The world's fastest sailing machine

Paul Larsen tells us how Vestas SailRocket 2 hit 60 knots...

This week is lining up to be a significant one for sailing speed records. Namibia is where it is all happening with the Luderitz Speed Challenge for the windsurfers and kiteboarders, while some 500km to the north in Walvis Bay, Paul Larsen and the Vestas SailRocket team on Monday established a new outright record (although not official) for ‘boats’ when their radical speed sailer averaged 54.08 knots down the 500m course, demolishing the previous record of 51.36 knots set by Alain Thebault’s team on L’Hydroptère in 2009.

Still elusive remains the ultimate sailing speed record of 55.65 knots, set two years ago in Luderitz by American kiteboarder Rob Douglas, but not for long, believes Larsen. On Monday’s final run, in 25-30 knots of wind, their peak speed topped out at 61.92 knots and for five seconds on this run their average speed was a whopping 59.08 knots.

“I thought we had it yesterday, as you would with a peak over 60 knots,” says Larsen. So later this week we could see the sailing speed record not just chipped away but, as Vestas SailRocket’s Aussie pilot puts it: “Being knocked out of the court”.

And boy, do they deserve it. Larsen, Helena Darvelid and the Vestas SailRocket team have been plugging away at this record solidly for a decade now and during this period they have seen the world record over the 500m course increase by 9 knots, including, four years ago, it being taken through sailing’s ‘sound barrier’ of 50 knots by a pesky kiteboarder. However Larsen believes that Vestas SailRocket is now showing the potential to break 60 knots average speed.

Having notched up a peak speed of 56 knots on their second run of the day, on the third Larsen recounts: “She just went into fast forwards. You are always getting that extra lunge and all of a sudden you are above 50 and I was going ‘this is fast, this is fast, this is REALLY FAST’...and the water starts feeling quite hard and the boat comes alive, is really responsive, you feel everything is locked in and lifting up – it was a pleasure to drive, it feels better, stable at speed, it goes on rails... And I was counting off long seconds, like I was running out of time, but you know every one of those seconds is gold on your average, so it was ‘hang on, hang on, HANG ON’...”

However in the split seconds towards the end of the run, Larsen says he realised that he had never experienced stopping Vestas SailRocket from 60 knots and, fast running out of runway, he was forced to bail out. “She was going all the way. That wasn’t hitting the limit, it was just ramping up. It was on its way to 65 and above,” he enthuses.

Since heading out to Walvis Bay at the end of September, the season had been going surprisingly slowly for the Vestas SailRocket team, thanks to the lack of any decent breeze. In fact until Monday they had only managed nine runs.

The VSR2 Team (left to right): Alex Adams, Helena Darvelid, Paul Larsen, Ben Quemener, Ben Holder. Wally is taking the pic.

Monday’s performance came slightly out of the blue. So having managed to get a top speed of 54 knots out of Vestas SailRocket last season, how come the big speeds seem to be coming now all of a sudden? The big issue has been getting the configuration of the lifting foil beneath her main hull just right. This has evolved venturing into new territory, moving away from cavitating foils towards ones that are the exact opposite – they are designed to ventilate (and therefore not cavitate), shedding drag by sucking air down from the surface.

Last year with their new Vestas SailRocket boat, the team went down to Namibia with two foils, one a regular(ish) sub-cavitating foil, the other a wedge-shaped affair designed to ventilate. The latter appeared to show the most promise, but they were unable to get it above a peak speed of 54 knots. However when they presented this problem to hydrodynamists, they were advised that it was impossible and that their ventilating foil should not be able to achieve speeds of more than 30 knots – something they had obviously proved otherwise in practice.

The ventilating foil (foreground) and cavitating foil at Vestas SailRocket 2's launch last year

As Larsen puts it: “The hydrodynamicist insisted that we would not be able to get ventilation of the foil all the way down to the tip. But when we looked at their theories and their numbers, we realised that that didn’t explain the performance we saw last year with our first try with one of these foils.”

One issue was that they had imagined the foil would ventilate on its low pressure side, but this proved not to be case with both sides of the foil remaining wetted and the ventilation occurring down the foil’s blunt trailing edge and at its base. “When we started researching why it was doing that, we realised that the foil was simply way too big to ever lose grip enough to ventilate,” says Larsen. So in the name of research towards the end of last session they decided to shortened their foil's horizontal lifting plane 15cm at a time.

“We went out with 15cm lines marked on the foil and a big hacksaw!” recounts Larsen. “You don’t chop up a big expensive foil like that lightly, but you realise if it is not working that was the next option.” Despite shortening the board eventually by 45cm, down from roughly 1m to 55cm, Vestas SailRocket was still starting up okay and was still hitting just over 51 knots. This done, they then returned to base to digest all the data they had accumulated.

At one stage in their discussions they were considering returning to a cavitating foil, but to get the desired drag numbers would require going ultra-thin and the type of foils necessary were going to be all but unbuildable, even in high tensile steel. “They had the drag qualities that we wanted, but structurally they were unfeasible and we kept coming back to this ventilated thing, of sucking air down the back of the foil to release all that drag,” says Larsen.

“The vapour pressure of liquid is almost a full negative atmosphere of pressure, almost a full vacuum. So if it is around -14psi, then you only have to have a foil like we have, with 18sqin on the back of it, and all of a sudden you have 100kg of drag, like pulling four or five water skiers off the back. That is no joke. But if you release that pressure by introducing air into it, it is like rupturing a vacuum bag – all of a sudden there is no vacuum, all those water skiers disappear.”

Another issue with their first ventilating foil was that the hole in the water down the back of the foil would occasionally suck in water. As Larsen explains: “Every now and then we’d be sailing along at 50 knots and you’d get this sensation like you are running along the bottom of a concrete boat ramp - this horrible crunching, grinding sound and that ventilating cavity at the back of the foil would be choking, it would cavitating - forming and collapsing, forming and collapsing like a pulse jet at a high frequency and the drag from that would just pull the boat down to 30 knots. It was quite alarming.”

Lacking data to model a new foil accurately using CFD and getting unhelpful advice from hydrodynamicists, ultimately Larsen and his design team set about creating a new base-ventilated foil in-house. The new foil ended up being designed by their aerodynamicist, Chris Hornzee-Jones from Aerotrope, who has carried out much of the design work on the new Vestas SailRocket.

“So we basically backed this horse that some people didn’t think was the right way to go,” continues Larsen. “Effectively what we are doing with the base ventilated foil is like going from a nice long fine displacement J-Class style hull to a planing hull where you get separation off the transom. So the thickest part of the foil is the transom and once you get that clean separation off the back there is no drag penalty there for having it.”

The end result is a much scaled-down version of last year’s ventilating foil, around 60% of the original's area, with a depth of around 60cm submerged and a chord of 25cm at its maximum. It also has less camber than the previous foil, but going into any greater detail about the shape errs into trade secrets.

“It is designed to be the world’s fastest foil, although we’re not aiming for 70 knots,”continues Larsen. “We wanted to build a conservative, easy to build, reliable version to show us that his concept works.”

The foil is also designed so that they can fit add-ons to its trailing edge or base. “You just have to start thinking of the foil as a planing surface on both sides, so all the things you’d normally play with, like fences or you could add pipes... because the funny thing about this back of this foil is that it is no longer in the stream of water, so you can hang any sort of junk off there you want - cameras and pressure sensors, drinks holders!”

To glean some hard data about what is going on along the foil and particularly at its ventilating base, this season they have fitted highly accurate pressure sensors which they have hooked up to a Cosworth Pi data logger. “If it is cavitating deep down, then we want to know by how much and if we add these trips etc to make it ventilate better, then we want to see the affect they have and the only way we can do that is to put pressure sensors deep down on the back face of the foil and see how much pressure is there,” says Larsen.

And fitting a well-positioned fence on the foil seems to have done just the trick and is what enabled Vestas SailRocket to manage Monday's big performance.

As Larsen says: “We looked at the whole problem and we couldn’t work out what it was. It was obviously not the power, because if we sailed in more wind, the boat didn’t go any faster. So we looked at the problem backwards basically and we said that the only thing that was stopping it from going faster was cavitation. We knew that the new foil shouldn’t cavitate until we are doing 65 knots or at 52 knots, if we are overloading it by 40%. Then we had to work out why or how that might be and it had to be that the top half of the foil was ventilating [ie the wrong part of the top half of the foil was ventilating] and the bottom half of the foil was doing all the work and was overloading it.

“So simply by adding a fence in a key location we went out and that’s when we saw the performance of the boat jump hugely in light winds, and that worked all the way through to the top end. We just stopped that ventilation where we didn’t want it on the side of the foil and everything began working efficiently. Now it just seems to be doing exactly what it is supposed to do.”

The eureka moment...

“One of the most important instruments on the boat is the little wind indicator that sits in front of the cockpit. The whole fuselage of this boat is orientated into the apparent wind angle, so I know that should be blowing straight down the fuselage, but it has never been anywhere near that before, it has always been off to one side which basically means that we have been underperforming against our polars and I haven’t been able to drag the apparent far enough forward. Well, on the last few runs that has been streaming straight ahead, so the boat is doing exactly what it is supposed to do. It is a simple little test, but it tells us nearly everything we need to know.”

So what’s it like at 60 knots? “It depends on how close I get into the beach. If I stay out of the rough stuff, it is a short sharp bumpy ride, like on a high speed power boat. This thing doesn’t knife through the waves, it skips over the top of the small chop. At the back of the boat it is pretty good, just riding on a foil, it is pretty civilised. The visibility is brilliant. I have got no sunglasses or visor on. There is no spray coming into the cockpit, compared to the last boat. I only feel a little bit of spray just when I start up.”

So now the speed sailing juices are fully flowing in Walvis Bay. Vestas SailRocket is in the right trim to hit bigger numbers than have ever previously been seen in sailing and it looks like the next windows of opportunity to achieve this will be on Friday and Sunday.

“She is just gagging to do it,” muses Larsen. “She didn’t hit her own peak at 61.9 knots - I had to let the mainsheet go to stop the boat before I ran aground. It was accelerating hard. If you look at the speed graph of the boat on that last run, it is like the Matterhorn! It goes up until I drop the mainsheet and then it drops off again. Our boat has no place hanging back around in the low 50s and I am interested to see what its limit is. I am pretty sure it is well above what we have already hit.”

While after Monday’s run Larsen is bullish enough to feel that breaking the outright sailing speed record is now almost a formality, but part of the game to coax the big numbers out of Vestas SailRocket will be getting used to her at warp speed. Larsen says that the early 50s is now familiar to him, but 60 knots might present some new challenges.

“This is all new territory to me,” Larsen admits. “Yesterday’s run was the first time I have ever gone 60 knots and everything goes into fast forward and your thought process has to keep up with it. You are thinking a lot of things that once you get experience there, you won’t need to consider, like ‘can I stop’? And you are trying to judge things and you are looking how high the pod is flying and seeing that the foil is riding high and it is all going mental and you are looking forward and making sure the ropes are in the right place, etc.”

Fortunately Larsen now knows that he can stop Vestas SailRocket, in fact he says that she was very well behaved when he finally dumped the sheet and dialled down at the end of Monday's last run. “I am pretty confident that we can get up to a 60 knot average. I always say ‘we haven’t done it yet’. Well yesterday we did it really. Now we just have to make a cleaner run. It is not about whether the boat can or can’t do it, the boat can do it, it’s now down to us humans not to fumble the ball. Everything is working well and the boat isn’t stressed at 60 knots. It is not made to explode at 65 knots, it is made to go well over 70 with a safety margin.

“So Friday. I predict a riot!”