Shear and buckling: if you are building what should be the fastest catamaran ever to sail, these are your two dread words.
Imagine hurtling up a wave at 40 knots. At the crest, both 36.5m hulls become airborne. Crashing back into the water, one hull impacts before the other. Massive side loads wrench at the bow section - as much as 45t smashing down on no more than 5m of hull.
Team Philips, the £2.2M racing catamaran commissioned by Devon yachting hero Pete Goss for a non-stop, global circumnavigation, was sailing in relatively tame conditions when the port bow broke clean off during sea trials in March this year.
Chief designer at Goss Innovations, Barry Noble, calculates the structure failed under about a fifth of the load it will be subjected to in the savage Southern Ocean. Horrified, Goss and his crew limped back to Totnes where Team Philips was built to analyse what went wrong.
The boat, designed by Adrian Thompson with structural engineering by SP Technologies, is a carbon fibre composite structure. Formed in a female mould, it consists of Nomex - a honeycomb of paper treated with phenolic resin - sandwiched between resin-bonded carbon fibre inner and outer skins. The hulls were designed as monocoque structures, with the 35mm thick shell stiffened by four carbon fibre 'strakes', or beams, 5mm thick, 500mm deep, running the full length of each hull.
These strakes provided roughly two thirds of the hull's load bearing capacity.
Noble, who designed Team Philips' rig and has masterminded the strengthening work needed to make the catamaran seaworthy, believes the original finite element analysis ignored the problems of eccentric loading on the bows. And build manager Gary Venning reports that an unprecedented problem in manufacture of the hulls exacerbated their inherent weakness.
The Nomex core has high shear tolerance and, securely bonded to the inner and outer carbon fibre skins, forms a rigid, shear resistant sandwich. Resin unifies the layers of carbon fibre fabric that make up the boat's skin. It also glues the skins to the Nomex core, running into the 3.2mm diameter cells to create a fillet bond with the cell wall.
'Wet' resins, cured through addition of a catalyst, are widely deployed in boat construction.
But, says Venning, it is impossible not to use too much. 'On a boat this size surplus resin could easily add a couple of tons.'
Pre-impregnated carbon fibre was used to build Team Philips instead. With 'pre-preg' construction, a vacuum is used to suck out air between the layers of material once lay-up is complete. The whole structure is then baked in an oven, melting the resin so it flows between the different carbon-fibre layers.
Around 60degrees is enough to create resin flow in most boats, notes Venning. However, for heat to penetrate the layers of material used to build Team Philips, a giant 500m 3, 85degreesC oven was built.
Venning and the team knew that air in the Nomex cells would expand during baking, posing the danger that core and skins would be forced apart and prevent a strong bond forming.
Materials suppliers advised him to pierce the carbon fibre at 60mm centres to allow expanding air to escape.
Over most of the hull this worked to plan. But on the inner skin, in the areas where the longitudinal strakes had been built in, the necessary shear bond was not formed. Venning believes the local thickness of carbon fibre - 9mm compared to 4mm for the rest of the hull - meant resin flowed over and blocked the piercings before the air was out.
'Although the surfaces were touching, a proper fillet bond was not formed in the area of the strake, ' Venning explains. Effectively, the strake was useless.
'There was very little rigidity in the bow, which started flexing 1m either side of the centre line.
Flexure led to total delamination of the strake, which meant there was then almost no stability at all. All loads were transferred to the shell, which was buckling.
Stresses concentrated in the weakest area.'
Compression fractures developed as the bow flexed back and forth. Finally the hull failed totally and the port bow broke off.
Repairing Team Philips, at a cost of £400,000, has involved inserting a 'space frame' akin to that of an aircraft. 'In some ways we've been fortunate, ' observes Noble. 'If you were presented with a monococque structure that was nearly strong enough it would be difficult to strengthen without creating too much structural redundancy. But because this is so far short of adequate we've been able to add ring beams and stringers for strength - the skin is now just for keeping water out.
Eschewing computers in favour of pencil and paper, Noble's strengthening regime has involved cutting into the catamaran's outer skin so that six longitudinal shear links can be installed on each hull. These are composed of Z-section carbon fibre beams bonded to the inner skin and packed with foam. A new outer skin is then laid over and bonded to the shear links, and filleted into the surrounding outer skin. More rigid 'by a factor of 100' than the original carbon-fibre Nomex sandwich, Noble says the shear links reduce the likelihood of buckling to virtually nil.
But for good measure, 150mm deep ring beams have been fixed inside the hulls at 600mm intervals. Thanks to the efficiency of the structural system used to beef up the hull and the extraordinary lightness of the carbon fibre elements used Noble expects to see little change in buoyancy.
Team Philips is taking to the water again next week. Strain gauges are being placed into the bows to monitor performance as the race, which starts on New Year's day, progresses. 'They are more for academic purposes than anything else, ' says Noble.
If the hulls do show signs of failing again, in the Southern Ocean, say, there is little the crew will be able to do to about it. 'But it gives us back real information - a bit of science that we can feed into future projects, ' says Noble.
And what chance of a second failure? Slim, he says.
On the spot
Name: Barry Noble Age: 53 Qualifications: None. I have never been to school and have had no formal education. I lived in various parts of Africa, where my father worked as a engineer, until I was 17. Every day from the age of seven I went to the workshop with him and my play was making things - my education was very practical.
Current job: Chief designer Goss Innovations Best thing about the current job: Complete freedom to explore the things I'm interested in, as long as they pay off in the end.
. . . and the worst: Not enough time.
Last job: Designing carbon fibre products for motorsports specialist Tom Walkinshaw Racing.
Ideal next job: Landmark achitectural project involving innovative structural solutions and composites engineering.
Most useful career qualification: The ability to make on the shop floor most of what I can design.
Most useful lesson learned:
Always to have the courage to scrap the work you've done and start again if a better idea comes along.
Most useful work tool: Pencil.
Advice to a young engineer:
Always to think more about art than engineering.
Anything else: I spent two years making cellos, double bass, and violins professionally, and still build instruments for fun.