Foiling Monohulls

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The curved DSS foils aboard the Open 60 Edmund de Rothschild do a lot more than just prevent leeway. Photo courtesy of Thierry Martinez/GITANA SA

The curved DSS foils aboard the Open 60 Edmund de Rothschild do a lot more than just prevent leeway. Photo courtesy of Thierry Martinez/GITANA SA

It’s easy to forget it was just three years ago that Team New Zealand turned the America’s Cup on its head when it persuaded its AC72 catamaran to lift off the water and fly, thus changing performance multihull design forever.

Now, both new multis and older ones retrofitted with foils are taking to the air in ever-increasing numbers. But why should multis have all the fun—what about monohulls, Moths aside? Fear not—their time is coming. In fact, it’s already here. In June, the Quant 23, a skinny scow-hulled sportboat with a pair of curved foils designed by Briton Hugh Welbourn, got airborne on a Swiss lake to become the world’s first foiling keelboat. It was the culmination of a decade of development for Welbourn and partner Gordon Kay’s Dynamic Stability System (DSS) project.

A crewman stands on one of the athwartships foils on the maxi Wild Oats XI. a crewman stands on one of the athwartships foils on the maxi Wild Oats XI. Photo courtesy of Andrea Francolini/Cruising Club of Australia

A crewman stands on one of the athwartships foils on the maxi Wild Oats XI. a crewman stands on one of the athwartships foils on the maxi Wild Oats XI. Photo courtesy of Andrea Francolini/Cruising Club of Australia

Look back to last December’s Sydney-Hobart race, and remember how the 10-year-old maxi Wild Oats XI trounced the brand new Comanche, the world’s most technically advanced—and probably most expensive—racing yacht. The Australian boat had just been fitted with its DSS foil, which tracks athwartships in a cassette below the waterline and is deployed to leeward. The foil provided some eight to 10 tons of vertical lift for Wild Oats XI, significantly improving the boat’s righting moment as its speed built; in short, it kept the boat’s bow up under spinnaker and made for longer and more controllable surfs in the heavy airs of the Bass Strait. This allowed the skinny Reichel/Pugh design, which hit 35 knots at times, to close the performance gap to the beamier and theoretically more powerful Comanche.

The DSS has been fitted to several boats, including the Quant 28 and 30 sportboats and three designs for Infiniti Yachts—the 36GT and 46R racer-cruisers, and a 100ft maxi. Farr Yacht Design has been commissioned to draw the latest in the series, the Infiniti 53, whose narrow hull is tailor-made for the benefits of the DSS and which can be ordered with a canting or fixed keel. However, says Welbourn, the increased stability offered by a DSS foil is such that a canting keel is hardly necessary. The faster the boat goes, the more lift the DSS foil provides.

A diminutive Quant 23 scow takes to the air during sea trials. Photo courtesy of Quantboats

A diminutive Quant 23 scow takes to the air during sea trials. Photo courtesy of Quantboats

Welbourn’s Quant 23 may not be the first fully foiling monohull—the foiling Moth and the Glide Free kit for Lasers predate it—but it is the first ballast keel-equipped monohull to foil. For this boat, Welbourn added winglets to the rudder and employed a pair of curved foils that partially retract upward into the hull. With a crew of three, the boat popped onto the plane at 8 knots and clocked over 18 knots, fully foiling in 10 knots of true wind. For a ballasted 23ft monohull, that’s extremely respectable, and there is bound to be more to come as the foils are tweaked and refined.

 DSS lateral foils help increase the righting moment aboard the Farr-designed Infiniti 53. image courtesy of Farr yacht design

DSS lateral foils help increase the righting moment aboard the Farr-designed Infiniti 53. image courtesy of Farr yacht design

The Quant 23’s curved foils obviously add plenty of lift, but a vertical foil (in this case, a keel) is still needed to prevent the boat from slipping sideways. French designers Guillaume Verdier and Van Peteghem Lauriot Prevost (VPLP) mulled all this over when the rules governing Open class monohull design were relaxed last year, and the results of their brainstorming are plain to see in the latest generation of IMOCA 60s. All six VPLP-Verdier-designed IMOCA 60s being built for next year’s Vendée Globe non-stop solo round-the-world race will be equipped with retracting L-shaped foils that replace the daggerboards traditionally used in conjunction with canting keels. Not only will these foils provide significant lift and therefore increase the righting moment, they will resist leeway.

 A rough conceptual drawing of how a leeward lifting foil works with the canting keel aboard the new cutting-edge Open 60. Image from Guillaume Verdier Edmond de Rothschild

A rough conceptual drawing of how a leeward lifting foil works with the canting keel aboard the new cutting-edge Open 60. Image from Guillaume Verdier Edmond de Rothschild

Dramatic footage of one of the first of the new breed, Edmond de Rothschild, blast-reaching in 30 knots of wind and seeming to skip across the waves, seemed to bear out Verdier’s prediction that “there will be a significant gain in speed; the foils could win two days on a Vendée Globe.” However, the Fastnet race in August saw older non-foil equipped IMOCA 60s finish ahead of the new boats, so there is still some work to be done. Specifically, they do not point as high as the daggerboard boats. Verdier again: “The real gains will be for reaching and downwind sailing. Upwind, it will be more complicated, but that only represents 10 percent of a Vendée Globe.”

The heavier airs guaranteed in this month’s Transat Jacques Vabre—from France to Brazil—will be the first true test for this new generation of ocean racers. Meanwhile, Welbourn reckons even the regular DSS could make the IMOCA class cheaper and more accessible, because the increased righting moment would do away with the need for a canting keel. With less need for form stability, the boats could be narrower—a la Wild Oats XI (interestingly, Comanche is also a VPLP-Verdier design).

The DSS has its benefits for cruising boats too—the boat will sail more upright, which adds to sail-carrying power, which means more speed. The lateral foil dampens rolling, both under sail and at anchor. As a New England sailor, I’d have to weigh that against the increased chances of snagging a lobster trap line…

 A conventional Laser retrofitted with a Glide Free foiling rudder and daggerboard. Image courtesy of Glide Free.

A conventional Laser retrofitted with a Glide Free foiling rudder and daggerboard. Image courtesy of Glide Free.

November 2015

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