Sailrocket

Strapped into the narrow carbon fibre cockpit of the Vestas Sailrocket, record breaking sailor Paul Larsen was making his 4th run across Walvis Bay in Namibia, in pursuit of the world speed sailing record. But, just moments after it reached a potentially record-breaking 52 knots, the craft back flipped into the air, somersaulted and smashed down into the waves, surprisingly Larsen and his craft are both still in one piece and back in the UK. Now the Sailrocket team are set to unveil a new boat that will push further into the unknown. Paul Larsen “Going from 50kts to 60 kts is like breaking the sound barrier. The water flowing past the hydrofoil starts to cavitate and you lose stability and control, but we think we have a way to live with it…”

Vestas Sailrocket was started by naval architect Malcolm Barnsley in 1999, working as a Senior Test Engineer for Vestas wind turbines he dreamt of building the world's fastest sailing boat. Indeed he had come across the 40 year old book that would define the Vestas Sailrocket's remarkable shape while studying naval architecture in Southampton. It all started in the early 50s, in the desert.

While posted to China Lake in California, naval weapons engineer Bernard Smith wanted to see how fast he could make a model sailboat go on the seasonal salt lake. He found the main problem with pushing traditional sailing boats faster is the side force on the sail trying to tip the boat over. Crews piloting fast yachts manipulate the weight distribution of these boats by leaning right out from the hull on the windward side; to try and extract as much power from the wind as possible.

Smith repositioned the sail so that it could be balanced by a hydrofoil, a conventional wing running in water, beneath the boat. This arrangement can in principle create a vessel where all the wind force is converted into thrust leaving no net torque on the hull. Following his initial work he published "The 40 Knot Sailboat" in 1963; and continued working on small boats and models, but it wasn't until 2007 that the Vestas Sailrocket became the first boat based on his concepts to reach 40 Kts, and then 50Kts later in the same year.

Launched in 2004 the boat began testing at Weymouth; it very quickly showed its promise, easily reaching speeds of 31kts. But the wind conditions were proving a limitation on performance, and a move to Namibia provided the conditions necessary to unleash its full potential. By autumn 2007 the team were ready for the record campaign in Walvis Bay. “The boat took off when it hit that amazing mile. In 15 seconds it went faster than it had ever been... and promptly spun out of control doing significant damage. But we just rebuilt the boat and went out again and again, you're always in search of a better run.”

It had been a hard slog to 40 knots, but when they got there Vestas, who had made the project possible by providing a home for the team, decided to actively sponsor Sailrocket. “The jump to 50 knots was not smooth but it came relatively quickly now that we had funding. We were able to thoroughly engineer solutions rather than patch up what we had.” They returned to Namibia in 2008 and 2009 and over the boat’s 112 total runs they managed to reach record speeds of over 53Kts (98kmh), in wind speeds of only 23Kts (42kmh). But the record is for average speed over 500 metres and while the Vestas Sailrocket’s best measured average of 49.31kts secured a class record, they were still short of the French L'Hydroptere team’s 51Kts average.

Commenting on the challenges of such records, Richard Jenkins, pilot of the Greenbird wind powered land-speed record holder. “Breaking a wind powered speed record is roughly a 50% split between technical credibility and luck with weather conditions. The biggest challenge is to be technically perfect, at precisely the right time, to take advantage of a freak set of conditions. You need to have the vehicle, timing equipment and team ready to go, be in the right place at the right time and have the right people watching in order to set a record which will be verified. This is no mean feat and takes years for all the elements to simultaneously fall into place.”

Richard's team are currently waiting on that perfect confluence for the ice yacht record, after taking the land record last year. They originally planned to tackle the water record as well but focussed on a single adaptable vehicle for the hard surface records. “If we are successful in the ice challenge I may then return to the Water record challenge and try to give Paul a run for his money! In the meantime, I wish him and his team the very best of luck, he certainly deserves it.”

To appreciate the magnitude of the challenge facing the team we must properly understand how the boat works. Sails create thrust just like wings create lift, indeed the Vestas Sailrocket uses a rigid wing rather than a fabric sail. As the wind flows past a sail “lift” is produced in a horizontal direction, usually a diagonal, pushing the boat forward but also rolling it over. Sailrocket's wing, following Smith's principals, is inclined to one side so that it can be balanced by a parallel hydrofoil beneath the boat. When the sail tries to force the boat one way, the foil pulls back the other and only the forward thrust is left. Trouble is that there's only so much force you can get from a hydrofoil.

For the Sailrocket to reach over 50Kts it needs a given size of wing to extract enough wind power; this in turn requires an equivalent force from the hydrofoil. Both wings work the same way, the working fluid is forced over a cambered shape, making it travel faster on the convex side, reducing the static pressure, and creating lift in that direction. Above 50Kts the pressure on the convex side of the foil gets so low that the water starts to boil at ambient sea temperature, spontaneously forming bubbles of gaseous water. This is known as cavitation and the bubbles destroy the effectiveness of the foil and hence the balance of the craft.

The answer would seem to be to reduce the force required from the foil, this could be done by having more than one foil, or a bigger foil. But this would increase the water drag on the boat, which then means you'd need a bigger wing to achieve more than 50kts, which would require a bigger opposing force from the foil and you'd be back where you started with bubbles forming.

The “bubble barrier” is comparable to the problem of breaking the sound barrier and the solutions might be similar. When an aeroplane approaches the speed of sound air builds up in front of it as a compression wave. But it can also happen on top of the wing, where the airflow is accelerated. This happened to subsonic aircraft when they started to push towards the sound barrier, and the resulting shockwave would destroy the lift and stability of the wing. Amongst the solutions was a move away from the classic curvy aerofoil to specific supersonic aerofoils, and it maybe that a similar change in hydrofoil shape might help. However, while Supercavitating hydrofoils have been studied for high speed boats (in this case the aim is to form a predictable layer of gas over a large part of the foil) they are much less efficient as only one side is providing lift. The extra drag this causes could stop a wind powered craft even reaching the speed when these foils become effective. Vestas Sailrocket hope they may be able to push the record higher without having to actually break the bubble barrier, and simply live with a small amount of cavitation. But either way they're pushing the limits of design, no one has tried to make hydrofoils work like this before, and the consequences of getting it wrong could be another, faster, spectacular crash.

The new boat will build on the experience of the record attempts and will apparently feature innovations in design that will maintain control and stability when cavitation starts to appear. But Larsen remains tight lipped about what these changes might be. “We’ve worked long and hard on the project, and in such a competitive field it would be awful to reveal our edge only to see another team beat us with our own ideas!”

The team plan to unveil the new craft in August; following which they hope to break the record before the end of 2010. Sadly this is just too late for Bernard Smith who died earlier this year at the age of 99. Larsen and team however were in regular contact with him. “When we broke 40 knots for the first time I thought it would be nice to contact his family as I didn't expect him to still be alive. It stunned me when he answered the phone.” He was 97 at the time and shared the excitement of the team, offering ideas and advice until the end.