Real results from sailing simulation
Image credit: Stefano Gattini, Team Ineos UK
America’s Cup boats are fast. Very fast. Speeds can top 50 knots (93km/h) as these boats apparently fly over the surface of the water. At this, the highest end of the sport, the engineering can split the winners from the also-rans.
Reigning champs Emirates Team New Zealand are, at the time of writing, preparing to defend the trophy they won in 2017 against the Luna Rossa Prada Pirelli team from Italy. A local pandemic-protecting lockdown in Auckland, New Zealand, has delayed the start of the final series, which is still due to be concluded on 15 March 2021 (weather and lockdowns permitting). Luna Rossa qualified for the final series having beaten the Ineos Team UK’s boat, Britannia, in the Challenger Series final, the Prada Cup – itself the culmination of qualifying stages to determine who gets to go head-to-head for the big prize.
Ineos Team UK, with four-time Olympic champion Sir Ben Ainslie at the helm, fast-tracked through the qualifying with early victories but in the final Challenger Series it lost out to Luna Rossa 7-1 in a best of 13 series. However, the British boat had proved it had a turn of speed that could compete with the very best, and that doesn’t happen by accident.
It also doesn’t happen easily, especially given the rules for the America’s Cup state limited amount of prototyping and pre-race time in the water. One of the quirks of the America’s Cup is that the winner sets all the rules. Thus, it was the New Zealand team who determined the venue, sailing and competition rules and the basic boat specifications. This AC75 Class Rule, as it is known, determined that the boats be 20m monohulls (they were catamarans at the 2017 America’s Cup), and set strict limitations on the number of components that can be built, including hulls, masts, rudders, foils and sails, thus encouraging teams to do more R&D in simulation and subsequently less physical construction and testing.
“We feel we have done a great job containing costs on certain aspects, while leaving the rule open enough for the America’s Cup to continue as the driving force of innovation and technology in sailing,” said Dan Bernasconi, design coordinator for Emirates Team New Zealand on announcing the rules in 2018. “The AC75 Class Rule sets the parameters for the teams to develop and race the fastest sailing monohull on Earth.”
The technical challenge was extended by one of the new rules. Nick Holroyd, chief designer, explains: “Teams have got used to being able to change parts such as the foils and reconfigure boats according to the weather on race day. For the next America’s Cup, we have a fixed configuration, which we are required to declare five days in advance. As racing is expected to last for ten days that means that we need to go for a rather general design which will sail well with any wind speed, both on the straight and going around corners.”
A foil optimised for low speed in light wind is very different from one designed to withstand strong winds and high speed, so the team had to work around average wind speeds in Auckland in March.
From receiving the class rules in April 2018, the team spent five intense months focusing solely on design, with construction beginning in August that year. It took 90,000 design hours and 50,000 construction hours to get the boat on the water. “We designed from scratch but we could draw on all the simulation tools and expertise that we already had in place,” says Holroyd.
“We relied on our simulation environment while the first boat was being built,” Holroyd continues. “One of the first requirements after launch was therefore physical testing to validate what our simulation showed. We expect all our simulation to be within a 2 per cent margin; even so, there’s always a surprise, and one of the fun aspects of the job is to turn such nuggets of information into an opportunity for our design.”
Under race rules the team is limited to developing two boats with a maximum of three rigs, four rudders and three pairs of wings, so again the team needs to rely on simulation. “Because we cannot develop spares, the fidelity of our computational solutions is paramount,” says Holroyd.
“A racing yacht sits at the interface between two fluids, wind and water, deriving all its power from that dynamic by minimising drag,” says Holroyd. “However, we can’t work with that in isolation. We have to consider structural and mechanical features, and for the boat to ‘fly’ we need to get the foils up to speed very quickly; at high wind we quickly run into cavitation, when pressure reduces too much over the foil.”
Ineos Team UK relies on Simcenter STAR-CCM+ software for computational fluid dynamics (CFD) to model the lift forces acting on the foil. Modeling shows how the flow is developing – changing, for example, from laminar to turbulent, and how it might separate, creating a drop in pressure and creating cavitation or a “boiling” effect in the water.
CFD engineer Max Starr spends 90 per cent of his day using the CFD software. “When we receive a CAD file the geometry is pretty complex and often not the cleanest. We need to throw some mesh on before we can solve. We can very quickly facilitate meshing issues without having to fix the geometry manually, and that means we achieve a very low failure rate.” According to Starr, the team tends to run hundreds of short, one-hour simulations in parallel. “We gain much more from these than from one long simulation that takes weeks,” he says.
Teams are not allowed to test in any closed environment such as a wind tunnel, so the pressure is back on simulation. “The challenge is to work out the combined forces acting on the boat and calculate the loads on the foils, the hull and the crew,” Holroyd says. “The calculations we gain from our software solution form the basis for waves and gusts introduced to a motion platform on which the sailors train, complete with virtual-reality headsets and graphics.”
The sailing team, which began using this simulator in 2018, are in continual dialogue with the design team regarding the feel and behaviour of the boat. Starr adds: “We can gather feedback from over 100 different points on the boat to work out the pitch, roll and yaw then convert that data into a sensitivity reading, positive or negative, for each of the components.”
The team’s second and final boat hit the water in the summer of 2020, and in early 2021 sailing began in earnest. “The rules and regulations push our research and development further into the digital world and our success depends on what decisions we made at the start regarding our broad technical strategy and where to concentrate effort,” says Grant Simmer, chief executive of Ineos Team UK. “To be competitive we are constantly balancing associated risk with performance reward and the resources that we can apply.”
Ultimately the light winds of the Prada Cup and the excellence of the Italian team proved too much for Britiannia to become the first British boat to win the America’s Cup, the world’s oldest international sporting trophy. Skipper Ainslie promised that it was back to the drawing board for another four years. It sounds like many thousands of simulation hours lie ahead.
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