Ben Ainslie’s BAR crew fend off Emirates Team New Zealand during the AC World Series which they won. Raced on the one-design AC45F foilers the World Series delivered plenty of action this tight, but few expect as much close combat once the America’s Cup proper begins in May when the technical teams also ‘line up’ against each other. While Artemis and Oracle fought it out in January’s practice races, SoftBank were also looking fast – Dean Barker’s team clearly benefiting from access to Oracle’s design technology
operator error onboard. So there could be races where the most proficient teams get around the entire course with their hulls never touching the water: the holy grail of foiling cat racing.
At the top end, AC50s may not be as supersonic as expected, says Emirates Team New Zealand design co-ordinator Dan Bernasconi: ‘I think you’ll regularly see the boats doing well over 40kt depend- ing on conditions. Maybe you’ll occasion- ally just touch 50kt in a bear-away…’ Part of the reason for this top speed ‘envelope’ is that a crew would need to be very brave indeed to go to the line with boards optimised for ultimate speed at the top end of the wind range. Plus there is the significant potential issue of boards cavi- tating once they reach 40-45kt. ‘At high speeds the water effectively boils on one side of the foil,’ explains Bernasconi (who faced this problem while working on Paul Larsen’s successful Vestas SailRocket speed sailing campaign – which eventually managed to overcome it). ‘That presents a physical barrier in terms of how fast you can ultimately go – once the foil starts to cavitate there is huge drag associated with that. You can design section shapes to try to delay the onset of cavitation, but it will always remain a trade-off between that and not hurting performance at lower speeds.’
No margin for error
As to the limits that America’s Cup foil design has reached in this cycle, Bernasconi summarises: ‘In the last Cup, at the start, it was all about just trying to get foiling and trying to be stable.
‘We [Emirates Team New Zealand] approached that from quite a different
34 SEAHORSE
perspective from Oracle: we were looking at something fundamental in the foil shape to give us foil stability, whereas Oracle had less stable foils to start with and were try- ing to work more on their control systems. Then we improved our control systems and they took some of our ideas on making the foils naturally stable – over time we converged somewhere in the middle. ‘This time everyone knows the equa- tions and it is more about how far you can push that. Last time chord length of the foils was coming down and down and everyone was trying to see how low you could go. This time teams quite quickly found about the right place in terms of chord length; since then they’ve been push- ing for higher aspect ratios, more like the very slender, glider wing-type aspect ratios rather than short, stubby, stronger foils. Today you are seeing how thin and how light you can make them, while keeping reasonable confidence in their strength.’ And it is here that the most significant compromises
face each team’s hydro -
dynamicists, engineers and foil builders. As Bernasconi observes: ‘It is a very coupled hydrodynamic and structural problem: you are trying to make the foils work with the best possible hydrodynamic shape, while making them stiff and strong and light. All of those things trade off against each other and require a thorough understanding.’ En route to this position all the teams are believed to have broken boards – some teams several – although in the case of Team New Zealand they have only broken one, which came with the Luna Rossa AC45F they acquired as their test boat. ‘We have been fairly conservative,’ admits Bernasconi. ‘From a campaign point of view, we can’t afford to be breaking many
boards, so maybe we have taken a weight penalty in building stronger boards.’ However, with the accuracy of the modelling ever increasing, alongside the advancing definition of the load cases, tol- erances and safety margins have come tumbling down. ‘We are now at the limit of the structures,’ admits Michel Ker- marec. ‘The progress has come from more knowledge, validated with better measure- ment using optical fibres, of what happens on the water – we have much more feed- back in terms of real-life loads than when we all started out. The coupling between the hydro part and the structural part, that is much better now, so we have become much more accurate in our predictions.’ However, so fine are the tolerances now it would be relatively easy for a board to be broken by user error. Dan Bernasconi observes: ‘It is quite easy to have either a sailing mistake or a mistake in the control system or hydraulic logic, which could lead to the foil seeing loads it shouldn’t. ‘It is something that is always high in our minds: if you press the wrong button you can break a foil. Trim it wrong and it can break. In every manoeuvre there is a risk of breaking a foil and when you only have two race pairs, that’s scary.’ Fortunately each AC50 is fitted with an array of load sensors, linked to displays and alarms, so there should be plenty of warning if someone hits the wrong switch or something is about to blow up. Then again, back in the era of the long and powerful V5.0 ACC monohulls, the structural engineers used to regularly chas- tise their sailors, that the red load warning lights they had so diligently installed were just that, warning lights. Not load targets. Next month… Design convergence
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