Sailing off with a world record!

Sailrocket 2 is the fastest sail-powered boat on the sea – and its developer, Paul Larsen, is looking to take the concept further. Lou Reade reports. Sailrocket 2 took 11 years to develop – and less than 15 seconds to break a world record. In 2012, it became the world’s fastest sailing boat, exceeding 65 knots over a 500m run along the Namibian coastline. On land, that would be fast enough to break the speed limit – equivalent to 75mph.

It may be described as a boat, but Sailrocket 2 looks more like an aircraft wing with a sail perched on the end. Composites feature heavily in the design – and, in many ways, were crucial to its success.

“Weight is not super-critical on a high speed vessel – as long as you’re going in a straight line,” says Paul Larsen, who ‘piloted’ the vessel on its record-breaking runs and was also pivotal in its development. “But you still need acceleration – and that’s why we used composites.”

Though Larsen is not an engineer, he is fascinated by boats and how fast they can go. So when he first met Malcolm Barnsley – and saw his design for Sailrocket 1 – he knew that he had to get involved and help him take the idea forward.
“Malcolm always says that something will only work if it’s properly engineered,” says Larsen. “He always insists on the hard numbers.”

Talking to Larsen, it appears that the 11-year project was an ongoing series of crisis points, as smashed prototypes, hospitalisations and design ‘blind alleys’ conspired to frustrate the team. But each was eventually overcome – culminating in a series of runs that went successively faster.

It was this determination – of constantly crashing, then coming back for more – that attracted the attention of the company that would finally sponsor the project and build the boat: Vesta. Vesta’s involvement gave the Sailrocket team access to much better facilities – in terms of both design and composites expertise. And the company stipulated that the boat should be totally redesigned – hence Sailrocket 2 was born.

“We finally had money to throw at the design team – and carry out a feasibility study,” Larsen says.
Once the team had tested the feasibility, it set itself what seemed an impossible target: to build a boat that would hit 65 knots.

“Nobody had come close to speeds like that. We were competing with kite surfers.”
Odd as it may sound, the record for a sail-powered craft at that time – of just over 55 knots – was held by a kitesurfer. In theory, a boat designed to hit 65 knots at peak speed would be able to achieve 60 knots – and break the record comfortably.

Reaching boiling point

One design problem proved almost impossible to solve. And no wonder: it had to do with a theoretical ‘ceiling’ that sail-powered craft seemed incapable of breaking.

A physical effect called cavitation causes drag on the vessel, and seems to limit its upper speed to around 52 knots. The low pressure generated behind the hydrofoil causes the water to literally boil – causing the drag. One way to solve it is to bring ambient-pressure air down to cancel out the cavitation. This is known as ventilation, but is difficult to control. It would be achieved by getting the design of the hydrofoil right – but there was no consensus on how this would be done.
“We had a structural engineer who wanted the foil to be thick, and a hydrodynamicist who insisted that it had to be thin: there was no way out,” states Larsen.

This caused the design phase to go round in a ‘loop’ for some time until – almost by chance – Larsen found a technical paper written by an enthusiast of radio-controlled boats which said that a wedge-shaped design – like an axe head – was far more efficient at getting air from the surface than a typical ‘teardrop’ foil design.

There is another precedent for this: fast powerboats, with propellers half in and half out of the water, also have a wedge-shaped design – with a square trailing edge. “But we don’t have a propeller,” Larsen asserts.

The idea of making a thicker foil had two positive effects. “Firstly, the foil has to withstand huge forces – so making it thicker was a great help.”

The other bonus was the method used to make the foil. Because it was now thicker, it could be made using a much cheaper production method.

“People expect the foil to be made of high-modulus prepreg, but now it wasn’t necessary.”

Larsen says that Dan Emuss, of Bristol-based Independent Composites, was confident of making the thicker part by infusing it at room temperature – with no need for an autoclave.

“Dan did a great job on the foil. He has lots of experience with infusion – and had to talk us into it.”

With one problem solved, another reared its head: transporting the foil to Namibia, where the team was anxiously waiting. Larsen says that only one airline would take the part as excess baggage – as long as it was less than 30kg.
“We managed that, but only just. We couldn’t even paint the part, or it would have made it too heavy,” he reveals.

Where composites count

Aerotrope was the primary design office behind both Sailrocket 1 and 2, with both boats built using Gurit’s composite materials throughout. The main hull and three pods were made of prepreg and Nomex. Uni-directional fibres were used in key areas. The wing had ribs made from biaxial carbon. Formaplex provided the tooling and complete CNC-milled foils for Sailrocket 2’s record-breaking session.

“When you are sailing a boat, weight is not really an issue,” explains Larsen. “If you’re going in a straight line it’s fine. It’s only when you need to turn that it becomes important.”

There is also the practical issue of actually transporting the vessel. At the same time, the high strength of composites ensure that the structure does not break apart under the huge forces it must withstand.

When the team finally got the boat out onto the water, they were dismayed to find they were hitting the dreaded 52-knot ceiling. However, computer simulation revealed something positive: the boat was actually performing way beyond expectations.

“When we fed the design back through CFD analysis, it only predicted a top speed of 28 knots – that’s when we knew we were on to something.”

The problem was solved using another surprising piece of improvisation: the team fished a piece of plastic from the scrap bag and attached it to the suction side of the foil, to make a ‘fence’. This was designed to help ventilation take place at the correct point. After this according to Larsen, the boat “turned on like a light switch”.

“The problem when you’re sailing is that you’re not sure exactly where the interface between the water and the air is. We took a guess when we stuck the fence in place – and that’s where it is to this day.”

The next run was 54 knots. After that, it was 60 knots. And – still not satisfied – the team went into its final run and broke through 65 knots in “perfect conditions”.

It could be argued that a small piece of carbon fibre fished from the waste was ultimately more important than any other composite part on Sailrocket 2.

Making future plans

As one challenge ends, another begins. For Larsen, the follow-up is Sailrocket 3. The concept – though rooted in the distant past – is slightly surprising: using sail-powered boats to transport cargo. Most cargo is now transported in enormous sea-going vessels. But Larsen says that Sailrocket 3 could transport smaller payloads – at speed.

“It’s like a return to the age of sail-powered transport,” he claims.

The new vessel – which is already in the design phase – takes concepts from Sailrocket 2 and puts them in a new context. Not only is the design capable of combining speed with load carrying ability, but making the hydrofoil with the relatively cheap infusion process could be scaled up to make larger craft, such as 100ft boats.

“You could build a boat with one-third less tooling – using infusion rather than prepreg – and cheaper methodology,” Larsen concludes. “It’s designed to cross oceans and carry payload.”

Sailrocket 2 was a dedicated speed sailing boat; Sailrocket 3 will be going at that speed across the ocean. And there’s also the prospect of using this type of boat for simple pleasure, crossing small stretches of water in a totally different way. So, in the near future, you may choose to cross the English Channel by ferry – or Sailrocket 3.

Credit/copyright: All images © Helena Darvelid/Sailrocket

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