The People’s Foiler: A Design Brief on the UFO Foiling Catamaran

The father-and-son design team of Steve and Dave Clark develops the UFO, a stable, affordable, production-built foiling beach catamaran.

UFO openerDave Clark

To design an affordable hydrofoil, Steve and Dave Clark realized they couldn’t just strip down an existing high-performance model. Their foiler also needed to be efficiently produced, durable, easy to launch, and forgiving to amateur sailors.

It’s June 2015. I’m chatting on the phone with my tugboat designer friend Bill: “Dad and I have been kicking around the idea of using a set of decent hydrofoils to get a simple, cheap boat to really perform without any of the pain, hassle, and expense we’ve all become accustomed to as the price of admission for foiling under sail. I can’t get it out of my head. Dad’s certain it’s possible to build, and I’m convinced people would like it.” I go on for a bit, describing the basic concept work we’ve done to date. Bill cuts in, his voice suddenly hard and assertive.

“Do it!”

“You think so?”

“Do it. Test it under cover of darkness. Tell nobody. Don’t sit on it. You’ll never forgive yourselves if 10 years down the line you see somebody else make it something more than a drawing.”
And we did it.

The job could have been simple: create a detuned civilian version of a proven high-performance foiling craft. This design project would normally start with identifying and removing every bit of costly exotic material and excess labor from the existing model we were detuning. But we were certain that with small foilers there’d be conversion issues beyond expense, including storage, transportation, and difficulty of use.

The competitive boats we were looking to emulate were designed to achieve performance goals but with little regard for convenience and userfriendliness. Informing those designs was thinking that went something like this: When you’re aiming to be the world’s fastest Moth, what does it matter that it takes two hours to put together and you need to walk shoulder-deep into the water towing a capsized boat with nonretractable foils to launch? What does it matter that the boat has a permanently unstable foot-wide waterplane? When you’re trying to build the world’s fastest A-class, what does it matter that the boat won’t collapse and takes up more space than a J/24 at the club? What does it matter that it’s nearly as expensive as a Porsche to own and just as costly to maintain? When you’re trying for absolute outright speed, what does it matter if it crashes, bites you, and requires constant tuning changes? Surely all those things are user-error. Just get that gash in your face stitched up, grow five more testicles, and realize the speed you’re going for. C’mon!

We recognized that to really distill the fun of foiling—one of the most profound pleasures in sailing—we’d need to take the self-harm out of it. We’d have to do more than simply reduce costs on an existing foiler precisely because existing foilers were developed by today’s foiling sailors, who are exceptional. Ours needed to bring a certain degree of civility and simplicity to the foiling experience.

The final brief was:

  1. A compact sailing hydrofoil that delivers credible performance at all points of sail in a broad wind range.
  2. Dimensions of less than 12′ long and 6′ wide (3.7m x 1.8m) and a vessel weight of 110 lbs (50 kg).
  3. Able to be rigged, launched, and retrieved upright, and sailed and maneuvered in shallow water.
  4. Forgiving—That is, not dependent on advanced sailing skills.
  5. Inexpensive and durable enough to be accessible to a wide variety of users.
  6. Easy to store and transport.
  7. Carrying profit margin sufficient for the builder to remain in business with active promotion.

We drew on a few main ingredients. First were the millions of hours the International Moth class has put into iteratively testing and perfecting centerline T-foils and their control systems. Second was a heavy dose of foil, sail, and aero-package studies my father, Steve Clark, had recently commissioned from Paul Bogataj for the 2015 Little America’s Cup campaign, a program that included a conversion of the C-class cat to full foiling, a sealed trampoline, end-plating, and canting the wing—all sorts of fun stuff. (For more on C-class development, see “Flight Plans,” by Steve Killing, in Professional BoatBuilder No. 151.) As is often the case, there’s no boat better than the one you’re going to build next. The nascent UFO (Unidentified Foiling Object) had the good fortune to be the next boat after the C-class campaign. Finally, there’s the normal grab bag of development knowledge picked up over the years in the A-class, the IC (international canoe), various skiffs, and Dad’s career manufacturing 3,000 boats a year at Vanguard. As always, the path you follow is a function of who you are and what you know.

The Hull

My father has had a fondness for solid-decked catamarans since he received an Alcort Catfish for Christmas 52 years ago. It’s an underexploited hullform that keeps the part count exceptionally low and allows for a high degree of performance and stability. It fit the bill in low cost, high stability, and ability to attain the speed necessary for takeoff.

Our first critical task was to keep the surface area low enough to yield a strong, light, easily built shape. Getting it right took several iterations at the modeling phase as well as careful laminate design and testing. We settled on a vinylester-infused laminate of Soric foam core skinned with E-glass cloth and minimal strategic reinforcement with unidirectional carbon. This kept internal framing to a bare minimum, saving man-hours and keeping the build uniform. A lot of our success in this endeavor is down to exploiting the physicals of high-quality vinylester and the reduced panel-stiffness require­ments for something that is mostly airborne while under way, where the fluid is roughly 800 times less dense than water.

ufo drawingDave Clark

Profile and plan views of the UFO (Unidentified Foiling Object).

UFO particulars:

  • LOA 10’6″ (3.2m)
  • Beam 5’7″ (1.7m)
  • Light ship 110 lbs (49.9 kg)
  • Sail area 84 sq ft (7.8m2)

The second, most obvious judgment call was to make the boat from two parts, bonded with a single straight lip joint for the whole perimeter. Hull + deck + external flange joint + Plexus = done and dusted. Conventional rolled-lip joints were developed for an era when weaker glue required higher glue area, and coffee-can joints that offer a “seamless” look are an invitation to countless hours of hand-finishing everything. More often than not, they violate the promise of gelcoat by requiring the joint to be faired and repainted. When building small production boats, if you aren’t pulling polished, gelcoated, essentially done parts out of the mold, you won’t be cost-effective.

ufo renderedDave Clark

The Clarks chose a solid-deck catamaran hullform for its low cost, high stability, and ability to attain foiling speed. At 70 lbs (31.5 kg), it’s also easy to handle and has dimensions and a bottom that lend themselves to cartop transport.

No matter how small you make the surface area, the hull will be the largest, most expensive part of a dinghy. With that in mind, you often need to put function first to control costs. The light, cheap, durable, and reliable solution for a hull-to-deck joint is a single half-inch (12.7mm) flange. It looks a bit funny in places, but it’s a small price to pay for a lightweight bond that eliminates the need for any secondary work.

first infusionDave Clark

The Clarks simplified production by designing the UFO’s hull and deck parts to be joined with a single straight lip. The vinylester-infused laminate of Soric foam core and E-glass includes minimal carbon reinforcement of the beams for stiffness.

Another detail on the UFO is the absence of tight areas where it would be difficult to reach into when building the boat. Even the bows are built with a wide stem radius, to make stacking the laminate easy and reliable.

I’m a firm believer in attacking a problem from both ends. Though we did an FEA (finite elements analysis) on all parts of the boat, we also performed what I call an additive guess-and-check FEA as well. This consists of creating a hull as intentionally underbuilt as possible, putting a ton of hours on it, patching it where it breaks, and counting the patches. It’s an empirical way of finding the hot spots in the build and serves as an excellent complement to the FEA for designing the final laminate.

Locating the mainfoil trunk just forward of the mast provides a couple of immediate benefits. First, the foil may be withdrawn between the hulls and clear of the rig for launching and landing, something that’s not possible on most foilers. On most, the lifting surfaces of their choked-up foils are sticking out the sides, jammed against the bottom of the hull, and in the way of the dolly. Not so for the UFO, whose foils pull up well clear of the waterline.

Because the hulls provide good lateral resistance, they enable a “beach cat mode” where the boat will operate normally, tacking, jibing, and achieving all points of sail with the mainfoil up and the rudder partially down. This idea dates back to a far earlier conceptual cousin of the boat that my dad doodled back in 2006. Second, it makes sealing the sail to the deck even easier.

Sealed or “endplated” sails gain apparent aspect ratio and feature a lowered center of effort and far less tip vortex drag at the bottom of the sail, thus driving up the performance a notch. It’s seen a spike in popularity lately, and part of the game is getting the “turntable” that the sail sweeps across as clear as possible. Moving the daggerboard out of there helped with that. Placing the board forward of the mast may seem abnormal to some, but one does not make a foiling omelet without breaking a few conceptual eggs. In practice, this board location has been proven for decades on all boats with canting keels and requires just basic math to get the balance right.

The Rig

More often than not, the variable cost center of a dinghy is aloft. It’s quite easy to lose all the budget victories you’ve made in hull design and process control by choosing the wrong spars and cloth. The easy solution would be to go down the Laser route and put a small full sail on cheap aluminum tubes, but that will not perform well enough for a foiler, or meet weight targets. While a fully battened Mylar sail can be made cost-effectively enough, a freestanding carbon spar would murder the budget. We found that a long composite sailboard spar, built in volumes that dwarf all other spars, is priced at an exceedingly seductive level but, on its own, lacks the stiffness to do the job. In addition, foilers work best with high leech tension, so much so that the booms and tension vangs on foiling Moths are historically plagued by breakage.

Without question we had a spar problem that required a creative solution. I found the answer quite by accident while looking at the 1930s Her­reshoff Amphi-Craft on display at the T.F. Green Airport (Warwick, Rhode Island). The boat featured a primitive straight-sparred wishbone boom system, held at its forward tips by wire diamonds and short wire tethers running straight aft to the mast.

herreshoffboatCourtesy Mystic Seaport Museum

Dave Clark was inspired to design the jumper-strut wishbone-boom rig after he spotted a 1930s Herreshoff Amphi-Craft like this one on display at his local airport. The adjustable rig allows for changes in mast stiffness and leech tension to be made by the helmsman while under way.

For a moment I mistook the tethers for spreaders, and realized that diamonds, plus compression struts (spreaders), plus wishbooms married to the tips of those spreaders would allow a cascade of benefits. You could swap out the stiffness needs of the spar with tension strength in Dyneema jumper stays, and replace the tension needs of the vang with compression strength in the booms. This would allow these forces to efficiently cancel one another out, resulting in a conventional stiff spar with a tight leech, while still being based on that inexpensive, flexible sailboard spar. This is the core concept of our original “jumper strut” arrangement, which, finding no prior art, we’ve provisionally patented. We find that it supports and works the sail in all the ways it should and has the added benefit of enabling a lighter-weight sailor to “turn off” the rig stiffness by releasing tension on the jumpers, effectively depowering the boat in windier conditions. This yields a surprisingly wide tuning range for a freestanding rig.

The other risk when designing a sailboat for yourself is the budget-crippling shopping spree at the chandlery. While riddling the boat with on-the-fly tuning systems and little bungee-driven take-ups is a complete blast, and leaves the boat completely caked in bling, it also costs more than any reasonable customer can afford. For beginners, this often booby-traps the boat, physically ensnaring them in a nest of systems they don’t want or understand. So we kept the deck layout pointedly sparse. There is a Cunningham and a mainsheet. Oh, and a tiller. Go sailing.

The Foils

We did our best to keep the hydrofoil system conventional, because we had no interest in subjecting our customers to an ego-driven science experiment. The UFO runs on a Moth-type system. The daggerboard (mainfoil) and the rudder are fitted with horizontal wings on their bottom ends. The mainfoil has a flap on its trailing edge actuated by a surface-sensing wand at the bow. When the hull is low to the water, the wand drags aft, and a series of bell cranks and connecting rods attached to it push the flap down, generating increased lift. When the boat is flying on the foils and the wand is not touching the water, the opposite conditions apply, and the system corrects downward. As a secondary consequence, this controls the pitch of the whole boat.

These components combine to determine the flight path of the boat at a set altitude, dictated by the length of the wand. This basic system was originally patented in May 24, 1955, and has been applied to a range of foilers since then. (See Jim Brown’s feature about Sam Bradfield, “Takeoff Window,” PBB No. 108.) Since 2002, this configuration has been vigorously refined by competitors in the ever-growing International Moth class, which has leveraged the technology to yield unquestionably the world’s fastest single­hander.

ufo on shoreDave Clark

With foils pulled up between the hulls, the UFO can be easily launched via a beach-style dolly.

We commissioned Nat Shaver, a friend who currently designs foils for the French America’s Cup team, to design the foil shapes. The result is a very gentle, high-lift package. Our objectives of modest and forgiving foiling performance aligned nicely with our cost objectives, as the less you prioritize absolute top-end speed, the easier the foils become to manufacture. Extruded aluminum vertical struts coupled with vinylester, glass, and carbon compression-molded lifting foils achieve 80% of the Moth foil package’s performance at a cost that’s lower by two orders of magnitude. Again, picking out our own design ground yielded benefits over merely value-engineering what already exists.

In Perspective

What we get in the end is a boat that fulfills the brief and has made sailing fun again on a personal level. Having grown up with some of the fanciest sailing toys that money and custom fabrication could produce, I can be as jaded as anyone about going out for a spin. Not with the UFO. At some point last summer I, having felt a puff, was rushing off to rig up for a test flight. As I hustled to get my gear, Dad pointed out, “I haven’t seen you this excited to go sailing in a long time.”

bowshotDave Clark

The UFO was designed to bring foiling to the masses. It has also made sailing fun again for Dave Clark.

He was right. There’s something truly special about foiling. You build speed, suddenly start rising up as the boat goes dead quiet and gains another burst of speed as all your hull drag and most of your strut drag vanishes. You are the wind. Being able to do that without wasting time rigging anything fussy or going through some sort of convoluted launching process, or fighting breakages and impossible maintenance and finish standards, has been completely liberating.

I’m glad we did this, and it’s a 
complete pleasure sharing it with everyone.

About the Author: Dave Clark began his apprenticeship under Steve Clark (Vanguard Sailboats 1985–2007) in 2004. He has been making an ass of himself and wasting expensive materials ever since.

Editor’s Note; The above article was originally published in Professional BoatBuilder No. 166.

For more about the UFO:

Video: Dave and Steve Clark’s UFO Foiling Catamaran