Sailrocket unloaded

I WAS BUSY messing around with my own boat back in November, so failed to note the fantastic achievement of Paul Larsen, who finally, after 11 years of work, succeeded in smashing the world sailing speed record in Vestas Sailrocket 2. By now everyone knows the number: 65.45 knots. That was Sailrocket‘s average speed over a 500-meter course on Walvis Bay in Namibia on November 24 in 25 knots of wind. Almost 10 knots faster than the previous record (55.65 knots on a kiteboard), the biggest single leap forward in the history of speed sailing… and Larsen claims he is confident that Sailrocket, in her current configuration, can easily top 70 knots.

So how does this weird-looking vessel manage to go so fast? Everyone has seen pictures of it, but I don’t think too many people understand much about how the boat actually works.

First consider Sailrocket as viewed in profile and from above:

Sailrocket drawing

Illustration by Francois Chevalier

She is utterly asymmetric and has no hull, in the sense that there is no main body part that actually floats on the water. Instead the main structural elements are a crossbeam, which supports the canted hard wingsail, and a fuselage with cockpits forward for crew. Both these parts are cocked at odd angles to the direction of travel. To reduce drag the fuselage is offset so that it is aligned with the apparent wind direction (about 20 degrees to starboard) when the boat is traveling at speed. The crossbeam lies perpendicular to the apparent wind direction at speed and is shaped like a foil to create lift when the boat is moving. Note that the boat is always sailed on starboard tack, with the fuselage to windward.

The only parts of the boat’s structure that are aligned with the direction of travel are the three small floats under the beam and fuselage and the main hydrofoil itself, which is situated aft on the fuselage, directly opposite the attachment point for the crossbeam. There is also a small rudder under the forward float (it is controlled with a wheel) and a retractable skeg in the aft float (not shown in the drawing above) that helps control the boat at speeds below 20 knots.

To get a sense of the forces that make the boat move, next consider Sailrocket as viewed from ahead:

Sailrocket drawing

Illustration by Realise Design

The boat’s “engine,” of course, is its wingsail, set all the way to port, which has an angled extension at the bottom. This, together with the foil-shaped crossbeam, creates vertical lift that can be controlled with a flap on the extension’s aft edge. This lift is critical at very slow speeds, as the leeward float and the outboard end of the beam tend to burrow into the water as the boat starts moving. Once the boat is moving at speed the leeward float normally flies clear of the water.

The main part of the wingsail is canted to windward so as to offset the heeling force it generates (just like on a windsurfer), and its driving force is set against the action of the canted main hydrofoil, which is set off to starboard. If you want to oversimplify this diagram in your mind’s eye, just erase everything but the wingsail and the lower section of the foil, then move the foil directly under the sail and draw in a windsurfer board between them with a guy hanging on under the sail.

The big breakthrough that made November’s blistering speed runs possible had to do with changes that were made to the length of the hydrofoil and its section shape. During speed trials in 2011 with a more conventionally shaped foil, Sailrocket would hit a wall at speeds over 50 knots and suddenly decelerate as the foil started cavitating. Larsen and crew intuited that part of the problem was that the foil was too long and they at once took a hacksaw to it.

Sailrocket foil alterations

Photo courtesy of Vestas Sailrocket

Back in the UK they also cooked up a radical new blunt-backed triangular section shape, and it was this foil that lifted Sailrocket into the record books this past November.

To get a much more detailed description of all this, and a look at that foil section, I urge you to trundle down to your local newsstand (remember those?) and pick up a copy of this month’s Yachting World, which has an excellent feature story by Matt Sheahan on the development of Sailrocket. You can find the primary illustration online, which is very helpful, but to get the whole scoop you should really read the print story.

Meanwhile, Paul Larsen, never one to rest on his laurels, is currently aboard a replica of the lifeboat that Ernest Shackleton sailed from Antarctica to South Georgia Island almost 100 years ago during his famous survival ordeal.

Paul Larsen

Paul Larsen in his Vestas wetsuit in Namibia

Alexandra Shackleton

His current ride, setting out from Elephant Island in Antarctica

Alexandra Shackleton location

His current location, approaching South Georgia Island

Larsen and five others are recreating both the voyage to South Georgia and Shackleton’s grueling hike across the island’s mountainous interior. You can follow their progress at the Shackleton Epic website.

Just for the hell of it, though I know you’ve probably already watched it, here’s the viddy of Larsen’s record run on Sailrocket. No matter how many more adventures he has in his life, I have a feeling this is the day he will always remember most.

For the record: Vestas Sailrocket 2 measures 40×40 feet, weighs about 400 pounds, and has a maximum sail area of 237 sq.ft. I have no idea how you would ever crunch performance ratios on her, as she really has no waterline. She also currently holds the record (55.32 knots) for highest speed averaged over a one-mile course. On her record 500-meter run she hit a top speed of 68.01 knots. Also, to clarify, in that viddy up there, where Paul draws a number in the sand, he is revealing the average 500-meter speed from his personal GPS, not the Trimble unit used for official record-keeping.

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