Beneath the surface: The present and future of composite workboats
Like Marmite, composite workboats have till now either been ‘love or hate’, however, there may be room to change a few minds, writes Stevie Knight.
Many people might think composites constructions are fine for leisure, but wouldn’t want to touch them for tough commercial builds. But composite builders disagree and there’s rising interest – take Norco’s landing of the contract for over 30 of the MoD’s 11m to 15m workboats.
To start with, there’s the somewhat over-simple idea that metal is more robust. This isn’t the whole story by any means. Firstly, as David Kendall, founder of Optima Projects and the engineer behind most of CTruk’s vessels explained: “The issue with aluminium is when you weld, you get a huge reduction in strength. As a result, most of them suffer from fatigue cracking especially around drive train and propellers; we just haven’t seen that happening in the same way with composite boats.” He added: “In fact, these are pretty much same materials as the wind turbine blades themselves – and you don’t often see them cracking despite the huge forces involved.”
So, while aluminium’s characteristics are fixed at the mill, FRP’s qualities are “determined by the construction of the vessel” said Kendall. Therefore many of FRPs perceived weaknesses “are overcome by good design”.
Equally, he pointed out, “if you don’t engineer the composites properly, you’ll get a boat heading toward structural failure”. So, for example, the alignment of the filaments is of enormous importance, and that means exerting a greater degree of control over the construction.
“It’s a more refined process than metal,” said Kendall. “You have to be careful; composites often have very high tensile, but lower compression strength.” In short, very much like rope, these fibres will tend to react less to pulling, while pushing or twisting will tend to force them apart, breaking the matrix which holds them. “To mitigate against that you have to control the orientation of the filaments,” he said: “It all involves making sure the yard builds exactly what you design – you need very good communication.”
The ability to refine the design yields large benefits. As James Bottoms of Norco explained: “With aluminium boats, all you can do is change the shell thickness... but with laminates you can customise the hull for different qualities.” Are Søreng of Umoe Mandal added being able to engineer the product “gives local strength where you need it, not global strength as that just adds weight and cost.”
It wasn’t always this way. The market used to be dominated by short, chopped glass fibres that were sprayed or wet laid into the composites willy-nilly “and as you couldn’t control anything, it meant adding a lot more bulk just to make sure you met the requirements”, said Søreng.
This has changed dramatically: “Long fibres can yield far better mechanical properties,” he explained and added that Umoe Mandal utilises a woven fabric laid up in different fibre directions in a vinylester matrix, sandwiching a PVC foam core; the final product exhibiting “all-round” resilience.
It’s been important in the building of the company’s three Wavecraft CTVs, high-speed air-cushion catamarans manufactured from carbon fibre, one of which drew attention on Seawork’s pontoon last year. “These crew transfer boats work long transits in the North Sea: the air cushion effect brings them partly out of the water to get up to a 40kn service speed - one of the fastest in the industry – but for that, you need a really lightweight hull,” said Søreng.
However, the CTVs still have to engage with the turbines up to 40 to 60 times a day, “and they are not always light kisses either” he underlined, needing an extremely robust design. The answer to fulfilling both sets of requirements “is to do the calculations around the forces the bow is designed to absorb... then strengthen with a higher density foam core, an extra laminated layer or even another bulkhead”.
Mixed layers can also yield interesting characteristics, especially when playing to their strengths. For example, PE Composites manufactured the FRP hull and deck structures for around 50 of the RNLI Atlantic 85 RIBs: this was made up of carbon and epoxy resin sandwiching a Divinycell foam core, with – interestingly – a final GRP outer and hefty pure GRP element to the very front of the bow.
Mark Russell of PEC explained that this outside layer “was designed to take the rough treatment from beaching as carbon is actually hard to repair if the long fibres are completely cut”. By comparison, “GRP is quite easy to repair, so it’s almost a sacrificial layer”, he explained. The same goes for the solid GRP at the bow as this can sometimes suffer from forceful contact: “Again, this allows for straightforward mending,” he said.
One thing is worth noting here: even though GRP makes for a lighter build than alloy, this construction has pared down the weight by 25% again compared with traditional polyester composite structures: with twin 115hp outboards the 8.44m RIBs make around 35kn.
There’s also room for other hybrids composites: occasionally glass is paired with polyaramids such as Kevlar: Kendall explained that one of his patrol boat designs for Turkish waters, incorporated Kevlar reinforcements in the bottom panels to increase strength and punch-through resistance – polyaramids like Kevlar being less sensitive to impact loading than carbon.
The resin plays an important part. Traditionally at the bottom end of the scale are the cheaper polyesters, but they can have an issue with picking up water. Then slightly more expensive but giving better resistance to water absorption are vinylesters. Epoxy resins are at the top of the range, but three times the price – and some people don’t like them because of the health and safety issues. However, there could be new formulations coming ‘downwind’ from turbine blade manufacturers, for example, a polyurethane resin that’s not only eco-friendly, the manufacturer also claims it’s done even better than epoxy in tests.
Further, as Geraint Havard of R-Tech Materials added “while at present composites have a bad rap when it comes to fire, it depends on the resin chemistry”. He pointed out that while metals can actually propagate fire by heat transmission, there are phenolic resins that are self-extinguishing, “so as soon as you take away the source of the problem, the flames go out”.
Production methods have come a long way from the days of wet or sprayed layups - to a certain extent pushed by safety and environmental regulations. Wind blades have further forced the evolution of fast production, closed mould, vacuum infusions often cured by autoclave; resin manufacturers have responded by developing low-viscosity products with enhanced flow to suit.
However, the nature of boat components, low volume and just plain bulky, has resulted in the rise of vacuum-bag methods – involving hand laying the dry fibre on the form first – which do a good job of compacting fibre and resin while minimising voids. Are Søreng explained that the hulls of boats such as the Wavecraft series are often placed in a hot tent, rather than oven, to enhance the positive mechanical properties of the resin with a moderate, post-cure treatment.
One can’t avoid mentioning the rise of pre-pregs, another technology in which Norco has invested. Typical hand-lay laminates, even when vacuum bagged, end up with a significant amount of excess resin which increases brittleness and reduces the overall benefits. By contrast, prepregs come ready impregnated with pre-catalysed epoxy to an optimal 2:1 fibre/resin balance. These still should be vacuum processed and they also need to be heat cured, typically at 100°C but despite the extra costs, James Bottoms added “it’s worth it if you want that extra level of control over the resin to fibre ratio... there are gains in strength, weight and uniformity.”
Resins and production methods have developed enormously since the early days and it should be mentioned that Umoe Mandal’s military vessels are still going strong after a quarter of a century. However, there’s still a focus on mitigating any remaining issues with delamination and micro-cracking from fibres pulling apart under stress and breaking the matrix.
According to Geraint Havard, there are a number of emerging solutions that hold promise “which will probably bleed down into the marine industry as the automotive sector picks up the technology and makes it more accessible”.
One process, tufting, means stitching a polymer thread through the layers (the needle only coming in from the top), although its main drawback is making sure the resin flows and consolidates properly around these tufts. Another, Z-pinning, forces tiny spikes of pre-cured composite through the layers, these are aligned along the filaments; using an ultrasonic hammer also softens up the tip to further mitigate against breaking the strands. Other, recent developments have taken this further: for example, there’s been research on using carbon nanotubes for the pins.
More, it’s now possible to programme a machine to precisely lay tapes, or to weave fibres into a 3D mesh: “Although it takes sophisticated systems, this gives you strength in every direction,” explained Havard.
Interestingly, there is one technology aimed at repairing micro-cracking. Self-healing composites are now being developed by the aerospace industry: within the structure there are tiny pockets of unset resin that release if there’s damage, flowing through the cracks and sealing them. Most interestingly, the healing puts strength right back – the healed composite having very strong interlaminar properties, so it’s even less likely that cracks will appear in the future.
More, while many industries are already putting sensors on structures, Umoe Mandal has been integrating sensors inside the FRP on the WaveCraft CTVs to record stress “especially as turbine operators want to know how much force the boat has transferred to the tower”, explained Søreng.
But according to Havard, there is one further development – some are looking to use the composite as the transmission medium itself, “so there’s no need to embed the sensors – and this way, there’s no reduction of mechanical properties”.
Initial costs are an issue admitted Kendall. “If you are looking for a one-off vessel, then it may be cheaper to build in aluminium. But if you are looking at a series of three or four, then you can offset the investment in the mould. So with multiple boats, there’s no reason not to go for a composite.”
Especially, that is, if you take the operational costs into account. “A lightweight composite vessel is significantly cheaper to run than its equivalent in aluminium, although that doesn’t always get so much attention as it should because there are often some quirky contracting clauses that mean the person paying for the fuel isn’t the person paying for the charter,” explained Kendall.
But, he added, it’s a ‘virtuous circle’ that reaches every element of the design, as a lighter build requires lower engine power, again giving a lighter total weight, less fuel in the tank, and so on.
As Are Søreng concluded: “When I started working with them 25 years ago, composites were the future. They are still the future.”
By Stevie Knight
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