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Frequent Flyer

Jarrett Bay, Princess and SEAir are just a few builders experimenting with foils to increase speed and efficiency. Alan Harper reports on the trend and if it will take off.

It was at the 1958 America’s Cup trials off Newport, Rhode Island, that Dick Bertram had his eureka moment. As the story goes, he was the sail trimmer aboard Vim in challenging, choppy conditions that had the chase boats struggling to keep up, so the way that one particular 23-foot powerboat was able to maintain its speed through the steep seas was all the more noticeable.

The boat was one of C. Raymond Hunt’s first deep-V designs. Bertram sought him out, sea-trialed the boat and commissioned Hunt to design a 30-footer for the Miami-Nassau offshore powerboat race. Bertram won, the superiority of the deep-V concept was clearly demonstrated and the rest—as they say—is history.

The new R35 from Princess Yachts has two retractable T-shaped foils installed in the hull to create lift or downforce as needed.

The new R35 from Princess Yachts has two retractable T-shaped foils installed in the hull to create lift or downforce as needed.

Bertrand Castelnérac had a similar eureka moment in Quiberon Bay in 2015. An intrepid, daredevil yachtsman of the sort that France seems to have a knack for producing, he was sailing a GC32—one of those hydrofoil catamaran sailboats for which insanity seems to be the principal qualification in a crewmember—when he noticed that his team’s tender, a RIB containing tools, spare sails and the long-suffering coach, was falling behind in the rough water. “So I thought, ‘why not have foils on the RIBs, too?’” he recalls.

Hydrofoils are those counterintuitive wings that when fitted to boat hulls jut out into the waterflow, yet somehow improve efficiency. It’s all about lift, of course: Whatever drag the foil might produce, once it has performed its function and raised the boat’s body out of the water, it reduces the far greater drag caused by the hull’s wetted surface area.

The principle has been documented for decades, and there are many hydrofoil fast ferries in service today around the world. But the underwater wing has been slow to find favor in leisure craft—until now. Recent improvements in electronics and materials technology, combined with big-budget sponsorship for sailing events, have led to a rekindled interest among naval architects and marine engineers in their search for superior performance and speed. Just check YouTube. From South Africa we have the Hysucat system in which a hydrofoil wing spans the gap between the hulls of a catamaran. In Slovenia, Quadrofoil has developed an extraordinary electric-powered personal watercraft that looks like a two-seater convertible on four legs. There are surfboards that ride along on single T-shaped foils while seemingly confounding all known laws of physics. And then there are the sailboats, with terrifying craft like the America’s Cup monohulls and GC32 catamarans at one end of the spectrum, and foiling versions of the Moth and Optimist sailing dinghies at the other.

This 18-foot prototype RIB built to SEAir specs by Zodiac features a foiling system that retracts into the helm when not in use.

This 18-foot prototype RIB built to SEAir specs by Zodiac features a foiling system that retracts into the helm when not in use.

Jarrett Bay, builder of custom sportsfishing boats, is perhaps a surprising name to find on the hydrofoil roster. But as reported in Power & Motoryacht’s July 2018 issue, the builder’s new 90-footer sports a foil—a 14-foot wing slung beneath the hull, to add lift and increase top speed to a barely believable 50 knots. Tests of a fixed protoype foil made of aluminum and carbon were apparently successful; now the plan is to design a retractable version.

In England, Princess Yachts has also had a rush of blood to the head in the form of its new R Class debutante, the R35, to be unveiled this fall. The R stands for Revolution. Two retractable T-shaped foils, each 39 inches long and 10 inches wide, are installed in the hull of the 35-footer, which even without them will have no trouble attracting attention with its all-carbon structure and swoopy Pininfarina styling. This head-turning machine sports a pair of 430-hp Volvo gas V8s on Duoprop sterndrives that will reportedly make it the first-ever 50-knot Princess, while the active foils, mounted 3 feet in front of the transom, rake forward or aft by plus or minus 5 degrees, creating lift or downforce as needed.

“When first pushing the throttles forward, you expect the bow to lift and the stern to dig in until the boat starts to plane,” explains R35 project manager Paul Mackenzie. “It’s what normally happens. But on the R35 there’s no hump—it stays flat, as the foils supply lift to the stern.” Keeping the stern up translates into efficiency, with the R35 able to plane at lower speeds than conventional hulls.

As the foils move independently, they can influence lateral trim as well, for ride comfort. The system will also include a sport setting, allowing greater angles of heel and a more dynamic ride. The software has been devised by Ben Ainslie Racing, the Olympic and America’s Cup yachtsman’s high-tech firm in Portsmouth, England, which also worked on the R35 hull design.

The SEAir system is based on pre-preg carbon fiber foil wings and a carbon horizontal stabilizer fitted to the leg of an outboard.

The SEAir system is based on pre-preg carbon fiber foil wings and a carbon horizontal stabilizer fitted to the leg of an outboard.

Originally developed in Switzerland but now funded and built by Enata in the UAE, the 33-foot Enata Foiler is easily the most dramatic and perhaps the most ambitious of this new breed of leisure hydrofoil power craft. A diesel-electric hybrid, the Enata is fitted with two 300-hp diesels that power two electric motors and also charge the batteries to allow the boat to cruise, briefly, in pure electric mode. Two huge forward wings, which retract and fold out of the way when docking, combine with a pair of vertical-deployment foils aft, each fitted with a torpedo-shaped electric drive—solving the problem of getting the propellers into the water while the hull is flying above it. The foils are designed to deploy once the hull reaches 17 knots, and with the boat lifted clear of the water on its foils the Enata’s maximum speed is reported to be 40 knots.

Like Princess’ Revolution 35, the Enata Foiler’s structure is molded entirely in carbon—the parent company’s specialty—and with an eye on the superyacht tender market, it has been designed to fold up neatly for stowage on board the mother ship. Its wingspan reduces from 24 feet to 11 feet with the foils retracted, and stowage height comes down to just 8 feet.

It was a chance meeting with fellow Frenchman and telecom entrepreneur Richard Forest that gave Bertrand Castelnérac the chance to test out his ideas for a foiling RIB, starting with an 18-foot technology demonstrator.

Alongside the dock it looks a lot like any ordinary rigid inflatable, for the foils retract almost completely into their housing in the helm console, with just the blade tips visible beneath the tubes. This prototype was built and adapted to SEAir’s specifications by Zodiac, which will also produce the first few production boats sold through SEAir, but eventually the foiling system will be made available to any boatbuilder who wants it. The RIB was fitted with a 115-hp Yamaha outboard when I caught up with it on Lake Geneva this summer, although SEAir would actually recommend an 80-hp unit as easily powerful enough, and more economical on fuel.

The foil wings on the 18-foot RIB prototype are handmade in solid carbon fiber, a process which took two weeks. Production foils will be built of pre-preg carbon in a mold and UV-cured in a controlled atmosphere, which will take two days. The larger foils on the next model, a 23-footer, will have a foam core.

The conventional look and straightforward helm layout of the prototype are reassuring. “It needs to be simple,” says Forest. “Just one button. People don’t need to know how the self-parking thing on their car works, they just need it to work.” Castelnérac explains that the wings account for 80 percent of available lift when the boat is flying, while the carbon horizontal stabilizer fitted to the outboard leg, rather like an outsize cavitation plate, supplies the remaining 20 percent.

If you’ve ever taken the helm of a boat with trimmable surface drives, such as Arnesons, driving the SEAir RIB will feel strangely familiar. Engine trim is key to performance. Set the throttles with the boat on the foils, trim the engine out a little, and the acceleration that results is like the surge you get when raising surface drives into thinner water. The cause is different, though: On the SEAir RIB it’s all about finding the correct angle of attack for the foils, so they produce maximum lift and minimum drag. There’s a sweet spot that reveals itself after a little trial and error. If you overdo the angle of attack as I did, and then apply some helm, one foil can stall like an aircraft wing. The resulting lateral oscillation was intense enough to remind me to handle this protoype with care.

On production boats it will be possible to adjust the rake of the forward foils while under way, giving more control over their angle of attack and refining your trim options. In the meantime, SEAir is also working with NKE Marine Electronics on sensors and automated ride-control software which, they say, will take all of the guesswork out of driving the boat.

Independent tests pitting a conventional 18-foot Zodiac against the SEAir foil version revealed higher speeds across most of the rev range and some significant gains in fuel efficiency. But the other thing that foils are good for is the reason Castelnérac wanted to build a foil RIB in the first place: the quality of the ride. With the hull out of the water there’s no slamming as you charge through rough seas, which should enable those unfortunate sailing coaches to keep up with their flying, foil-born charges. Obviously on Lake Geneva I couldn’t put this to the test, but the RIB’s gentle demeanor as we crossed the wakes of the big lake steamers suggested that he has a point; there was a distinct cushioning effect.

So, are hydrofoil powerboats going to make history in the way that the first offshore deep-Vs did? It’s going to be fun finding out.

This article originally appeared in the October 2018 issue of Power & Motoryacht magazine.