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The foil force

Last year Paul Larsen broke the world speed sailing record three times. Matthew Sheahan reports on how his craft’s unconventional hydrofoil technology could change the course of design for the entire boating world

June 15 2013
Matthew Sheahan

The world’s fastest sailor sounded anxious as he made one last push down the speed sailing course in Walvis Bay, Namibia. Despite more than a decade spent chasing the world sailing speed record, this run – the last of his 2012 campaign – was different, and the Australian was struggling to temper his burning ambition with prudence. Having suffered several serious crashes in his previous boat, one of which left him hospitalised, Paul Larsen knew the risks as Vestas Sailrocket 2 took him to the edge once again.

While he coaxed himself and his extraordinary craft to the end of the 500m run, his voice over the team intercom was as jarred and distorted as a rally car navigator’s on a forest stage. Travelling, on average, almost as fast as the UK motorway speed limit, with only the wind to propel him, every small wave felt like hitting a speed bump followed by a pothole.

“I was nervous about the potential of the day. It could all end in a number of ways,” he recalls. “There are crash scenarios for this boat that I’m pretty sure would be lethal.” Indeed, so concerned had he been the night before that he lay awake wondering whether he should write a last note and stash it somewhere, just in case. After wrestling with his emotions, he defaulted to his more familiar confident self. “Just get it right, Larsen,” he told himself. And a few hours later, he did.

On December 4 2012, VSR2 achieved a top speed of 65.45 knots (75.32mph), a performance that didn’t just nudge the bar higher, but obliterated the previous record set by kitesurfer/kiteboarder Rob Douglas in 2010. Larsen’s run had added almost 10 knots onto Douglas’s speed – an 18 per cent improvement and the biggest single step between competitors in the outright sailing speed record since its beginnings in 1972.

This impressive achievement was major news and a big deal for high-performance yacht design. But for many of the experts who had followed the progress of Larsen’s 10-year quest, the record, and the manner in which he had broken it, was a turning point for amateurs, too.

In fact, this was the third time Larsen had broken the world record in two weeks. His first victory on the 500m course came on November 16. Two days later, he added just over a tenth of a knot to this speed. But, in a separate attempt on the same day, he’d also proved that VSR2 was more than a short-course sprinter, as he demolished the one nautical mile record, adding more than 5 knots to it with an average speed of 55.32 knots.

Larsen and VSR2 were on fire. After years of thrills, spills, disappointment and hardship, in a campaign that at one point took him close to financial ruin, the planets had aligned for Larsen and his team. Now they were notching up the records as confidence in the bright orange beast grew.

But this achievement was about more than posting multiple entries in the record books. For years, breaking through 50 knots had become sailing’s sound barrier. The closer teams got, to this speed the more there was talk of strange things happening to the underwater control of the boat. Even when the barrier was finally broken in 2008, the speed increases that followed were generally small, incremental steps – proof, it was thought, of sailing’s speed ceiling.

So Larsen’s spectacular performance at the end of 2012 represented a true breakthrough, which proved some of the world’s leading hydrodynamic experts wrong. He had provided unequivocal proof of a new concept and one that could change the course of boat design. There is no question that VSR2 is a radical machine. One of the key features of her innovative design is her underwater appendages. These are crucial in balancing the driving force with the need to prevent her from taking flight by keeping her locked onto the water’s surface.

This may sound a long way from the requirements of a typical family cruiser, but when it comes to keels, rudders and other underwater control surfaces, the principles of lift and drag remain the same whatever speed a boat travels at. Like wings under water, a keel or rudder’s shape and cross section are as important as those of any aircraft’s wing. And just as the design of aerofoil shapes has improved dramatically over recent decades, so the technology involved in the underwater appendages of sailing boats has produced better-balanced boats for cruising and racing. VSR2 marks the next big step forward.

The recent development of keels, rudders and centreboards (or “foils”, as they are generally termed) has been a big focus for designers and has seen overall performances increase as dramatically as safety and efficiency have in the automotive industry. Foils are in vogue, but there are some that are taking the technology and speeds to completely new levels.

High-performance foils are particularly popular at present with those who want to go faster than the waves will allow. A set of hydrofoils raises the boat above the water’s surface, providing a very appealing solution to the problem. The basic technology has been around for a very long time; its early development started more than 100 years ago when Italian Enrico Forlanini succeeded in reaching 36.9 knots (42.5mph) with his 60-horsepower airscrew-driven boat in 1906.

Achieving speeds on water to rival that of many of the cars of the day made several engineers sit up and take notice, including the Wright Brothers and Alexander Graham Bell, both of whom experimented with foil-borne craft. But it wasn’t until the 1930s that sailing boats got up onto foils, with Americans Robert Gilruth and William Carl among the first to manage the same feat as Forlanini (albeit more slowly) under sail.

Among the many foiling projects that currently exist, a renewed interest in sailing hydrofoils has been rekindled by the rapid growth of the International Moth class, single-handed 11ft racing dinghies that fly on slender struts a metre above the water as they slice their way silently around the course. Never before has foiling been available to so many at club-racing level. The latest 72ft America’s Cup catamarans, which will compete this summer in San Francisco, will also now climb up on their hydrofoils to reach speeds nudging 40 knots. All this current focus on following technology means new lessons are being learnt fast.

The key to Larsen’s project has been anchoring his boat to the water with a foil rather than lifting it clear, yet the fundamental issue of developing high forces at high speed using hydrodynamics is the same. Whether you’re lifting the boat or holding it down, there’s a problem that has held back foil designers for years. Push a foil fast enough and at some point the water will be unable to follow the shape of the underwater section. A gap will appear, causing a vacuum, which can result in a great deal of drag. In some cases the foil stops working altogether, with disastrous consequences similar to those suffered when you get a puncture on the front wheel of a bicycle travelling at speed. And so, cavitation, as it is known, is a serious problem for high-speed foils.

One solution is to allow the foil to suck air down from the surface to prevent the vacuum from developing. It was this “ventilation” principle that Larsen’s design team used to prevent its foil from succumbing to cavitation. But instead of following conventional wisdom and sucking air down onto the side of the foil, it decided to draw the air to the trailing edge. This innovative technique proved successful, but the team had certainly learnt the hard way.

Its first boat, Vestas Sailrocket, proved a nightmare to handle at speed, although not because of foil issues. “We were having a lot of crashes. VSR1 was a traumatic thing to sail and we flipped it several times,” recalls Larsen. “She was a weapon, but she had a nasty edge. It was like trying to fly an arrow backwards.”

The design had a number of structural and balance issues that were causing serious problems, including a spectacular cartwheel in December 2009, when VSR1 took off and performed a backward flip in the air. Miraculously, Larsen escaped unscathed. Later on, after VSR1 had been repaired, a major structural failure at speed knocked Larsen unconscious in the water and saw him hospitalised with head and neck injuries.

At this stage, the issues were more to do with the balance of the boat and, as the team struggled to keep its boat and sailor in one piece, it still had little idea of the foil problems that lay ahead. The second boat, VSR2, was launched in 2011. From the outset she was a big improvement, a more stable vessel, less prone to spectacular wipeouts.

“It was like going from a Sopwith Camel to a Starfighter,” says Larsen. But as he gained confidence in pushing her and the speed started to build, so the boat started to behave strangely. “We’d get her up to around 50 knots and there would be this grumbling noise and violent shaking and her speed would drop to 28 knots,” continues Larsen. “We knew we had lots of power in the sail and couldn’t figure out what was happening.”

The answer was that the foil was cavitating, and after plenty of head scratching and number crunching, Larsen’s team decided that ventilating the foil would be the best solution. “The experts said it wasn’t possible to build a ventilated foil like this, and even if it were, it would create loads of drag,” he explains. “When we said we were going to proceed anyway, the hydrodynamicists didn’t want to be associated with us any more.”

After experimenting with the first foil and arming themselves with a stack of new data, the team went back to the UK, where aeronautical engineer and Sailrocket team designer Chris Hornzee-Jones drew a new ventilated foil. The key to the foil was having a more triangular wedge shape that naturally formed a void off its wide trailing edge and drew air down along its length.

This was the breakthrough. From the early trials in 2012, the team felt more comfortable at speed and, as its confidence grew, the top speeds climbed rapidly and into the record books. But how could such a refined component, designed and built to operate at speeds that would be incomprehensible for anyone other than a speed sailor, provide any wider benefit?

Until now, developing high-performance foils has meant making them thinner while ensuring they are strong enough to carry high loads. This becomes increasingly difficult and building them often requires expensive materials and processes. “Thick foils, like the one we developed, are good structurally and they’re easier to build,” says Hornzee-Jones. “In addition, creating a foil that ventilates at low speeds means that at, say, 20 knots you’re already into low drag.” For a boat, this would be similar to a car’s gearbox that allowed you to engage top gear at 20mph and accelerate through to 100mph without changing it (though sailing boats also have the benefit of gathering more power from the wind the faster they move). Versatile, low-drag, well-mannered foils whose creation doesn’t require costly materials and precision engineering could indeed transform the behaviour of boats in the future.

Just as the performance of the average family saloon car has improved dramatically over the past couple of decades, so has that of typical family sailing cruisers. While these boats won’t get close to the speeds that Larsen achieved, the design of VSR2’s foils could provide a big step forward in performance without compromising handling or increasing costs. Small, easy-to-build foils that present little drag could be the next generation of rudders or daggerboards, but the applications could extend even further. Efficient, low-drag stabilisers, like those that have transformed modern passenger ferries, could significantly reduce the rolling of a boat when sailing downwind. For long-distance blue-water cruisers, a stable boat would be a godsend for many family crews. From day sailing to an Atlantic crossing, the ability to create a more stable platform could reduce seasickness significantly.

Larsen believes the repercussions could even travel beyond sailing. “No one’s flying around in supersonic any more, and yet few would say that the Concorde project was a waste of time because it was so extreme,” he says. “Important lessons were learnt and the technology trickle-down happened. Our discoveries may well filter into other areas, too, from the design of blades in, say, hydraulic pumps to those of turbines. Technical breakthroughs can spread wide and their applications are often the product of some strange coincidences.”

Larsen’s record might not be about cargo ships or cruising boats travelling at 60 knots, but it is about people being inspired and using the ideas and concepts in other ways. His team is certainly not resting on its laurels, as it is already considering heading away from the coast and into long-distance performance. “Our next stage is to start applying the concept to a more practical environment for offshore sailing. So far, we’ve built a no-compromise speed-sailing boat and succeeded. Now we have to build a boat that is compromised and can do all the things a normal boat needs to do. And therein lies the challenge. Now we have a boat that has disproved conventional thinking, what exactly is normal?”

See also

Sailing, Paul Larsen