Feature 2 | MARINE COMPOSITES IPE-300 Steel Carbon fibre Carbon fibre CFRP GRP Steel 115 (307)
33 (70) 100
Aluminium 96 747
363 100 160
holes, grooves and cuts. For a sandwich panel with glass-fibre
laminates the core is the dominating cost and all effort should be concentrated into finding less expensive core material. When it comes to weight the fibre is the dominating factor but as with carbon fibre there might not be much that can be done to reduce this. The resin in the core is the same as for the carbon- fibre sandwich.
Optimal stiffness geometry Since carbon-fibre laminates have different properties such as strength, stiffness and density the optimal geometry of a specified structure will differ for other material with other sets of properties. In Table 8 a standard steel beam is compared to a symmetrical top hat stiffener in carbon fibre with sandwich web. Since the carbon-fibre stiffener has
higher strength to stiffness ratio it can be built taller that the steel beam without reaching its strength limit in the flange. Te height will also give it geometrical stiffness. In theory it can be built so high that it will not need a flange at all, which is very attractive from a production point of view. Te factor that will limit the height is local buckling and/or tripping. Tis is also the limiting factor for the steel beam but at only 300mm height and more weight per meter.
Stiffener layout From a practical point of view it will in most cases not be possible to have stiffeners that are more than a meter high which means that they have to be smaller and by that not fully optimised. Hulls of catamarans are usually more
or less empty so in that case there is no problem having high stiffeners. The narrow hulls of catamarans will also make maximum bottom panel width
40 322
184 100 200
small (about 1m between keel and chinline, as in the example above) making longitudinal stiffeners for panel stiffening unnecessary all together, with the exception of engine girders.
Maintenance Low maintenance cost is one of the greatest advantages for composite material in marine application. The maintenance cost is an integrated part of the total life-cycle cost and will together with the lower weight and fuel consumption have a very positive effect on the total operating cost. Te most important factor affecting the
maintenance is the fact the composites do not corrode, see Figures 9 and 10. Tis will not only save the time spent on rust removal on steel ships but also means that less time and money can be spent on corrosion prevention. Examples of this are no special paints, no sacrificial
Table 8. Comparison of steel and carbon- fibre stiffeners.
anodes, no corrosion margin and so on. The lower weight of the composite
ship means that smaller engines can be used and these will have lower maintenance cost. A composite hull will provide smooth
surfaces with few stiffeners, brackets, etc., and will be very easy to keep clean. A steel hull requires the inside to be reinforced with longitudinal stiffeners and web frames, thus making the cleaning and painting more time consuming. One problem with the maintenance of steel hulls is the invariable corrosion in areas or compartments that are out of reach and therefore require extensive disassembly before maintenance and/or repair can be attempted. A steel hull will require internal insulation to maintain the required interior climate. Oſten this insulation is prone to damage and requires regular maintenance. Te insulation may also absorb water and cause widespread corrosion damage to the hidden steel surface. Te internal insulation of the sandwich structure will also result in less condensation and bilge water, which will in turn result in less humidity and better climate in the keelson.
Figure 9: Bottom structure of 40-year-old composite ship.
Conclusion Carbon-fibre catamarans have advantages compared to similar ships in steel, aluminium and glass fibre. The lower structural weight and fuel consumption and the lower maintenance will give a lower overall operating cost. Te different properties of carbon fibre, design rules, material cost and production methods will add a lot of complexity and demand for special knowledge when designing such ships. Te ever increasing fuel costs, the lower material costs and the greater knowledge in the composite industry have today made it possible to build and operate carbon-fibre passenger ferries with a profit. SBI
Reference 1. DNV, ‘Rules for classification of high-speed, light craſt and naval surface craſt’, July 2009.
Figure 10. Bottom structure of steel ship.
Te author of this paper, Måns Håkansson, is composite specialist at Kockums AB. He is responsible for process and material development of composite materials.
Ship & Boat International July/August 2010
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