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The following is a full, unedited article from Multihull designer Jim Brown, entitled, “TakeOff Window.” It’s officially about the lifelong work of Sam Bradfield in the area of hydrofoils.

More specifically, Jim writes about the 18-foot trimaran — OSPREY — that Sam spent the last years of his life making into an incredible hydrofoiler. The boat is a marvel of engineering and offers a sensation of flying over the water (according to those who’ve had a ride on it) that is absolutely amazing.

The edited version of this article, which includes photos and illustrations, comes from one with the same title — featured in Issue No. 139, October/November 2012, of Professional Boatbuilder magazine … you can see this issue advertised here (full access to back issues are only available to the magazine’s subscribers).

It’s a treat, therefore, for Jim to share the complete, unedited version of this article with us here. It is worth noting that soon after this article was published, Sam Bradfield passed away. Jim has been told that this piece, which he wrote as a tribute to Sam’s work, was the last thing Sam ever read.

The things missing from this written piece, of course, are the photos and illustrations that were published in the magazine’s version. Perhaps they can be added to this post in the future. Until then, enjoy the following from our friend Mr. Brown.


The Sailing Hydrofoils Of Sam Bradfield
By Jim Brown

The levitation is subtle, but the acceleration is overt. To stay with the boat, I crouch on the trampoline, my fingers clutching through the Spectra netting. I croak to no one, “It’s like flying in your dreams.” There is no spray, the water noise is gone, and I am soaring on a paper dart. Or a creature? Her name is OSPREY, but she flies more like an albatross, skimming over but hardly touching the waves, and I am crouching on her wing, gripping feathers.

Sheeting in, here comes another surge ahead and a quantum increase in apparent wind. Hanging my head “out the car window,” grinning open-mouthed to let the wind inflate my cheeks, I peer down to see the vertical “dagger” portions of the foils slicing the “road.” They make just the faintest white noise while leaving three thin lines of salt steam converging rapidly away. I cannot see the underwater “wing” sections of the foils which are – unreasonably it seems – lifting this little trimaran (and its elevated crew of three) well up above the water. I want more. More wind, more speed, more quiet. To see the underwater wings at work, I want to sail this thing at night in phosphorescent seas.

Fancy Into Fact….

Flying over water while being powered by the wind is not a new idea, for some success in this lofty endeavor was achieved as early as 1938. Indeed the Pacific Ocean was crisscrossed by David Keiper in his trimaran foiler WILLIWAW in 1995-6. Even various foiling consumer products have been offered over the years. Greg Ketterman’s biplane-rigged TRI FOILER, was produced by Hobie in the early 90s, Sam Bradfield’s RAVE, was introduced by Windrider in the 2,000s, and the foil-borne Moth-class dinghies are currently popular. Going big, the 78’ French hydrofoiler L’HYDROPTERE holds the current world’s sailing speed record (except for the kite boarders) at over fifty knots.

[The World Sailing Speed Records Council gives Hydroptere’s length as 78 feet. Other sources say as low as 50’.]

More to the point, it is Bradfield’s thirty year progression of working prototypes and his assiduous gathering of data from them, that now results in OSPREY. The design and construction of this little trimaran, with her duplex T-foils, and their automatic flight control system, all combine to embody – at least in this writer’s opinion – the current state of the wind powered hydrofoiling art.

Sam’s fascination with the subject originated with a 1960s book called, The Forty Knot Sailboat. Written by the visionary Bernard Smith, the book makes several very fanciful proposals for achieving truly controllable, highly efficient, yet simple and dependable flight for wind-powered watercraft. Bradfield resolved to turn that fancy into fact.

His career as a professor of aeronautical engineering, where he specialized in boundary layer studies, has provided obvious benefit in his foiling quest. Not so obvious have been the laboratory infrastructure available at the several universities where he has taught, and the willingness of eager students to take on the challenges he assigned. His work has also attracted the participation of gifted boatbuilder’s and sailors, in particular the long-term loyalty of Mike McGary and Tom Hayman, both of whom are continuing the research under Sam’s direction as he now ticks off his mid 90s.

How Foiling Works….

Hydrofoils are underwater wings with stilts. Basically, they counteract gravity by lifting a vessel’s hull(s), at least to some degree, up out of the water. Hull drag in water is reduced or eliminated, and is replaced by the far lesser foil drag. Consequently, there can be a significant increase in the vessel’s speed, and a far more efficient use of propulsive force (whether produced by sails or engines). Perhaps most important in the long run, foiling REALLY smoothes out the ride!

The engineering challenges of sailing hydrofoils are somewhat different, rather more complicated, than for an aircraft or submarine, neither of which – other than when landing or surfacing – is obliged to contend with more than one medium at a time. Furthermore, neither subs nor planes glean their motive power from the wind. The sailing hydrofoiler, by contrast, must obey “The Laws Of Flight” while operating in two fluids at once, and one is at least six hundred times more dense than the other. It must also contend with the destabilizing forces of sailing, all this while still escaping the usual tyranny of displacing tons of water in order to proceed.

Several types of foils have been used over time. After years of experimenting, which sometimes included two types of foils in the same boat, Bradfield has settled on the exclusive use of inverted “T” foils, one mounted in each hull of a trimaran vessel. The vertical “dagger” portions of the Ts pierce the water’s surface with minimal drag, and they provide both lateral resistance and directional control to the craft (the main hull’s transom-mounted foil is steer able). The horizontal “wing” or “lifter” portions of the Ts operate completely submerged to elevate the craft into the air and, as we shall see, control the distance of that elevation above the water’s surface.

Attention is drawn here to the sectional shape of hydrofoils. The early sections were like airplane wings, cambered on top and relatively flat on the bottom (plano/convex). When running at speed, this section generates lift upwards to hold the craft aloft. For very high speed aircraft, and now for hydrofoils, “symmetrical” sections – cambered both top and bottom – (convex/convex) are used by Bradfield and others. When fitted with an articulating aileron or flap, these foils can create lift in either direction, up or down, and so can exert a profound stabilizing influence on a sailing trimaran. Heeling effort in the sails, which would normally depress one side of the craft and elevate the other, is resisted absolutely by the downwind foil lifting up and the upwind foil pulling down. The craft stays dead flat, the mast stays plumb vertical, the usual spilling of wind from heeling is converted instead to thrust, and the power of the sails is fully utilized to propel a craft that is now largely relieved of the age-old drudgery of hull drag.

Bradfield and his associates did not invent, but have highly evolved, this “duplex” hydrofoil concept. From extensive wind tunnel testing and years of usage on their sailing prototypes, they have optimized the curvature of their hydrofoil’s cambered section. When running at speed in water, this camber avoids creating extreme “pressure peaks,” in its distribution of lift across the foil. Such peaks, caused by many foil shapes, can induce cavitation and/or ventilation of the foil, which destroys lift. This can result in dropping the hulls onto the water at speed, or in releasing the foils from the surface.

The goal, of course, is to maintain a reasonably consistent elevation of the craft’s three hulls above the water, and, consequently, a relatively consistent depth of emersion for the underwater wings (optimally 2.5 times the foil’s chord). To achieve this consistency, the flap’s articulation is adjusted continually and automatically during flight. In the Bradfield system, flap articulation is driven by a very simple, mechanical “sensor ” wand which trails along the water’s surface to “feel” its level relative to the boat. The wand pivots to follow that level, thereby actuating a very simple, mechanical linkage, which articulates the flap. The mechanics are robust, dependable and easy to maintain, and the action is immediate and powerful.

Adjustment of the vessel’s general flying height is made manually. Long bungee cords lead from the wand linkages to the helm, and these bungees exert pressure downward on the wands to vary their degree of contact with the water’s surface. Once bungee tension is set to create proper flying height in accord with ambient wind, waves and payload, then continuous control is left to the wand-to-flap connection, thereby achieving truly automatic, totally non-electronic, flight control.

No longer running into the displacement wall, foil-borne craft are able to sail at speeds at least twice the velocity of the ambient wind that propels them. By entering this regime, the vessel’s own speed, through oblique-moving air, can greatly amplify its power, and the combined effect of these efficiencies takes the sailing machine into a whole new realm.

“To Bring Me Pleasure”….

Sam Bradfield’s focus has been on recreational foilers, ones that he can imagineer and finance on his own. He has not sought much external funding, feeling that he needed the freedom to proceed without intrusion through his several phases of sailing hydrofoil development. “I’ve done all this just to bring me pleasure,” he says.

For a private initiative, the extent of his pleasurable investigation is remarkably comprehensive. For example, in this thirty-year sequence of successive refinements, the Bradfield Flying Fleet has included the following range of experimentation:

  • Both catamarans and trimarans are represented. Their length has varied from 16’ to 37’. Built of everything from laminated wood to skin/foam/skin composites – and even rotomolded polyethylene – these hulls and platforms have carried either soft sails or rigid wing sails.
  • Vastly different hydrofoil types have been used. The early “ladder foils” have been made of wood and later metal, the more contemporary “dihedral foils” have been used both with and without the “J” segments, and the latest duplex, “T” foil designs are made of either fiberglass, extruded aluminum, or carbon fiber/foam composites.
  • The sections of these T foils have been either plano/convex or symmetrical, some with and some without flaps or ailerons. Their flight control systems have been both manual and automatic, and included both front and back steering.

Throughout this quest, Sam has assiduously gathered performance data. Beginning with simple spreadsheets showing the known results of various individual craft, he recently has been aided by his son-in-law Leonard McCrigler, a computer engineer, in developing from Sam’s data a highly integrated Performance Prediction Program.

Besides Going Fast….

It was while pondering Sam’s computer screens that I was privileged to have him, together with Mike McGary and Tom Hayman, all try to explain the essence of their endeavor to this surface bound, non-mathematical sailor. With my voice recorder running, I asked them, “Besides just going fast, what are you guys really trying to do here?

We are trying to make foiling practical. And in order to do that, we have had to collect a lot of know-how and hard data just to understand what really happens when a sailboat flies on water. Devising this program has really helped in that understanding, to the point that we can now confidently design new boats that we know will achieve specific performance criteria.

Right, We’re not about just top speed. The French are now foiling at over fifty knots, but it takes something like an 80’ foot boat and maybe forty knots of wind, blowing over flat water, for them to go that fast. We’re trying to attract ordinary sailors into foiling, but who wants to wait until its blowing a gale to go sailing?

Most recreational sailing happens in winds of around ten knots. This new boat we have now, the OSPREY, is only eighteen feet long, and we’ve sailed it just a few times so far, but it is living up to its takeoff window prediction. Sam’s early boats needed at least twelve knots of wind to fly, but OSPREY will fly in just seven, and to us that’s almost doubling performance.

We’ve learned that our boats must perform adequately as surface trimarans in very light airs, and they do, but the real challenge is to make a boat that spends very little time not flying.” Of course, high speed is fun, and going by the program, plus a lot of experience in our other boats, we expect OSPREY to fly at over forty knots in a twenty knot wind.

Well, that assumes that I have the guts to sheet it in that tight. The way we’ve built this one to emphasize light airs takeoff, if we push it to forty knots I think we might be right on the edge of structural problems.

Yes, we have a lot to learn from this boat before we push it that hard, The loads on these things are terrific, and we’d like to identify potential failure modes before they happen.

It seems the downwind foil has to lift much of the weight of the boat, but also the added load of the upwind foil pulling down! How do you make the foils strong enough?

That’s it, The lifting portions have to withstand 5,000 pounds of load, and that’s in just this 400-pound boat.

These foils for OSPREY are made of carbon fiber and foam, and they’re molded in one piece, with solid carbon at the juncture of the T. But still they flex a lot, as much as three inches in their five foot span.

“Wow, you’d think they’d break.”

Well, we’ve broken a few foils over the years, including the ones made from helicopter blades, but we’ve learned they have to bend in order to keep from breaking. And that means the flap hinges have to tolerate bending without binding up. That’s why we split the flap into two segments, port and Starboard.”

But how about pitch? There are no flaps or sensors on some of your rudder foils. What controls the boat’s fore ‘n aft attitude?

The wing on the rudder foil changes its angle of incidence with the boat. If the bow pitches up, the rudder’s wing follows it like the feathers on an arrow, and lifts the stern, too. Even in real waves, pitch control can be inherently automatic. But these boats have to be designed to sustain what we call crashing, the sudden loss of lift. If it can happen, it will, but we find that when the foils ventilate, the flow usually becomes re-attached very quickly, before a crash occurs.

Well, we’ve done it both ways, with flaps and without flaps on the rudder’s wing. OSPREY has a manually controlled flap there, which can be used to encourage takeoff at low speeds even with a good load aboard. But the rudder foil also might be developed to automatically control nose diving in a crash. I think it needs to be further explored.

Light weight in the structure is so very important to you guys that I wonder about crew weight and other payload. Isn’t payload part of practicality?

Once these things are up on the foils, you can pile on the weight and they’ll keep flying. Getting off the water in light airs is the challenge, and that explains our focus on light weight and on the takeoff window.

Well, you can also take off with auxiliary power, or with a more powerful sail. In the last Americas Cup, the big trimaran ORACLE carried a rigid wing sail, and it won easily over the soft-sailed competition. That was a no-brainer to us because, as Sam’s program clearly shows, there is an almost thirty percent increase in thrust from the rigid wing compared to a soft mainsail. And now, all the Americas Cup boats have wing sails. They’re not easy to take down at night, but the AC boats must have them in order to compete.

But we don’t have to have them! So far, wing sails are impractical, and they’re terribly expensive. We can gather plenty of data without them, and we can’t sell boats, or even sell our ideas, if they’re that much trouble and expense. You just have to fight all the time to keep foiling from getting too complicated.

And so far, wing sails can’t be reefed, Our boats have to be able to handle the rough stuff and the strong winds, and we don’t think wing sails are that far along. For instance, In SCAT, our 37’er, we crossed the Gulf Stream in the Miami/Nassau race when conditions were rather bad. The boat was new, and it had been built somewhat overweight. We were close reaching at high speed, and reefed down, and our main hull was skimming the crests. Things were scary and wet, but we did just fine. We took second place behind a big racing catamaran with a superstar crew, and our consolation was that they got seasick and we didn’t.

That’s important, These craft really level out the ride. In big swells, they conform to the general surface, but the foils seek a median between the troughs and crests of ordinary wind waves. That means the mast and sails run much more steadily in the wind, which increases the power they produce.

Well, as you guys know, I like to play prophet, and it seems to me that the real benefit of foiling, and perhaps the real achievement of your work, is that it has the potential to solve the number one problem in taking human beings to sea, and that is sea sickness.

With that, the discussion strangely turned to talk of lunch, which we pursued with gusto. Later on, I found myself trying to place hydrofoils in the history of modern multihull development. By messing about in multihulls for over fifty years, I have seen them progress from the “lunatic fringe” to the “up side” of the boating industry. Now “the official yachts of official yachting,” the Americas Cup catamarans are foil-assisted, and by next year’s contest they may be completely foil-borne. Therefore, I find it fascinating to imagine what another fifty years may bring. There seems little doubt that hydrofoils are the next big thing. Surely they are destined to appear on many recreational multihulls because sailing with the laws of flight is such a blast. Moreover, the increased efficiencies of foils – and the decreased riding motion – is likely to dictate their usage on commercial and military vessels.

For example, contemporary aeronautics feature jet aircraft; they require a great burst of fossil energy to get them off the ground and up to that rarified altitude where they can run out their distances with great energy efficiency and speed. Coincidentally they fly above the weather to practically eliminate air sickness. So it seems to me quite reasonable that future oceanautics could include surface vessels that work like jetliners. They would require bursts of stored-aboard energy to boost them onto their foils and into that rarified realm of sailing on self-generated wind, but from there they could run out their distances as futuristic motorsailers, propelled at ocean-liner speed largely by wind power. Coincidentally, their passengers would fly above the waves in comfort.

For sure, hydrofoils have some nagging disadvantages, like collecting weed and striking obstacles. These seem minor relative to the obstacles of aviation; we already have side-scanning Sonar, but not yet the underwater whale whistle. Still, in an energy-smart, earth-smart future, it is not unreasonable to suspect there may again be need for us to utilize the power of the wind for transportation. If that means hydrofoils, it also mean some form of auxiliary power for becalmed craft to keep schedule, and to keep their takeoff window always open. Applying auxiliary power to a sailing hydrofoiler is a daunting challenge, but so many other challenges already have been met! Indeed, because of pioneers like Sam and his colleagues, it seems that much of the technology for the future’s Flying Ship, starting at the size of something like a jumbo jet, is already on the shelf. Judging by the pace at which current events are now driven by eco-nomics, we – Sam included – just might live to see such vessels flying through that window. In such a future, I like to think the flagship of the fleet would proudly bear the name: FS SAMUEL E. BRADFIELD.