powerboat hulls. In particular, developments in materials technology, manufacturing methods, structural configurations, design methods and quality assurance are discussed in detail. The emphasis is on racing sailboats, since these are still the major application for this technology, but powerboat applications are also discussed, as are possible uses for this technology outside the world of offshore yacht racing.
2 CHARACTERISTICS OF THE INDUSTRY There are several factors peculiar to the racing sailboat industry which have influenced the rapid and widespread acceptance of advanced composites. These factors, which have contributed to significant advances in the fields of design, materials and manufacturing methods, are:
(i) Desire for maximum performance within a given budget. (ii) Short project timescales - - typically less than six months from start of design to commencement of sailing trials. (iii) Concentration on a small number of racing classes and competitions with a reasonable number of individual projects in each class, e.g. yachts built to the International Offshore Rule (IOR) for competitions such as the Admiral's Cup, Maxi Raters (Maximum size rating allowed under IOR rules) for the Whitbread Round the World Race, and yachts built to the new America's Cup Rules. (iv) Increasing influence of sponsorship to fund development programmes for competitions such as the Whitbread Round the World Race and the America's Cup. When these factors are put together, a situation results which is ideal for
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rapid development. The desire for increased performance leads to a willingness to try new ideas which may give a competitive edge. A large number of projects allows a steady flow of individual small improvements with relatively small technical risk. Finally, short project timescales give rapid 'Real World' feedback on technical developments. Consequently, a considerable amount of technical development can be compressed into a short timescale. An analogous situation applies in the field of racing car development and its relationship with the volume car manufacturers. 3 NEW DEVELOPMENTS IN MATERIALS Improvements in materials technology have been directed to three main requirements: (i) Improved structural performance. (ii) Better handling and processing characteristics. (iii) Increased value for money. Developments relating to resin systems, reinforcements and core materials are discussed below. 3.1 Resin systems
Epoxy resin systems have replaced polyesters as the preferred resin system for high performance structures. Significantly better adhesion, higher strain to failure, longer working times, and the elimination of degradation due to osmosis have been the main reasons for the change to epoxies. Increased cost of epoxies compared to polyester resins has not been significant when compared to costs for a complete project and the performance benefits which can be achieved (Table 1). Early developments of epoxy resin systems had a number of drawbacks. Particular problems were high viscosities making it difficult to wet out reinforcements, various health and safety problems, and limited working times. Current epoxy resin systems which have been developed for hand layup of large components now have much improved health and safety characteristics, much lower viscosities allowing good wet out of tightly woven or heavyweight reinforcements, and gel times of up to 12 h. Table 2 and Fig. 1 show data for a typical epoxy optimised for large vacuum bag layups. Figure 1 shows how the viscosity time history has been modified compared to a normal wet layup system. It can be seen
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TABLE 1 Project Costs
/OR 1 tonner Leading dimensions Length (m) D i s p l a c e m e n t (tonnes) Structure weight (kg) Project costs Structural design Materials Labour Equipment Fully fitted on water Research & Development Ca mpaign Totalproject costs N u m b e r of boats in p r o g r a m m e D o m i n a n t Cost o
Whitbread Maxi
12 5 500 £3000 £28 000 £100 000 -£200 000 Nil
America's Cup
Production powerboat
22 a 2-3000
13 11 3500
24 36 6000
£10 000 £18 000 £150-160 000 £250 000 £400 000 ---£1-1-5 million £1.1 million £1-4.5 million £6-14 million
£50 000 £250 000 1
£2-6 million £10-20 million 1 4
Boat
Campaign
R&D
£20 £50 £280 £120 £450
000 000 000 000 000
Nil £15 million 30 Production
data not available.
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Advanced composites for high performance marine craJ~ TABLE 2 Epoxy Resin Mechanical Data (a) Typical mechanical properties achievable with different cure schedules Cure schedule (time/temp)
Hardener used Flexural strength (MPa) Flexural modulus (GPa) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%)
5 h/80°C
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4 weeks/18-30°C
Slow 1:1 Fast Slow 1:1 Fast Slow 1:1 Fast 161 160 159 160 157 187 102 130 135 3.2 3.6 3.6 3.7 3.8 3.7 3-6 3.7 3.7 78 77 74 80 80 87 63 69 77 3-1 3.3 3.3 3.4 3-4 3.4 3.6 3.6 3.6 6.4 7.2 7.0 5.6 5.8 5.5 3.3 3-4 3-4
(b) Typical component properties Property
Resin
Appearance Viscosity cps 25°C Specific gravity
Water-white liquid 730 1.15
Hardener Fast
Slow
Pale amber 1750 1-06
Dark amber 210 1.01
(c) Typical mixed system properties Property
Mixed viscosity cps Gel time (150 g @ 25°C) min Tg 2 (after 5 h @ 80°C) °C Tg2 (ultimate) °C Inter-laminar shear stress (13 plies AR250) MPa
Hardener Fast
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940 15 78 82 N/A
510 200 84 96 61
that the viscosity rise d u e to resin g e l a t i o n is d e l a y e d to give the m a x i m u m w o r k i n g w i n d o w p r i o r to c o n s o l i d a t i n g the l a m i n a t e with a v a c u u m bag. At the p r e s e n t state o f d e v e l o p m e n t , the m o s t c o m m o n f o r m o f d a m a g e in the l a m i n a t e p r i o r to failure is m i c r o c r a c k i n g , a n d it o c c u r s w h e n the resin c r a c k s a w a y f r o m the fibres a n d t h e s e c r a c k s s p r e a d t h r o u g h the resin. It c a n o c c u r at strain v a l u e s significantly b e l o w t h o s e a c h i e v e d at u l t i m a t e failure for the l a m i n a t e . C u r r e n t w o r k o n i m p r o v i n g structural p e r f o r m a n c e o f l a m i n a t e s
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includes an examination of the effects of resin microcracking on environmental degradation of laminates, and the implications of microcracking on design allowables for mechanical strength, particularly for fatigue loadings. Also, ways of modifying epoxy resins to delay the onset of microcracking are being investigated. For brittle resin systems, such as most polyesters, this point is a long way before laminate failure. Recent tests have shown that for a polyester/ glass woven roving laminate, microcracking typically occurs at about 0.2% strain with ultimate failure not occurring until 2.0% strain. As the ultimate strength of a laminate in tension is governed by the fibres, these microcracks do not immediately lower the ultimate properties of the laminate. However, in an environment such as water or moist air, the microcracked laminate will absorb considerably more water than an uncracked laminate. This will then lead to an increase in weight, moisture attack on the resin and fibre sizing agents, loss of stiffness and a subsequent drop in ultimate properties. As can be seen from the above, for a laminate to be used in a marine environment, it is vital that these microcracks do not occur under normal working loads. Work from various sources has shown that factors which affect the microcracking performance are resin toughness, resin/fibre interfacial adhesion, level of residual stresses on cure due to shrinkage on cure, and weave depth and fibre crimp. Generally, epoxy systems perform better than polyester systems on the first three factors. Fatigue of composite laminates is an area requiring much work, both to understand the mechanisms involved, and to derive fatigue design allowables for particular fibre/resin combinations and fibre orientations. Stress cycling of a laminate to levels above the onset of microcracking will obviously generate cumulative fatigue damage in the laminate. There is some evidence to suggest that there is a link between the strain at onset ofmicrocracking and the residual strain to failure after high cycle fatigue.
3.2 Reinforcements
Improvements in reinforcements have mainly concentrated on fibre hybridisation to achieve the desired combination of properties, and the development of modified forms of reinforcement construction for better matching with the production processes. Stitched multiaxial fabrics allow much heavier weights of reinforcement to be laid down in each layer. E glass reinforcement weights of 2400 g/m 2 have been used successfully, giving a laminate thickness of 2. 5 m m per layer. It would be difficult to manufacture a woven material
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of this weight and it would have unacceptable fibre crimp. Also, bunching of the fibre bundles would inhibit full wetting out with the resin system. With the stitched multiaxial, fibre crimp is minimal, drapeability is good, there is no fibre bunching to inhibit wetout even on the heaviest reinforcements, and the facility to be able to combine several fibre directions (e.g. 0/90/+45) in a single heavyweight layer can save considerable time on thick laminates. A further development being considered for the stitched multiaxials is fibre hybridisation with different fibres being used in each layer, or even combining two or more fibres within a single layer of the multiaxial. Current fibre combinations being investigated are Carbon/Ultra High Molecular Weight Polyethylene (UHMPE) for improved impact resistance at lower cost. Unidirectionals are also now available in heavyweight form (typically up to 800 g/m 2) and widths up to 0.5 m. Several forms of construction are available, the choice depending on the required handling characteristics. For pre-pregging, a continuous filament web over the entire surface of the reinforcement is used to hold the fibre tows in place. For wet layup, a construction using an open, crimp cancelling weft insertion to hold the fibre tows is generally recommended. With both forms of construction, fibre crimp is minimal and the fibres remain parallel during handling, which is very important when draping long lengths of reinforcement over large hull mouldings.
3.3 Pre-impregnated systems In the one-off construction world, the latest 'state-of-the-art' for marine advanced composites is the use of low temperature curing epoxy 'prepreg' materials. The system can be used with unidirectional, multiaxial and woven reinforcements in carbon, glass, aramid and hybrids of these fibres. Until recently, pre-impregnated materials have been based on 120°C curing epoxy systems and have required high processing pressures to ensure adequate resin flow and consolidation in the laminate. These processing characteristics have limited the successful application of 120°C pre-pregs mainly to the aerospace industry, since expensive tooling and heating equipment, usually in the form of large pressurised ovens (autoclaves), is required. The largest available autoclaves will generally not take a 25 m boat hull. The current generation of low temperature curing epoxy pre-pregs have curing temperatures of 75°C, and require consolidation under vacuum pressure only. Tooling requirements are also simplified. This level of technology can be readily achieved by a good boatyard.
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Pre-pregs are attractive for a number of reasons: (i) Superior laminate properties are achievable, because better resin systems, which may not be suitable for hand layup, can be used. (ii) Control available over fibre/resin ratio in manufacture, allowing a higher fibre volume fraction to be achieved, and giving significantly higher mechanical properties. (iii) Resin and hardener are pre-mixed and reinforcements are already fully wet-out, improving the quality of construction, reducing variability, improving weight control, and improving the health and safety aspects of construction. (iv) For the designer, it is possible to specify sophisticated, optimised layups, since the 2-3 day working time will give the builder time to build up correctly a complex ply sequence. The first application of this technology was the large multihull yacht 'Steinlager 1' (Fig. 2). To date, the largest project built using the same technology has been a 25 m Maxi yacht called 'Windward Passage', (Fig. 3). It is likely that this hull is the largest single-piece pre-preg structure ever built, the previous record being held by the loading bay doors on the NASA Space Shuttle. This yacht out-performed the existing
Fig. 2. Steinlager I.
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Fig. 3. Windward passage. By R. Tomlinson.
Maxi Fleet to such an extent that the aluminium structured yachts have become obsolete. Details of the construction methods used on a project such as this are discussed in Section 4. 3.4 Core materials
Sandwich structures are used almost exclusively for high performance hulls. The reasons for using sandwich instead of solid skin laminates with frames and stringers is discussed in Section 5. Both foam and honeycomb cores are used, depending on the loading conditions. A typical yacht hull may use honeycomb for the majority of the hull, with foam below the waterline from the keel forwards to resist slamming loads. As honeycomb properties improve, these areas of foam may be replaced by honeycomb. Powerboats generally use foam cores for the higher mechanical properties and better resistance to impact damage. The current core materials are: • Rigid crosslinked PVC. • Linear (non crosslinked) PVC. • Nomex Aramid fibre/Phenolic honeycomb.
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Foam densities range from 60 to 250 kg/m 3, with the linear foams having higher toughness, but slightly lower mechanical properties, compared to a rigid PVC of the same density. Typical Nomex densities are 48 and 64 kg/m 3.
4 MANUFACTURING METHODS A well defined manufacturing process has been developed for the construction of large one-off racing hulls using sandwich structures. Typical features of the build process for a large racing hull similar to 'Windward Passage' and built with low-temperature curing epoxy prepreg are as follows: (i) Wood male hull plug sheathed in glass/epoxy, faired and painted. (ii) Prefabricated tent or box oven with hot air blowers and recirculating fans. Heater blankets can also be used. (iii) High capacity vacuum system for rapid air removal from the vacuum bag, and for coping with small leaks. (iv) Inexpensive low temperature vacuum bag and disposables such as peel plys, bleed cloths and breather fabrics. (v) Adequate number ofthermocouples on the laminate to monitor curing temperature at each cure stage. At 75°C it is possible to work for short periods inside the oven, checking the temperature distribution with a hand-held temperature probe when the oven is being calibrated. (vi) Three-stage lay-up p r o c e s s - inner skin, core bonding and outer skin, with a vacuum bag cure at each stage. (vii) During the cure cycle, as the resin temperature rises, its viscosity drops dramatically, and under vacuum pressure it flows and thoroughly wets out the fibres and the laminate is consolidated. After a short time the resin starts to gel and full cure is achieved typically after 5 h at 75°C. (viii) Main advantage of using a male plug instead of a female mould is speed of build and reduced cost for a one-off. It is easier to achieve good fibre alignment when draping reinforcements over a male plug. Also, fitting of thick core materials (typically 50 mm for a Maxi) is much easier, and less core bonding adhesive is required. (ix) Hull outer laminate is faired and finished with lightweight epoxy syntactic foams, followed by priming and painting. The
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quality of the outer laminate surface is improved by being careful with reinforcement overlaps, and fairing is now a rapid operation which adds very little weight to the hull. In a production situation, where a number of hulls is required, similar manufacturing methods can be used. In this situation the wood male plug is made to the hull outside profile, and production glass/epoxy female moulds are taken from this plug. An alternative method for curing the pre-preg is to use heated moulds incorporating an electrical resistance heater mat laminated into the mould layup. Fairing and finishing on the outside of the hull is no longer necessary, but time saved on this operation is offset by increased difficulty in working with laminates and cores in a female mould.
5 DESIGN ASPECTS A full understanding of composite materials technology and manufacturing techniques, as well as skill in designing with highly anisotropic materials, are all necessary if one wants to exploit the considerable potential of advanced composite materials. This integrated approach, covering structural engineering, resin development, reinforcement design, mechanical testing, process engineering and liaison with the builder, ensures that the design on the drawing board can become a practical reality.
5.1 Structural design Designing a high performance structure requires design for structural integrity and consideration of performance-enhancing features of the design. When designing for structural integrity in a sailing yacht, the main loading cases considered are fore and aft hull bending due to rig loads, transverse rigging loads, sail control loads, keel and crew weight righting moment, slamming wave pressures and rudder loadings. For a powerboat, slamming, impact, rudder and inertia loads are likely to be the critical cases. Performance-enhancing features for a sailing yacht are low structural weight for reduced hull drag, light bow and stern sections for reduced radius of gyration in pitch, and high hull bending and torsional stiffness for better rig and sail control. For a powerboat, reduced weight means better acceleration and higher speeds. If the weight saving is significant
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in the hull, this can have a knock-on effect, allowing a reduction in engine size and fuel capacity. Most racing yachts use a h o n e y c o m b sandwich structure for hull and deck with a small n u m b e r of internal bulkheads or frames and no stringers. The keel is usually attached to a longitudinal girder structure which can be a l u m i n i u m alloy or composite. Hull layup is mainly carbon U / D or c a r b o n / a r a m i d U / D with a layer of c a r b o n / a r a m i d hybrid fabric for impact resistance. A typical layup is shown in Fig. 4. Powerboat hulls use different structures depending on the application. A particular challenge was posed by an assault craft, the Smuggler 384, as this craft had a prime requirement that it could be driven on to a beach fully loaded at a speed of 15 kts. For this craft, slamming, impact a n d abrasion as well as high speed and manoeuvrability are the design drivers, and a foam sandwich structure with glass/carbon/aramid/epoxy laminates is most suitable. As with the racing yachts, the m i n i m u m a m o u n t of framing is used 1.2 (Fig. 5). A high performance lifeboat such as the R N L I Fast Carriage Boat (Mersey Class) requires more substantial bulkheads for flooding considerations. Speeds are lower, so s l a m m i n g pressures are reduced, but impact protection with resistance to hull penetration becomes important. Production requirements also influence materials selection and heavyweight multiaxial pre-pregs are used with a thick foam core. A production run of these hulls justifies the use of female moulds.
5.2 New structural concepts As higher performance composite materials have been developed, skin thicknesses in hulls and decks have been progressively reduced. This has led to the situation where a closely framed structure has become less attractive, since the skin thicknesses b e c o m e unacceptably thin and delicate.
5.2.1 Monocoque hull structure An alternative is the ' m o n o c o q u e ' concept which allows material to be taken out of the internal frames a n d bulkheads and some of it to be put back into the skins. This concept has been developed on projects such as the Smuggler 384, and applied successfully on projects such as ' W i n d w a r d Passage'. The reasoning b e h i n d the m o n o c o q u e concept is that s l a m m i n g loads are an energy type of impact. W h e n loaded, a hull panel will elastically absorb a n d then release the strain energy. A closely framed structure is stiff, so for a given energy applied, deflections will be low, and loads high.
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Conversely, if panel sizes are increased, the structure b e c o m e s more resilient and the higher deflections reduce peak loads. Also it is k n o w n that s l a m m i n g pressures peak only on a small area, and reduce w h e n the panel size is increased. 3"4
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Fig. 5. Smuggler 384.
As the panel size increases, core shear can become critical. However, as Fig. 6 shows, a small increase in core shear strength allows a significant increase in allowable panel span. In the monocoque concept, a higher density core (higher shear strength) is used, and core thickness is also increased. Core thickness is typically 50 mm on a Maxi. Additional transverse unidirectional reinforcements are incorporated in the hull shell layup. With this construction method, the frames can be assumed to be built into the shell, and the result is high strength with much higher energy absorption. As an example of the potential weight savings for this monocoque concept in advanced composites, the all-up structural weight of a raceboat such as 'Windward Passage' is less than 4000 kg, compared to a conventional closely framed aluminium construction weight of 8500 kg. An additional benefit of this form of construction is that elimination of frames and bulkheads results in a significant reduction in layup and assembly time.
5.2.2 Foam supported shell A further development which can be used in conjunction with a monocoque structure is the foam supported shell for hull structures.
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5.3 Loads and design allowables Design allowables for laminate mechanical properties are generally derived by following the aerospace practice for analysing test data. Since most variability is found in strength values, mean values are used for moduli, and strength data is analysed on a statistical basis to derive design allowables for a given confidence level. These statistically derived allowables take into account actual laminate properties achieved in manufacture. It may also be necessary to include a factor to account for variability in production. Proof and ultimate factors applied to the limit load cases allow for variability in specification of loads and accuracy of structural idealisation during analysis. Different reserve factors may be used, depending on the particular application. A different situation applies with, say, a flat-out raceboat designed for 'round the buoys' inshore racing with safety boats in attendance, compared to a lifeboat designed for arduous service over a long working life. In the former case, the operating environment is more controlled, and the consequences of structural failure are generally not life threatening. Consequently, it is accepted that reserve factors and margins can be reduced as a trade-off for increased performance. Specifying accurate sailing loads for hulls is still problematical, particularly slamming, impact and inertia loads. Until these have been better quantified it will be difficult to get away from empirical development or conservative design based on scantling rules and design structures which exploit the full potential of the technology. One of the spin-offs from the America's Cup programme may be detailed information on hull and rig loads. Each syndicate is planning a programme based on up to four boats. The first boats will be heavily instrumented 'trial horses' with strain gauged hulls and rigging. They are intended to generate this type of loading information to allow subsequent boats in the programme to be designed with structures more closely matched to realistic loads. While some of these spin-offs, financed by sponsors of America's Cup programmes, may read across to the needs of power craft, parallel investment in instrumentation in the latter field, especially for defence applications, is equally necessary and is overdue.
5.4 Computer aided design and drafting Designs for structures such as monocoque hulls have developed partly by empirical means, improving the concept over a number of projects,
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based on feedback from the sailing trials or the racecourse. Detailed analysis of monocoque sandwich hulls with complex and highly anisotropic layups is only possible using Finite Element (FE) techniques. The development of thick sandwich shell elements and the availability of FE packages to run on desktop computers will allow more use of FE as an interactive design tool to develop the structural configuration. So far, FE analysis has been generally applied only when much of the structural design has been finalised. The use of powerful analytical tools in the design process is no substitute for having good design ideas in the first place, but it should help in developing ideas into workable structures. Conventional engineering drawing methods have developed around metallic materials and there are no conventions for drawing complex, multi-layered components. Computer drafting programs are ideally suited to the production of layup drawings for composites since they work on a layered structure. Each layer can be developed individually, while maintaining an overview of the complete structural design as it develops. Once design is completed, a hard copy can be printed for every layer of the drawing. Each sheet of the drawing will show the orientation and extent of an individual layer of reinforcement or core material (Fig. 4 ) . A simple process control technique for a complex layup is to produce the individual layer drawings in booklet form in the correct layup sequence. By working through the book, the builder can reduce the chances of missed layers or an incorrect sequence.
6 QUALITY ASSURANCE When considering quality assurance for raceboat construction, the main consideration is the development of a procedure which is suitable for one-off construction. The main features of such a procedure are: (i) Materials and manufacturing methods reasonably tolerant to variations in processing conditions. (ii) Skilled workforce, experienced in the use of materials and techniques. (iii) Clearly defined, documented and controlled process. (iv) Visual inspection at each layup stage to ensure that any faults are found at a stage when they can be rectified. (v) Monitoring of resin mix ratios, cure cycle and vacuum pressure during construction.
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(vi) Analysis of cured resin samples using a Differential Scanning Calorimeter (DSC) to assess accuracy of mix ratios and cure state. (vii) Mechanical testing of laminate samples taken from component. Ultrasonic or other forms of non-destructive testing of the finished component is generally not possible, due to cost, the large surface areas to be covered, and the difficulty in calibrating equipment for a one-off layup. A tap test with a coin is still a good alternative for detecting debonds, although it will not detect small voids within the laminate. Wet layup is the most difficult process to monitor since the laminate quality depends very much on the skill of the laminators. This situation can be improved by pre-impregnating reinforcements with the laminating resin using a simple impregnating machine consisting of a resin bath and nip rollers. This can improve consistency of wet/out and fibre/resin ratios. The use of pre-preg further reduces the level of layup skill required, but the process engineering capability must be higher, requiring more know-how, planning and equipment. Core bonding is another difficult area to check during construction. Foam cores have holes punched through to ensure that no air is trapped under the foam. This can be inspected after cure by checking that resin has flowed up through the holes during vacuum bagging of the core. Honeycomb adhesive fillets can be checked visually after bonding to the inner skin. For the outer skin layup, a single lightweight woven fabric is first bonded to the honeycomb. After vacuum bagging and cure, this layer is visually inspected for lack of bonding to the honeycomb. Any rectification is carried out prior to laying up the remainder of the outer skin layup. In production, all of the preceding quality assurance techniques can be used with the following additional techniques: (i) Pre-cut kits of reinforcement, pre-preg and core material. (ii) Mixer pumps for dispensing resin or adhesive mixed with hardener. (iii) Monitoring of resin, reinforcement and adhesive usage. (iv) Weighing of finished, trimmed components to monitor weight variations. (v) Ultrasonic and other forms of non-destructive testing. (vi) Measurement of void content and fibre volume fraction on laminate samples. (vii) Mechanical testing on components from production - - either by testing to destruction on complete component, or cutting up for
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various mechanical tests, followed by correlation with mechanical tests on laminate samples and DSC results.
7 PROJECT COSTS Table 1 shows the project cost breakdown for a representative range of advanced composite boats. The leading dimensions for each boat are also included to allow a size comparison. Some points to note are: (i) The contribution of material costs to total project costs is small even though some of the projects use the highest performance and, therefore, the most expensive advanced composite materials available. (ii) Man hours costs are greater than materials costs, so selecting materials and construction methods which reduce layup and assembly times can be more cost effective than going for the lowest cost materials available. (iii) Dominant cost is different for each of the projects. (iv) A new America's Cup rule has replaced the '12 Metre' rule based on aluminium hulls; the rule is now written around advanced composite hulls with sandwich structures similar to the current Maxi/IOR boats.
8 VALUE ANALYSIS ON MATERIALS When designing in advanced composites, the materials can be assessed against various selection criteria such as: (i) Specific performance requirements - - strength/unit weight, stiffness/unit weight, impact strength, etc. (ii) Suitability for proposed manufacturing methods. (iii) Cost to achieve a specific requirement. As discussed previously, when comparing materials options the real cost of a laminate should include resin and reinforcements plus processing costs, which include man-hours costs for layup as well as tooling and equipment, disposables costs, curing costs, etc. Figure 7 shows a comparison of specific material properties for various reinforcements with epoxy resin. Also a value analysis has been carried out to show what level of performance can be achieved with each
D. S. R. Ness, D. A. Whiley
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material for a given cost. The cost is based on reinforcement plus resin prices plus man-hours costs for layup. For simplicity, ancillary processing costs such as tooling, disposables and cure, etc., have been omitted. Figure 7 gives some indication of the best materials options for this specific requirement. In some cases, pre-pregs are shown to be cheaper when layup costs are included. For a direct tensile load, U / D carbon is shown to be the most cost effective solution. This conclusion might not be expected since this is the most expensive of the chosen materials. For other structural requirements such as shear stiffness, or for a more isotropic laminate, a different material may be more cost effective. However, this graph shows that basic raw material cost can be a misleading criterion when selecting the most cost effective material to do a particular job. 9 CONCLUSIONS This article has reviewed the state-of-the-art in the advanced composite high performance marine craft design and construction. The industry
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has developed the skills and materials for designing and manufacturing large one-off high performance advanced composite structures. There will be further developments in raceboat technology, driven by high budget programmes such as the America's Cup competition but similar investment in other areas, especially the defence field, is vital to ensure that the technology is not restricted to the one-off raceboat industry. If this investment is forthcoming, the following important developments of the technology can be expected: (i) Use of the technology in a production environment: this is starting to happen with high performance lifeboats, assault craft and production racing powerboats. (ii) Application to other high performance marine craft, e.g. surfaceeffect catamarans. (iii) Use of prototyping skills for advanced composite production prototypes. (iv) Application of the technology outside the marine industry, e.g. for offshore and wind energy structures. Advanced composites have the potential to improve most large high performance structures; the necessary technology is now proven and available for design and fabrication of very large composite structures.
REFERENCES 1. Editorial, Ship & Boat International, Dec 1986. 2. Editorial, Ship & Boat International, Jan/Feb 1987. 3. Allen, R. G. & Jones, R. R. A simplified method for determining structural design limit pressures on high performance marine vehicles. In Proceedingsof the AIAA/SNAME Advanced Marine Vehicles Conference, San Diego, 1978. 4. Heller, S. R. & Jasper, N. H. On the structural design of planing craft. Trans., Royal Institute of Naval Architects, 103 (1961) p. 49. 5. Cripps, R. M. Pressure and impact tests carded out on panels of different materials, RNLI Report, June 1987. 6. Paffett, J. A. H. The Material Revolution, The Lifeboat, Spring 1989.
Advanced Composites for High Performance Marine Craft D. S. R. N e s s & D. A. W h i l e y aStructural Polymer Systems Ltd, Love Lane, Cowes, Isle of Wight PO31 7EU, UK
ABSTRACT Advanced composite materials, as distinctfrom conventional GRP, have been used to construct high performance raceboats and marine craftfor a number of years, and these materials have come to dominate the market for racing sailboats, to the virtual exclusion of all otherforms of construction. Also, their use in advanced composite powerboat projects has shown that advanced composite construction compares very favourably with conventional structures for weight, cost, impact strength and stiffness. This paper reviews developments in the industry which have led to the building of numbers of large, high performance, advanced composite sail and powerboat hulls. In particular, developments in materials technology, manufacturing methods, structural configurations, design methods and quality assurance are discussed in detail. Advanced composites have the potential to improve most large high performance structures, and the necessary technology is proven and available as a basis for developing a wide range of very large, advanced composite structures. Key words: advanced composites, construction methods, materials, design, raceboats, quality assurance, costs.
1 INTRODUCTION A d v a n c e d composites, as distinct from c o n v e n t i o n a l G R P , have b e e n used to construct h i g h p e r f o r m a n c e raceboats a n d m a r i n e craft for a n u m b e r o f years. Starting in 1979 with the first high p e r f o r m a n c e 111 Marine Structures 0951-8339/90/$03.50©1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain.
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multihull built with carbon/aramid reinforcements, epoxy resins and Nomex Aramid honeycomb cores, advanced composite structures have come to dominate the market for racing sailboats, to the virtual eS~clusion of all other forms of construction. Racing and high performance powerboats still favour a conventional GRP or a welded aluminium structure, but a number of advanced composite powerboat projects have shown that advanced composite construction compares very favourably with conventional structures for weight, cost, impact strength and stiffness. This paper reviews development in the industry which have led to the building of numbers of large, high performance, advanced composite sail and powerboat hulls. In particular, developments in materials technology, manufacturing methods, structural configurations, design methods and quality assurance are discussed in detail. The emphasis is on racing sailboats, since these are still the major application for this technology, but powerboat applications are also discussed, as are possible uses for this technology outside the world of offshore yacht racing.
2 CHARACTERISTICS OF THE INDUSTRY There are several factors peculiar to the racing sailboat industry which have influenced the rapid and widespread acceptance of advanced composites. These factors, which have contributed to significant advances in the fields of design, materials and manufacturing methods, are:
(i) Desire for maximum performance within a given budget. (ii) Short project timescales - - typically less than six months from start of design to commencement of sailing trials. (iii) Concentration on a small number of racing classes and competitions with a reasonable number of individual projects in each class, e.g. yachts built to the International Offshore Rule (IOR) for competitions such as the Admiral's Cup, Maxi Raters (Maximum size rating allowed under IOR rules) for the Whitbread Round the World Race, and yachts built to the new America's Cup Rules. (iv) Increasing influence of sponsorship to fund development programmes for competitions such as the Whitbread Round the World Race and the America's Cup. When these factors are put together, a situation results which is ideal for
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rapid development. The desire for increased performance leads to a willingness to try new ideas which may give a competitive edge. A large number of projects allows a steady flow of individual small improvements with relatively small technical risk. Finally, short project timescales give rapid 'Real World' feedback on technical developments. Consequently, a considerable amount of technical development can be compressed into a short timescale. An analogous situation applies in the field of racing car development and its relationship with the volume car manufacturers. 3 NEW DEVELOPMENTS IN MATERIALS Improvements in materials technology have been directed to three main requirements: (i) Improved structural performance. (ii) Better handling and processing characteristics. (iii) Increased value for money. Developments relating to resin systems, reinforcements and core materials are discussed below. 3.1 Resin systems
Epoxy resin systems have replaced polyesters as the preferred resin system for high performance structures. Significantly better adhesion, higher strain to failure, longer working times, and the elimination of degradation due to osmosis have been the main reasons for the change to epoxies. Increased cost of epoxies compared to polyester resins has not been significant when compared to costs for a complete project and the performance benefits which can be achieved (Table 1). Early developments of epoxy resin systems had a number of drawbacks. Particular problems were high viscosities making it difficult to wet out reinforcements, various health and safety problems, and limited working times. Current epoxy resin systems which have been developed for hand layup of large components now have much improved health and safety characteristics, much lower viscosities allowing good wet out of tightly woven or heavyweight reinforcements, and gel times of up to 12 h. Table 2 and Fig. 1 show data for a typical epoxy optimised for large vacuum bag layups. Figure 1 shows how the viscosity time history has been modified compared to a normal wet layup system. It can be seen
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TABLE 1 Project Costs
/OR 1 tonner Leading dimensions Length (m) D i s p l a c e m e n t (tonnes) Structure weight (kg) Project costs Structural design Materials Labour Equipment Fully fitted on water Research & Development Ca mpaign Totalproject costs N u m b e r of boats in p r o g r a m m e D o m i n a n t Cost o
Whitbread Maxi
12 5 500 £3000 £28 000 £100 000 -£200 000 Nil
America's Cup
Production powerboat
22 a 2-3000
13 11 3500
24 36 6000
£10 000 £18 000 £150-160 000 £250 000 £400 000 ---£1-1-5 million £1.1 million £1-4.5 million £6-14 million
£50 000 £250 000 1
£2-6 million £10-20 million 1 4
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Campaign
R&D
£20 £50 £280 £120 £450
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Advanced composites for high performance marine craJ~ TABLE 2 Epoxy Resin Mechanical Data (a) Typical mechanical properties achievable with different cure schedules Cure schedule (time/temp)
Hardener used Flexural strength (MPa) Flexural modulus (GPa) Tensile strength (MPa) Tensile modulus (GPa) Elongation at break (%)
5 h/80°C
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Slow 1:1 Fast Slow 1:1 Fast Slow 1:1 Fast 161 160 159 160 157 187 102 130 135 3.2 3.6 3.6 3.7 3.8 3.7 3-6 3.7 3.7 78 77 74 80 80 87 63 69 77 3-1 3.3 3.3 3.4 3-4 3.4 3.6 3.6 3.6 6.4 7.2 7.0 5.6 5.8 5.5 3.3 3-4 3-4
(b) Typical component properties Property
Resin
Appearance Viscosity cps 25°C Specific gravity
Water-white liquid 730 1.15
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Slow
Pale amber 1750 1-06
Dark amber 210 1.01
(c) Typical mixed system properties Property
Mixed viscosity cps Gel time (150 g @ 25°C) min Tg 2 (after 5 h @ 80°C) °C Tg2 (ultimate) °C Inter-laminar shear stress (13 plies AR250) MPa
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Slow
940 15 78 82 N/A
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that the viscosity rise d u e to resin g e l a t i o n is d e l a y e d to give the m a x i m u m w o r k i n g w i n d o w p r i o r to c o n s o l i d a t i n g the l a m i n a t e with a v a c u u m bag. At the p r e s e n t state o f d e v e l o p m e n t , the m o s t c o m m o n f o r m o f d a m a g e in the l a m i n a t e p r i o r to failure is m i c r o c r a c k i n g , a n d it o c c u r s w h e n the resin c r a c k s a w a y f r o m the fibres a n d t h e s e c r a c k s s p r e a d t h r o u g h the resin. It c a n o c c u r at strain v a l u e s significantly b e l o w t h o s e a c h i e v e d at u l t i m a t e failure for the l a m i n a t e . C u r r e n t w o r k o n i m p r o v i n g structural p e r f o r m a n c e o f l a m i n a t e s
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includes an examination of the effects of resin microcracking on environmental degradation of laminates, and the implications of microcracking on design allowables for mechanical strength, particularly for fatigue loadings. Also, ways of modifying epoxy resins to delay the onset of microcracking are being investigated. For brittle resin systems, such as most polyesters, this point is a long way before laminate failure. Recent tests have shown that for a polyester/ glass woven roving laminate, microcracking typically occurs at about 0.2% strain with ultimate failure not occurring until 2.0% strain. As the ultimate strength of a laminate in tension is governed by the fibres, these microcracks do not immediately lower the ultimate properties of the laminate. However, in an environment such as water or moist air, the microcracked laminate will absorb considerably more water than an uncracked laminate. This will then lead to an increase in weight, moisture attack on the resin and fibre sizing agents, loss of stiffness and a subsequent drop in ultimate properties. As can be seen from the above, for a laminate to be used in a marine environment, it is vital that these microcracks do not occur under normal working loads. Work from various sources has shown that factors which affect the microcracking performance are resin toughness, resin/fibre interfacial adhesion, level of residual stresses on cure due to shrinkage on cure, and weave depth and fibre crimp. Generally, epoxy systems perform better than polyester systems on the first three factors. Fatigue of composite laminates is an area requiring much work, both to understand the mechanisms involved, and to derive fatigue design allowables for particular fibre/resin combinations and fibre orientations. Stress cycling of a laminate to levels above the onset of microcracking will obviously generate cumulative fatigue damage in the laminate. There is some evidence to suggest that there is a link between the strain at onset ofmicrocracking and the residual strain to failure after high cycle fatigue.
3.2 Reinforcements
Improvements in reinforcements have mainly concentrated on fibre hybridisation to achieve the desired combination of properties, and the development of modified forms of reinforcement construction for better matching with the production processes. Stitched multiaxial fabrics allow much heavier weights of reinforcement to be laid down in each layer. E glass reinforcement weights of 2400 g/m 2 have been used successfully, giving a laminate thickness of 2. 5 m m per layer. It would be difficult to manufacture a woven material
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of this weight and it would have unacceptable fibre crimp. Also, bunching of the fibre bundles would inhibit full wetting out with the resin system. With the stitched multiaxial, fibre crimp is minimal, drapeability is good, there is no fibre bunching to inhibit wetout even on the heaviest reinforcements, and the facility to be able to combine several fibre directions (e.g. 0/90/+45) in a single heavyweight layer can save considerable time on thick laminates. A further development being considered for the stitched multiaxials is fibre hybridisation with different fibres being used in each layer, or even combining two or more fibres within a single layer of the multiaxial. Current fibre combinations being investigated are Carbon/Ultra High Molecular Weight Polyethylene (UHMPE) for improved impact resistance at lower cost. Unidirectionals are also now available in heavyweight form (typically up to 800 g/m 2) and widths up to 0.5 m. Several forms of construction are available, the choice depending on the required handling characteristics. For pre-pregging, a continuous filament web over the entire surface of the reinforcement is used to hold the fibre tows in place. For wet layup, a construction using an open, crimp cancelling weft insertion to hold the fibre tows is generally recommended. With both forms of construction, fibre crimp is minimal and the fibres remain parallel during handling, which is very important when draping long lengths of reinforcement over large hull mouldings.
3.3 Pre-impregnated systems In the one-off construction world, the latest 'state-of-the-art' for marine advanced composites is the use of low temperature curing epoxy 'prepreg' materials. The system can be used with unidirectional, multiaxial and woven reinforcements in carbon, glass, aramid and hybrids of these fibres. Until recently, pre-impregnated materials have been based on 120°C curing epoxy systems and have required high processing pressures to ensure adequate resin flow and consolidation in the laminate. These processing characteristics have limited the successful application of 120°C pre-pregs mainly to the aerospace industry, since expensive tooling and heating equipment, usually in the form of large pressurised ovens (autoclaves), is required. The largest available autoclaves will generally not take a 25 m boat hull. The current generation of low temperature curing epoxy pre-pregs have curing temperatures of 75°C, and require consolidation under vacuum pressure only. Tooling requirements are also simplified. This level of technology can be readily achieved by a good boatyard.
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Pre-pregs are attractive for a number of reasons: (i) Superior laminate properties are achievable, because better resin systems, which may not be suitable for hand layup, can be used. (ii) Control available over fibre/resin ratio in manufacture, allowing a higher fibre volume fraction to be achieved, and giving significantly higher mechanical properties. (iii) Resin and hardener are pre-mixed and reinforcements are already fully wet-out, improving the quality of construction, reducing variability, improving weight control, and improving the health and safety aspects of construction. (iv) For the designer, it is possible to specify sophisticated, optimised layups, since the 2-3 day working time will give the builder time to build up correctly a complex ply sequence. The first application of this technology was the large multihull yacht 'Steinlager 1' (Fig. 2). To date, the largest project built using the same technology has been a 25 m Maxi yacht called 'Windward Passage', (Fig. 3). It is likely that this hull is the largest single-piece pre-preg structure ever built, the previous record being held by the loading bay doors on the NASA Space Shuttle. This yacht out-performed the existing
Fig. 2. Steinlager I.
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Fig. 3. Windward passage. By R. Tomlinson.
Maxi Fleet to such an extent that the aluminium structured yachts have become obsolete. Details of the construction methods used on a project such as this are discussed in Section 4. 3.4 Core materials
Sandwich structures are used almost exclusively for high performance hulls. The reasons for using sandwich instead of solid skin laminates with frames and stringers is discussed in Section 5. Both foam and honeycomb cores are used, depending on the loading conditions. A typical yacht hull may use honeycomb for the majority of the hull, with foam below the waterline from the keel forwards to resist slamming loads. As honeycomb properties improve, these areas of foam may be replaced by honeycomb. Powerboats generally use foam cores for the higher mechanical properties and better resistance to impact damage. The current core materials are: • Rigid crosslinked PVC. • Linear (non crosslinked) PVC. • Nomex Aramid fibre/Phenolic honeycomb.
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Foam densities range from 60 to 250 kg/m 3, with the linear foams having higher toughness, but slightly lower mechanical properties, compared to a rigid PVC of the same density. Typical Nomex densities are 48 and 64 kg/m 3.
4 MANUFACTURING METHODS A well defined manufacturing process has been developed for the construction of large one-off racing hulls using sandwich structures. Typical features of the build process for a large racing hull similar to 'Windward Passage' and built with low-temperature curing epoxy prepreg are as follows: (i) Wood male hull plug sheathed in glass/epoxy, faired and painted. (ii) Prefabricated tent or box oven with hot air blowers and recirculating fans. Heater blankets can also be used. (iii) High capacity vacuum system for rapid air removal from the vacuum bag, and for coping with small leaks. (iv) Inexpensive low temperature vacuum bag and disposables such as peel plys, bleed cloths and breather fabrics. (v) Adequate number ofthermocouples on the laminate to monitor curing temperature at each cure stage. At 75°C it is possible to work for short periods inside the oven, checking the temperature distribution with a hand-held temperature probe when the oven is being calibrated. (vi) Three-stage lay-up p r o c e s s - inner skin, core bonding and outer skin, with a vacuum bag cure at each stage. (vii) During the cure cycle, as the resin temperature rises, its viscosity drops dramatically, and under vacuum pressure it flows and thoroughly wets out the fibres and the laminate is consolidated. After a short time the resin starts to gel and full cure is achieved typically after 5 h at 75°C. (viii) Main advantage of using a male plug instead of a female mould is speed of build and reduced cost for a one-off. It is easier to achieve good fibre alignment when draping reinforcements over a male plug. Also, fitting of thick core materials (typically 50 mm for a Maxi) is much easier, and less core bonding adhesive is required. (ix) Hull outer laminate is faired and finished with lightweight epoxy syntactic foams, followed by priming and painting. The
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quality of the outer laminate surface is improved by being careful with reinforcement overlaps, and fairing is now a rapid operation which adds very little weight to the hull. In a production situation, where a number of hulls is required, similar manufacturing methods can be used. In this situation the wood male plug is made to the hull outside profile, and production glass/epoxy female moulds are taken from this plug. An alternative method for curing the pre-preg is to use heated moulds incorporating an electrical resistance heater mat laminated into the mould layup. Fairing and finishing on the outside of the hull is no longer necessary, but time saved on this operation is offset by increased difficulty in working with laminates and cores in a female mould.
5 DESIGN ASPECTS A full understanding of composite materials technology and manufacturing techniques, as well as skill in designing with highly anisotropic materials, are all necessary if one wants to exploit the considerable potential of advanced composite materials. This integrated approach, covering structural engineering, resin development, reinforcement design, mechanical testing, process engineering and liaison with the builder, ensures that the design on the drawing board can become a practical reality.
5.1 Structural design Designing a high performance structure requires design for structural integrity and consideration of performance-enhancing features of the design. When designing for structural integrity in a sailing yacht, the main loading cases considered are fore and aft hull bending due to rig loads, transverse rigging loads, sail control loads, keel and crew weight righting moment, slamming wave pressures and rudder loadings. For a powerboat, slamming, impact, rudder and inertia loads are likely to be the critical cases. Performance-enhancing features for a sailing yacht are low structural weight for reduced hull drag, light bow and stern sections for reduced radius of gyration in pitch, and high hull bending and torsional stiffness for better rig and sail control. For a powerboat, reduced weight means better acceleration and higher speeds. If the weight saving is significant
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in the hull, this can have a knock-on effect, allowing a reduction in engine size and fuel capacity. Most racing yachts use a h o n e y c o m b sandwich structure for hull and deck with a small n u m b e r of internal bulkheads or frames and no stringers. The keel is usually attached to a longitudinal girder structure which can be a l u m i n i u m alloy or composite. Hull layup is mainly carbon U / D or c a r b o n / a r a m i d U / D with a layer of c a r b o n / a r a m i d hybrid fabric for impact resistance. A typical layup is shown in Fig. 4. Powerboat hulls use different structures depending on the application. A particular challenge was posed by an assault craft, the Smuggler 384, as this craft had a prime requirement that it could be driven on to a beach fully loaded at a speed of 15 kts. For this craft, slamming, impact a n d abrasion as well as high speed and manoeuvrability are the design drivers, and a foam sandwich structure with glass/carbon/aramid/epoxy laminates is most suitable. As with the racing yachts, the m i n i m u m a m o u n t of framing is used 1.2 (Fig. 5). A high performance lifeboat such as the R N L I Fast Carriage Boat (Mersey Class) requires more substantial bulkheads for flooding considerations. Speeds are lower, so s l a m m i n g pressures are reduced, but impact protection with resistance to hull penetration becomes important. Production requirements also influence materials selection and heavyweight multiaxial pre-pregs are used with a thick foam core. A production run of these hulls justifies the use of female moulds.
5.2 New structural concepts As higher performance composite materials have been developed, skin thicknesses in hulls and decks have been progressively reduced. This has led to the situation where a closely framed structure has become less attractive, since the skin thicknesses b e c o m e unacceptably thin and delicate.
5.2.1 Monocoque hull structure An alternative is the ' m o n o c o q u e ' concept which allows material to be taken out of the internal frames a n d bulkheads and some of it to be put back into the skins. This concept has been developed on projects such as the Smuggler 384, and applied successfully on projects such as ' W i n d w a r d Passage'. The reasoning b e h i n d the m o n o c o q u e concept is that s l a m m i n g loads are an energy type of impact. W h e n loaded, a hull panel will elastically absorb a n d then release the strain energy. A closely framed structure is stiff, so for a given energy applied, deflections will be low, and loads high.
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As the panel size increases, core shear can become critical. However, as Fig. 6 shows, a small increase in core shear strength allows a significant increase in allowable panel span. In the monocoque concept, a higher density core (higher shear strength) is used, and core thickness is also increased. Core thickness is typically 50 mm on a Maxi. Additional transverse unidirectional reinforcements are incorporated in the hull shell layup. With this construction method, the frames can be assumed to be built into the shell, and the result is high strength with much higher energy absorption. As an example of the potential weight savings for this monocoque concept in advanced composites, the all-up structural weight of a raceboat such as 'Windward Passage' is less than 4000 kg, compared to a conventional closely framed aluminium construction weight of 8500 kg. An additional benefit of this form of construction is that elimination of frames and bulkheads results in a significant reduction in layup and assembly time.
5.2.2 Foam supported shell A further development which can be used in conjunction with a monocoque structure is the foam supported shell for hull structures.
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Fig. 6. Allowablepressure versus panel span. Instead of using stringers or frames to support the hull laminate, a thick foam layer is used, with a thin inner laminate to seal the foam surface. This concept has been developed on projects such as the Smuggler 384 and refined on the prototype advanced composite Fast Cardage Boat (FCB) for the RNLI. For these projects, resistance to impact penetration and flooding is of major importance. The thick hull shell can be considered as ~ structural double bottom. In-plane hull shell loads are carded by the outer skin which has a multiaxial glass/woven aramid/epoxy laminate for high impact strength. Resistance to hull laminate panel buckling is increased by the support from the thick foam core. If penetration occurs in the outer skin, there is still a thick layer of foam before the object reaches the inner skin. This is the final barrier and the inner skin laminate contains a layer of woven aramid fabric which will accept considerable damage to the laminate without itself tearing, thus preventing large scale water ingress. Impact and pressure tests on panels representing various forms of lifeboat structure have shown that this concept works well, with the benefit of reduced hull weight.5 On the RNLI Fast Carriage Boat this foam supported shell is allied to a semi-monocoque structure with a small number of watertight bulkheads, but no framing is required between the bulkheads. For lifeboat applications, an additional feature of the thick foam sandwich is the considerable amount of built-in buoyancy in the hull. 6
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5.3 Loads and design allowables Design allowables for laminate mechanical properties are generally derived by following the aerospace practice for analysing test data. Since most variability is found in strength values, mean values are used for moduli, and strength data is analysed on a statistical basis to derive design allowables for a given confidence level. These statistically derived allowables take into account actual laminate properties achieved in manufacture. It may also be necessary to include a factor to account for variability in production. Proof and ultimate factors applied to the limit load cases allow for variability in specification of loads and accuracy of structural idealisation during analysis. Different reserve factors may be used, depending on the particular application. A different situation applies with, say, a flat-out raceboat designed for 'round the buoys' inshore racing with safety boats in attendance, compared to a lifeboat designed for arduous service over a long working life. In the former case, the operating environment is more controlled, and the consequences of structural failure are generally not life threatening. Consequently, it is accepted that reserve factors and margins can be reduced as a trade-off for increased performance. Specifying accurate sailing loads for hulls is still problematical, particularly slamming, impact and inertia loads. Until these have been better quantified it will be difficult to get away from empirical development or conservative design based on scantling rules and design structures which exploit the full potential of the technology. One of the spin-offs from the America's Cup programme may be detailed information on hull and rig loads. Each syndicate is planning a programme based on up to four boats. The first boats will be heavily instrumented 'trial horses' with strain gauged hulls and rigging. They are intended to generate this type of loading information to allow subsequent boats in the programme to be designed with structures more closely matched to realistic loads. While some of these spin-offs, financed by sponsors of America's Cup programmes, may read across to the needs of power craft, parallel investment in instrumentation in the latter field, especially for defence applications, is equally necessary and is overdue.
5.4 Computer aided design and drafting Designs for structures such as monocoque hulls have developed partly by empirical means, improving the concept over a number of projects,
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based on feedback from the sailing trials or the racecourse. Detailed analysis of monocoque sandwich hulls with complex and highly anisotropic layups is only possible using Finite Element (FE) techniques. The development of thick sandwich shell elements and the availability of FE packages to run on desktop computers will allow more use of FE as an interactive design tool to develop the structural configuration. So far, FE analysis has been generally applied only when much of the structural design has been finalised. The use of powerful analytical tools in the design process is no substitute for having good design ideas in the first place, but it should help in developing ideas into workable structures. Conventional engineering drawing methods have developed around metallic materials and there are no conventions for drawing complex, multi-layered components. Computer drafting programs are ideally suited to the production of layup drawings for composites since they work on a layered structure. Each layer can be developed individually, while maintaining an overview of the complete structural design as it develops. Once design is completed, a hard copy can be printed for every layer of the drawing. Each sheet of the drawing will show the orientation and extent of an individual layer of reinforcement or core material (Fig. 4 ) . A simple process control technique for a complex layup is to produce the individual layer drawings in booklet form in the correct layup sequence. By working through the book, the builder can reduce the chances of missed layers or an incorrect sequence.
6 QUALITY ASSURANCE When considering quality assurance for raceboat construction, the main consideration is the development of a procedure which is suitable for one-off construction. The main features of such a procedure are: (i) Materials and manufacturing methods reasonably tolerant to variations in processing conditions. (ii) Skilled workforce, experienced in the use of materials and techniques. (iii) Clearly defined, documented and controlled process. (iv) Visual inspection at each layup stage to ensure that any faults are found at a stage when they can be rectified. (v) Monitoring of resin mix ratios, cure cycle and vacuum pressure during construction.
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(vi) Analysis of cured resin samples using a Differential Scanning Calorimeter (DSC) to assess accuracy of mix ratios and cure state. (vii) Mechanical testing of laminate samples taken from component. Ultrasonic or other forms of non-destructive testing of the finished component is generally not possible, due to cost, the large surface areas to be covered, and the difficulty in calibrating equipment for a one-off layup. A tap test with a coin is still a good alternative for detecting debonds, although it will not detect small voids within the laminate. Wet layup is the most difficult process to monitor since the laminate quality depends very much on the skill of the laminators. This situation can be improved by pre-impregnating reinforcements with the laminating resin using a simple impregnating machine consisting of a resin bath and nip rollers. This can improve consistency of wet/out and fibre/resin ratios. The use of pre-preg further reduces the level of layup skill required, but the process engineering capability must be higher, requiring more know-how, planning and equipment. Core bonding is another difficult area to check during construction. Foam cores have holes punched through to ensure that no air is trapped under the foam. This can be inspected after cure by checking that resin has flowed up through the holes during vacuum bagging of the core. Honeycomb adhesive fillets can be checked visually after bonding to the inner skin. For the outer skin layup, a single lightweight woven fabric is first bonded to the honeycomb. After vacuum bagging and cure, this layer is visually inspected for lack of bonding to the honeycomb. Any rectification is carried out prior to laying up the remainder of the outer skin layup. In production, all of the preceding quality assurance techniques can be used with the following additional techniques: (i) Pre-cut kits of reinforcement, pre-preg and core material. (ii) Mixer pumps for dispensing resin or adhesive mixed with hardener. (iii) Monitoring of resin, reinforcement and adhesive usage. (iv) Weighing of finished, trimmed components to monitor weight variations. (v) Ultrasonic and other forms of non-destructive testing. (vi) Measurement of void content and fibre volume fraction on laminate samples. (vii) Mechanical testing on components from production - - either by testing to destruction on complete component, or cutting up for
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various mechanical tests, followed by correlation with mechanical tests on laminate samples and DSC results.
7 PROJECT COSTS Table 1 shows the project cost breakdown for a representative range of advanced composite boats. The leading dimensions for each boat are also included to allow a size comparison. Some points to note are: (i) The contribution of material costs to total project costs is small even though some of the projects use the highest performance and, therefore, the most expensive advanced composite materials available. (ii) Man hours costs are greater than materials costs, so selecting materials and construction methods which reduce layup and assembly times can be more cost effective than going for the lowest cost materials available. (iii) Dominant cost is different for each of the projects. (iv) A new America's Cup rule has replaced the '12 Metre' rule based on aluminium hulls; the rule is now written around advanced composite hulls with sandwich structures similar to the current Maxi/IOR boats.
8 VALUE ANALYSIS ON MATERIALS When designing in advanced composites, the materials can be assessed against various selection criteria such as: (i) Specific performance requirements - - strength/unit weight, stiffness/unit weight, impact strength, etc. (ii) Suitability for proposed manufacturing methods. (iii) Cost to achieve a specific requirement. As discussed previously, when comparing materials options the real cost of a laminate should include resin and reinforcements plus processing costs, which include man-hours costs for layup as well as tooling and equipment, disposables costs, curing costs, etc. Figure 7 shows a comparison of specific material properties for various reinforcements with epoxy resin. Also a value analysis has been carried out to show what level of performance can be achieved with each
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10000
320 ~300 ~280 ~260 ~240 ~220 -200
9000
I Load/ Cost N/£
iiii ,'
4000
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.
3000 2000 1000 0
I
o
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Specific Strength J 160 I Tensile Strenglh/Density 140 ~120 --~-100 -:_-s o
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Fig. 7. Tensile load capacity versus cost.
material for a given cost. The cost is based on reinforcement plus resin prices plus man-hours costs for layup. For simplicity, ancillary processing costs such as tooling, disposables and cure, etc., have been omitted. Figure 7 gives some indication of the best materials options for this specific requirement. In some cases, pre-pregs are shown to be cheaper when layup costs are included. For a direct tensile load, U / D carbon is shown to be the most cost effective solution. This conclusion might not be expected since this is the most expensive of the chosen materials. For other structural requirements such as shear stiffness, or for a more isotropic laminate, a different material may be more cost effective. However, this graph shows that basic raw material cost can be a misleading criterion when selecting the most cost effective material to do a particular job. 9 CONCLUSIONS This article has reviewed the state-of-the-art in the advanced composite high performance marine craft design and construction. The industry
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has developed the skills and materials for designing and manufacturing large one-off high performance advanced composite structures. There will be further developments in raceboat technology, driven by high budget programmes such as the America's Cup competition but similar investment in other areas, especially the defence field, is vital to ensure that the technology is not restricted to the one-off raceboat industry. If this investment is forthcoming, the following important developments of the technology can be expected: (i) Use of the technology in a production environment: this is starting to happen with high performance lifeboats, assault craft and production racing powerboats. (ii) Application to other high performance marine craft, e.g. surfaceeffect catamarans. (iii) Use of prototyping skills for advanced composite production prototypes. (iv) Application of the technology outside the marine industry, e.g. for offshore and wind energy structures. Advanced composites have the potential to improve most large high performance structures; the necessary technology is now proven and available for design and fabrication of very large composite structures.
REFERENCES 1. Editorial, Ship & Boat International, Dec 1986. 2. Editorial, Ship & Boat International, Jan/Feb 1987. 3. Allen, R. G. & Jones, R. R. A simplified method for determining structural design limit pressures on high performance marine vehicles. In Proceedingsof the AIAA/SNAME Advanced Marine Vehicles Conference, San Diego, 1978. 4. Heller, S. R. & Jasper, N. H. On the structural design of planing craft. Trans., Royal Institute of Naval Architects, 103 (1961) p. 49. 5. Cripps, R. M. Pressure and impact tests carded out on panels of different materials, RNLI Report, June 1987. 6. Paffett, J. A. H. The Material Revolution, The Lifeboat, Spring 1989.