On 24 November 2012, Paul Larsen and his Sailrocket team rewrote our understanding of the physics of sailboats, stamping their names indelibly in the record books as they set a new record for the world’s fastest sailboat.

A little over a week earlier, at a spot called Walvis Bay on the coast of Namibia, Sailrocket 2 had pushed the outright sailing speed record up by the biggest-ever margin – from 55.65 to 59.23 knots. The performance on the 24th smashed it beyond all expectations though, a gloriously windy day that saw Sailrocket 2 deliver a 65.45 knot average officially becoming the world’s fastest sailboat.

It was a remarkable human achievement, piloting a boat down a 500m course at speeds that had previously been thought impossible. “Your job is to go 100% down that course, there’s no halfway about it,” Paul Larsen told me, almost a decade later. “By the time you’ve got a big team and all the momentum of that project going, your biggest fear is not going fast.”

The risks are inescapable though, as Larsen had revealed in a blog; “As I lay awake in bed that morning I considered writing a little note that I hoped would never be read and stashing it somewhere. Too morbid. Just get it right, Larsen.”

Growth of the world’s fastest sailboat

To put Sailrocket’s performance into context you need to consider the trajectory and history of the sailing speed record. It started back in 1972 with Tim Colman and Crossbow setting an opening mark of 26.30 knots.

By 1993, Yellow Pages had upped that all the way to 46.52 knots – an average improvement of almost a knot every year. But then something changes, progress halts for over a decade. The windsurfers and kiteboarders eventually start nudging it back up, but it’s 16 years before another yacht – Alain Thebault’s foil-borne L’Hydroptère – sets a new record, not even five knots quicker than Yellow Pages.

It was thought that the speed of sailing machines was reaching a ceiling, a physical limit defined by the cavitation point. If you have ever made a cup of tea at altitude then you will know that the boiling point (the transformation point where water changes from a liquid into a vapour), varies with pressure. The lower the pressure, the lower the temperature required for water to boil. So, at the top of Everest, water will boil at about 68°C.

There’s also low pressure on the leeward side of an aero- or hydrofoil. Foils provide a lifting force because of the pressure difference between one side and the other. This difference creates the force as the foil tries to equalise the pressure.

If a hydrofoil goes fast enough then the pressure to leeward will drop sufficiently that the water starts to ‘boil’ or vaporise. This creates a loss of lift, and instability as smooth flow turns chaotic, with vapour bubbles flowing down the foil to an area of higher pressure where they collapse.

It’s this speed limit that we see America’s Cup and SailGP foilers hit on a reach. Once the speed gets much above 50 knots the foils – which are designed to suppress cavitation for as long as possible – finally start to cavitate and the boats just can’t go any faster.

To get past this point a completely different type of foil is required, one that does not try to eliminate cavitation but instead tries to stabilise it, and this is the secret to the 65-knot speed of Sailrocket 2. “That’s the brilliant [foil] design that we settled on, with a lot of help from guys like Aerotrope and Chris Hornzee-Jones. Chris did amazing work behind the scenes on that project, including designing the final foils,” said Larsen.

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The team realised the foil didn’t need to be impossibly thin to suppress and avoid cavitation. Instead, they could encourage it and push past the cavitation point with a foil that would cavitate in a stable fashion.

“To make a dinghy or a powerboat analogy, it’s like when you get over that hump and the boat gets up on the plane. We all know when the water separates off the back of the boat, you don’t want your transom gurgling around at the back there with all that drag,” Larsen explains.

In a similar fashion, Sailrocket 2’s foil is able to shed the turbulent, draggy flow of early cavitation and replace it with a single smooth pocket of vapour around the foil as air sucks down from the surface. Larsen calls this a base ventilated foil, it’s also sometimes termed a super-ventilating foil.

“So you end up with these very shallow camber, base ventilated foils, and they’re not overly efficient but they don’t have a limit,” he explained. “They keep working. It’s like a jet fighter’s wings. They’re not efficient, but if you put a big jet engine behind them, they keep going where the others stop and hit the ceiling.”

Force alignment

The jet engine was the other part of the problem. How do you generate enough power from the aerofoil to push a horribly inefficient hydrofoil up to the speeds required to start cavitation, and then blow through that barrier?

The answer lay in a decades-old idea – force alignment. In conventional sailboats, be they dinghies, multihulls or yachts, the aerodynamic force created by the sails is both pushing the boat forward and pushing it over.

The force is resisted by a combination of a hydrodynamic force from a foil in the water, and weight – either the crew’s bodyweight or the weight of a keel. These two forces act at a distance from the centre of effort of the sail – creating opposing levers, with the forces of mass and hydrodynamic lift opposing the aerodynamic force generated by the sail (or wingsail).

The use of these forces to create a propulsive forward force demands a structure of a commensurate size and strength. So to go faster required more force and/or lighter overall weight, but also stronger structures. It was big improvements in the strength and weight characteristics of materials that allowed much of the jumps in speeds set through the 1970s, 80s and 90s.

But there was another way: by offsetting the forces and aligning them. “So [you] have the centre of effort of the aerodynamic forces, the sail or the wing, directly aligned with the opposing force of the foil,” explains Larsen. In other words, remove the levers by having the force from the sail directly oppose the force from the hydrofoil.

“We didn’t come up with that concept, that was written about in the 1960s by Bernard Smith in the book The 40-Knot Sailboat,” Larsen adds. Smith’s insights were so far ahead of his time that it took almost five decades for them to be fully realised in Sailrocket 2’s record.

Sailrocket 2 achieved the force alignment with a wing mounted on the leeward hull that was canted over the windward hull by 30°. The force it generated was driving the boat forward and trying to lift the windward hull out of the water.

This force was resisted by a foil under the windward hull. And so that foil was pulling down rather than pushing up. It’s a crucial distinction between Sailrocket 2 and the type of foiling craft used in the America’s Cup or SailGP. In those boats, it’s the leeward hydrofoil that pushes back against the sail force. It also lifts the whole boat up and out of the water.

These two breakthrough ideas – force alignment and super-ventilated foils – along with a ‘no guts, no glory’ attitude, took Larsen and his Sailrocket 2 team over 65 knots, a mark that has been held for almost a decade. But might the time have come for that record to be broken?

“I think we’ve sat on it for long enough and it’s definitely time for it to be challenged,” Larsen says. “There was a time I was quite protective and proud of it, and wanted to sit on that throne for a while. But right now I want to see what other people can do with it and see what their solutions might be. I’ll see if it motivates me enough to get back out there myself!”

New fastest sailboat challengers

There are two major challenges shaping up to take on the Sailrocket team’s record and both should take to the water later this year or early in 2023. One of them, Syroco, has been set up by Alex Caizergues, the first man to travel sail-powered at over 100km/h on water, and twice holder of the outright sailing speed record on his kiteboard. The other, SP80, has come out of the Swiss engineering school École Polytechnique Fédérale de Lausanne (EPFL).

Both are using the principles that Larsen established, and both teams think they won’t just break the record but will smash it. Syroco’s stated target is 150km/h, a breathtaking 80.99 knots. SP80 is also chasing the 80-knot barrier.

“I actually like where both projects are aiming,” said Larsen. “They’re definitely using the force alignment concept.” Both the SP80 and Syroco teams will use a kite, aligning its aerodynamic force with the hydrodynamic force from a foil. This should allow the generation of an immense drive force on a relatively light structure. They will need all the power they can get to push through the cavitation point.

The SP80 project is also using a super-ventilating, surface piercing foil like Sailrocket’s. “Vestas Sailrocket and the work done by Paul Larsen and his team was the main source of inspiration that we used to develop the boat,” said Benoît Gaudiot, one of the three founders of SP80.

They started throwing around ideas in 2017, building super-ventilated fins for a kiteboard. Gaudiot, an experience kitespeed sailor quickly got it to 41 knots. They were going to need a different approach to beat the record though.

“The body cannot handle the power that is required to reach more than 60 knots,” said Gaudiot.

Another of the founders, Xavier Lepercq, built a simulation tool, and they started developing designs. What they came up with was a trimaran powered by a kite, whose aligned force was balanced by a surface-piercing foil.

Once this was formulated the team quickly grew, with EPFL pledging its support and sponsors coming on board. “In the team, we have six full-time employees and almost 40 students from EPFL,” explained Mayeul van den Broek, the team’s project manager. They tested a prototype on Lake Geneva in 2020 and in June 2021 began construction of the full-size craft at Persico Marine.

The transition to a kite means that the biggest challenge to both teams is control – accurately balancing the aero and hydrodynamic forces. SP80 has tackled it with what they call the ‘power module’. “The idea behind this is to balance the force. The way we designed the boat, the main thing to achieve was stability,” said Gaudiot.

The exact mechanism of the power module is confidential, but it’s visible at the back of the boat in their visualisations and animations. It provides a direct link between the kite and the hydrofoil and appears to ‘trim’ the hydrofoil depending on the force vector coming from the kite. The shape of the foil and the linkage to the power module are key to the flight stability of the craft.

“It’s fully mechanical and it’s fully adjusting the balance by itself,” said Gaudiot. “The controls will be quite simple for the pilot. There will be no need for me to control the height, the elevation of the boat, just the direction. And the power of the kite.” The kite lines will run to the cockpit and be controlled with the hands, while the direction of the boat will be controlled with the feet.

The SP80 team plan to challenge the record from a base in the south of France early in 2023, and Paul Larsen is looking forward to it. “I think the SP80 is a more practical solution that has made compromises for practicality. And I think I can get my head around that one a bit more. I think SP80 is probably closer to getting results. And I want to see how a kite’s going to go against the [Sailrocket] wing, because historically wings are faster.”

Flight on water

Looking to spoil the Swiss party is Syroco, a French company that comes to the world sailing speed record with gold-plated credentials. Co-founder and CEO Alex Caizergues has already held the record on his kiteboard.

“Since Paul broke the sailing speed record, I knew that we had to change the software and the way to go fast on water. I knew that I had to assemble around me a team of people able to build this kind of craft,” Caizergues recalls.

Caizergues isn’t just an athlete, he’s a business school graduate with an entrepreneurial track record. Syroco was set up in 2019 with four co-founders and support from technology entrepreneurs and venture capitalists.

They want to do more than just break the record, building a technology business around the attempt. The team has about 15 people working in Marseille with specialists in fluid mechanics, structures, software and data analytics.

“Our l’aile d’eau concept… it’s a little bit like Sailrocket,” said Caizergues. The concept is very simple; a hydrofoil will ‘fly’ underwater, pulled along by a cable that’s connected to a kite flying in the air above it.

Suspended between the two is a capsule containing the pilot Alex Caizergues, and a co-pilot. The aero and hydrodynamic forces oppose each other in an almost perfect representation of the aligned forces concept that powered Sailrocket 2.

It should have the greatest speed potential because there is nothing extraneous. The capsule is only there because both the aero and hydrodynamic wings must be controlled, and the forces balanced by the pilots (not automated).

And that’s the tough part, controlling it, particularly keeping the foil in the water. “Nope,” responds Alex, quickly, when I mention this possibility. “The foil never goes out of the water.” The Syroco foil isn’t surface piercing, it runs below the surface, only connected to the capsule and the kite by a cable.

It doesn’t rely on dragging air from the surface to stabilise the cavitation around the foil. Instead, it will rely on the cavitation creating its own stable pocket of water vapour around the foil – this is called super-cavitation. When it occurs the water flows around the bubble of vapour as though it were a solid, significantly enhancing the performance of the foil – as long as the bubble remains stable.

The problem is keeping the bubble intact. Paul Larsen pointed out that the cable gives the air a pathway down to the super-cavitating foil. “How they’re going to stop air sucking down from the surface and rupturing the bubble, that’s the real trick. It’s a very dynamic problem to solve. It’ll be interesting to see how well their simulations live up to the reality of what they’re about to strap themselves into…”

The control mechanisms for the final craft are still being worked on, but they have flown a prototype, towed by a RIB rigged with a 5m-high mast that simulated the force from the kite. The team hopes to commit to a final design with construction starting in the spring.

Human element

And then of course, there will be the matter of executing the plan on the day. “If you’ve done your maths, you’ve done your engineering, you’ve been thorough, that gives you confidence when you stand up on top of that course on one of those big days and you’re not exactly sure what’s about to happen,” recalls Larsen.

“You know it’s probably just slightly above your top wind range but all the cameras are rolling and the drones are in the air and everyone’s waiting with their stopwatches. That gives you the confidence to say: ‘Yeah, I’m going to go and wring its neck.’”

“Any crashes I had [and there were several] usually all the systems I had in place [for safety] were still completely locked on among all the wreckage. You’d go and flick off that lever you were going to use to control something – because by the time you’ve realised what’s happening, it’s happened.”

“If we go again with Sailrocket, then safety will feature bigger. I wouldn’t get in that boat and go that speed again. We got away with it because we had to.”

“Safety is really important for us,” agrees Benoît Gaudiot. They have built a kevlar cockpit for protection, installed a six-point harness and an F1-inspired seat. Gaudiot will wear a helmet with oxygen that will switch on if the helmet detects water in contact with its mask. “I would be able to stay in the water for a few minutes to have a diver come and open it.”

“The critical point on the boat is the hydrofoil. If the hydrofoil breaks, the boat…” Van den Broek interjects. “…will take off,” Gaudiot finishes the sentence for him.

Their enthusiasm for the project is infectious, the words tumbling out. And no one wants the boat to take off. One big advantage that they have that Larsen did not, is that they can release the power source. “With a kite it’s a few lines and you can just cut it super-fast,” says Gaudiot. “You can do it by yourself. You can do it from a distance, from the chase boat. You can do it automatically.”

“I think both those guys [Caizergues and Gaudiot], they’ve got the mentality,” said Larsen. “They’re not going to get up there and be scared of what they’re doing or intimidated too much by the craft.”

And what if they do break the record that Paul Larsen and his team have owned for almost a decade?

“We opened the door on a whole new world full of potential. And so there is a part of me that’s curious as to what lies further down that path. We validated the concepts that could get above what people thought were the cavitation limits and the ceilings of speed sailing. We proved you could get beyond that. They can take you to new levels of physics.

“The boat [Sailrocket 2] is sitting there in perfect shape. It was made to last forever… we could rig that thing up and do 65 knots in a week or two.” And if his record goes, I wouldn’t put it past him to dust her off and do just that.

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