World's Fastest Sailboat: Quantum Leap
World's Fastest Sailboat: Quantum Leap
Tech Review: How did Paul Larsen's Vestas Sailrocket 2 peg 64 knots? James Boyd explores the turbulent path to the recent record blitz.
On one memorable occasion, Vestas SailRocket took off and performed a complete backflip, leaving Larsen upside down in the water and the boat once again in pieces. The video of this crash went viral on YouTube and has been played more than 400,000 times. But this was one of many incidents: “We had rounded up into the wind, smashed the wing, and folded up the beam at least four times before we even got to the flip,” he recalls. “Each one of those was a big crash, big repair, damaged wing, broken struts; once we got the boat going really quick, then she started to somersault.”
Amid all of this, the world record was being pushed further down the track by the kiteboarders with Douglas stealing it from the windsurfers and then Frenchman Sebastien Cattelan being the first sailor to break the 50-knot barrier. But Vestas SailRocket also made its mark. The same day as the backflip, SailRocket became the world’s fastest boat, as opposed to board, at a speed of 47.3 knots.
The following season Larsen and company realized time was running out for Vestas SailRocket. They had an unofficial run of 49.38 knots and a peak speed of 52.78 knots, but the runs were still very much do or die. Larsen endured another full backflip and a separate catastrophe when the forward beamstay broke, causing the beam to fly back into the main hull and the boat to fold up, putting the pilot in the hospital. “It went from over 47 knots to a standstill, and the beam came back at me like a cricket bat,” says Larsen. “I still rate that as the most violent crash in yachting yet.”
With Vestas SailRocket reaching the limit of its potential, the team was already deep into the design of Vestas SailRocket 2, harnessing all the knowledge they’d learned from the first boat.
While Barnsley spearheaded the design of the first boat, the principle designer of the second was Chris Hornzee-Jones, a structural engineer and aerodynamicist, who heads the company AeroTrope and designed the wingsail for the first Vestas SailRocket.
Launched in March 2011, Vestas SailRocket 2 incorporated all the fundamental features of the first boat: a hull to windward incorporating the all-important foil, a single crossbeam, and a wingsail inclined to weather by 30 degrees. In other ways, however, it was a significant step forward. At 40 feet long by 40 feet wide, it was slightly bigger, and the hull was now more like a glider fuselage sitting on two short floats at the bow and stern, with the rudder mounted on the forward one. To leeward the wingmast sat atop a third float.
Most noticeable was that while the floats pointed in its direction of travel, the fuselage was offset to starboard by 20 degrees to point into the direction of the apparent wind in order to minimize drag at high speed. They also “reversed the arrow,” putting the cockpit in the bow of the fuselage. They enlarged the wing from 172 sq. ft. to 193 sq. ft., added a hooked section at the bottom of the wing (giving it a hockey stick profile), which acts as an endplate for the wing and also provides some control over how high the leeward float flys.
In the cockpit, in addition to the steering wheel, the controls Larsen uses during a run are the mainsheet and the control for the flap on the outboard extension of the wing. There are also controls for raising and lowering the main foil and the low-speed skeg, and controlling the wing when stationary.
During the 2011 season, the team made solid progress. Vestas SailRocket 2 proved more controllable and stable than the previous boat, and in two seasons of use it experienced none of the same catastrophes that afflicted the first boat. However, regardless of the wind speed, the new boat couldn’t surpass the low 50-knot range. By this stage, Douglas had pushed the record to 55.65 knots.
The culprit proved to be the foil, mounted on a bracket well aft on the windward side of the fuselage.
In 2011, the team trialed two foils. Both were L-shaped, one a conventional asymmetric teardrop shape—with a similar section to an IMOCA 60/Volvo 70 daggerboard—the other a ventilating foil. With the former both the low- and high-pressure sides of the foil are put to use, but when traveling at speeds approaching 60 knots the foil cavitated. This is a common problem for propellers, caused when pressure on the low-pressure side of the foil becomes so low it causes the water to vaporize, effectively detaching it from the foil. With only one side of the foil working, the performance of the foil drops suddenly, with potentially disastrous effects.
A ventilating foil with more of bullet shape (a sharp leading edge, and a blunt trailing edge) is, in hydrodynamic terms, much less efficient: Its effective working area is much reduced, and it creates more drag. However, this shape theoretically removes the cavitation issue and allows the foil to operate smoothly at speeds well in excess of those where a conventional foil starts to struggle. During the 2011 season Vestas SailRocket was mostly being sailed with this foil, only it failed to ventilate properly. In desperation the team took out the grinder and progressively shortened the foil in 6" chunks, down from 3'3" to 1'9", before returning to base to consider the data.
Back in Great Britain, the team planned to build a new foil, but was unsure what exactly to build. Talking to the experts only caused more confusion. They were advised a ventilating foil shouldn’t be able to get beyond 30 knots, but they had achieved speeds in excess of 50 knots with it. So they reverted to their original concept of a ventilated foil, only a depth of around 2' submerged and a chord of 10" at its maximum—about 60 percent of its original area. They also fitted Cosworth data loggers to the foil to establish where cavitation or ventilation was occurring.
The eureka moment came not with the new foil on its own, but when they added a strategically placed fence to prevent ventilation in an area of the foil that shouldn’t have been ventilated. And the rest, as they say, is history. Initially they set a new record of 59.23 knots, and 10 days later Larsen managed 65.45 knots with a peak speed of 67.74 knots.
What’s it like at 60 knots? “It depends on how close I get into the beach,” says Larsen. “If I stay out of the rough stuff, it is a short, sharp, bumpy ride, like on a high speed powerboat. 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 civilized. 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.”
At present there are no plans to progress with Vestas SailRocket. The point has been proven. From the heavens Bernard Smith, who passed away on Feb. 10, 2010, can smile. Larsen is adamant the concept will go faster; in theory there is nothing to stop this genre of boat from hitting 100 knots. But it will require another foil. With his offshore background Larsen is intrigued to see if the neutral stability concept can be developed for more practical applications, but only if it makes boats like the 131-foot Banque Populaire maxi tri [the outright ’round the world record holder at 45 days] look like pedestrian dinosaurs.