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Sail-World.com : America's Cup: Steve Clark on Bows, Stress and Weight

America's Cup: Steve Clark on Bows, Stress and Weight

'BMW ORACLE Racing’s BOR90 suffered noticeable platform wracking at times, which will also be a challenge for Alinghi 5.'    BMW Oracle Racing © Photo Gilles Martin-Raget    Click Here to view large photo

Over the past two days, the sailing world has glimpsed some of the most advanced sailing technology ever seen.

Last Monday BMW Oracle Racing relaunched their trimaran, BOR90 after a period of substantial modification following several session sailing trials off Anacortes and San Diego.

Like Alinghi 5 revealed the day before in Villeneuve, Switzerland, BOR90 featured the wave piercer bow - one of several modifications from her previous mode.

Sail-World asked multihull and high performance sailing guru, Steve Clark how these bows work and that the designers are trying to achieve with this style.

Clark's pedigree goes way back into the early days of C-class catamarans, in fact his father Van Allen Clark sailed the US C-class cat Beverley against Hellcat (GBR) in 1962. Since then Steve has been involved in the development of C-Class catamarans culminating in the superb Cogito which won back the Little America's Cup in 1996. The C-Class catamaran have been the pioneers of much of the technology seen in these two America's Cup 'supermultis'. Wing masts for instance first appeared on C-cats over 40 years ago and the hard wingsail first appeared 35 years ago in 1974.

It's familiar technology for Clark, and the issues are well known.

'The raked back bows get called 'wave piercers' but that really is a misnomer,' he explains. 'The idea is to have the hull go right through the wave instead of rising over it.

Wave piercer bows are an obvious change from the pre-rebuild version of BOR90. -  www.sailkarma.com?nid=58754   Click Here to view large photo


'The notion is that you don't waste the energy making the boat go up and down ( heaving), and you don't incur pitching. The result is supposed to result net reduction in energy expended on sea-keeping thus making the vessel more efficient.

'If you look at the INCAT fast ferries, these are true wave piercing bows, and they make the ride more comfortable for the passengers.

'In sailing catamarans, the tendency has been toward wider waterlines and fuller bows and sterns. The NA (naval architecture) term for what you achieve with this is higher 'waterplane inertia (which means that the hull is less susceptible to trim changes.) That is to say it is harder to push the bow down.

'As a result, there is less need for reserve buoyancy forward, so freeboard can be cut down. Further you do not require as much or indeed any flare in the above water sections, so you can reduce surface area by pulling the deck beam in. As you do this, the stem starts to rake aft at the top, giving the Dreadnought profile.

'There is a further benefit of reducing the skin area forward, and the effect of that is that the wetted surface can't grow by as much, so the cat is slowed less if it is trimmed bow down for a short period. This makes it less likely to 'trip' over itself.'

In summary, the benefits of the so-called 'Dreadnought' bows are a reduction in pitching, and reduction in surface area of the bow, giving less is less drag.

BMW ORACLE Racing - original bow shape - the new ones should reduce drag and pitching -  BMW Oracle Racing © Photo Gilles?nid=58754 Martin-Raget   Click Here to view large photo


Turning to Alinghi 5, one of the features which created considerable comment was the lack of visible support for the bows - as her design team had opted for a two beam (main and aft beam) configuration rather than the more conservative approach which has been to add a third beam forward to tie the platform (hulls and structure) together

Steve Clark explains: 'The big structural problem with these big multis is the torsional stiffness of the platform. That is to say making it so that the bows rise and fall together. Photos of BOR90 show there can be quite a bit of wracking between the weather ama and the leeward ama.

'We solved this in the C Class by using a four stay shroud system. This, essentially, turns the mast into a king post truss and relieves the beams from providing much in the way of platform rigidity. But if you are setting jibs and big asymmetrics, you can't have the four stay layout. So the usual response has been to add a third beam, such that the bows and sterns are linked solidly together.'

Alinghi 5: Close up of dolphin striker under mast position. Note temporary opposing strut. The two Y aft running beams can be seen, which intersect with the forward prodder at the mast juncture. -  Bruno Cocozza@Alinghi?nid=58754   Click Here to view large photo


Alinghi doesn't have a third, forward beam.

'Jo Richard's innovation on Le Black (the 41 ft catamaran on which Alinghi 5 is said to be based) was to use a matrix of dolphin strikers and dolphin striker straps under the platform to achieve this torsional rigidity', explains Clark

' Essentially this system makes a tension structure under the trampoline do what the four shrouds on a C Class do above the trampoline. But on these America's Cup multihulls, there is more to it than just that, because there is also a honking great bowsprit to support, so 'web central' is quite a big deal.'

'The concept has been pretty well tested on Le Black, so the question is how well can it be scaled up to jumbo size?'

Alinghi 5 showing the placement of her two beams and no forward cross beam -  Stefano Gattini - Alinghi   Click Here to view large photo


Clark has long advocated that the America's Cup should be sailed in the ultimate sailing craft, and looks set to see his dream realised.

'Alinghi is a pretty striking beast.' he says. 'I hope we actually get to see both these boats sailed hard.'

'Of course the most important thing will not how up in photos. HOW MUCH DOES IT WEIGH?'

'The same question has to be asked of BMW Oracle Racing's BOR90 before we can start to figure who has what kind of advantage here.

'It may be simplistic, but in the C Cats the heavier boat rarely won a series, and on th eodd occasion when they did, that was usually because the lighter boat broke too often. The scales were the best predictor of the outcome,' he concludes.

'Of course, the best way to mitigate the sailing loads on a catamaran is to be very aggressive about keeping the weight down. The lighter the boat, the less stability, the less stability, the lower the loads, the lower the loads, the lighter the structure and so forth....' says Clark expounding the self fulfilling prophecy of weight reduction in yachts of this type.

Against this backdrop of the pursuit of lightness and general weight reduction, the concept of using water ballast seems to be an anachronism. However Le Black, the seed for Alinghi 5, carried water ballast.

Clark admits to being a little perplexed on this point. 'I don't have a clear picture for the benefit of water ballast in these boats', he explains. 'Given the monstrous loads, why make it worse? Given the brute force required to sail a machine like this, why not just add crew?

Alinghi 5 features very slim 'wave piercing hulls' -  Carlo Borlenghi- Alinghi?nid=58754 ©   Click Here to view large photo


'They move and can do more than just sit there, like water ballast. There is no requirement to sail with a fixed number of crew, so you can mode the boat for the day by adding monkeys.'

'It also occurred to me that at the speeds these boats are likely to sail, a 40 mile race isn't a particularly long event. The chances of getting caught out by a weather change are greatly minimized if the race takes 2-2.5 hours.'

The clear implication being that if you can predict the wind for the next three hours, then you should be able to mode the boat with the optimum crew weight to extract maximum performance for the expected wind condition. Which in turn points to the advantage/necessity of having venue which has steady and predicable winds.

Clark believes Le Black was used extensively by Alinghi’s desigbn team to evaluate and measure stresses -  George Johns-Alinghi?nid=58754   Click Here to view large photo


Despite having his skipper and lead designer, Duncan MacLane, from the Cogito project, consulting to Alinghi, Clark can only guess at the development process the America's Cup Defenders have worked through with the multihull now known as Alinghi 5.

'I'm pretty sure that a bunch of the sailing Le Black did over the last year, was loaded to the gills with strain gauges as Dirk Kramers refined their understanding of how the various components of the struts and strings are loaded', Clark surmises.

'I also suspect they tried several different configurations of trussing to determine if there was a better plan. I have a great deal of admiration for Dirk and would hang my ass out a 10th story window on a piece he said was strong enough! So I would pr




by Richard Gladwell

  

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12:35 PM Wed 8 Jul 2009 GMT






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