The latest America’s Cup foiling cats are providing a wonderful spectacle as they scorch round the racecourse, but how did foiling start and what are the issues involved? Matthew Sheahan investigates

Here’s a challenge. Design a way to support the weight of five saloon cars on a plate the size of your desk on the water. That was the task that faced America’s Cup designers in their bid to make the new breed of Cup boats fly.

If that’s a bit tough, here’s an alternative. Support a 70kg person and their boat in the water on a plank the size of a cricket bat. This one is much easier, the GCSE of hydrodynamics by comparison, with plenty of examples as to how it can be done thanks to the proliferation of foiling Moths around the world.

Getting boats to fly above the water surface is simple in theory, but tricky in practice and has challenged designers for over a century, but in the last decade one class appears to have cracked it.

On the face of it, International Moths on hydrofoils are sailing’s answer to the unicycle. But Moth sailors are far from being trick cyclists, there is a serious side to all this. A foiling Moth will reach 14 knots upwind and 20 knots downwind in just ten knots of wind, and in 20 knots of breeze they’ll be cranking along at 17 knots upwind and 25-30 knots down.

The International Moth has done more than any class to raise the profile of sailing hydrofoils. Photo: Thierry Martinez/Sea&Co

The International Moth has done more than any class to raise the profile of sailing hydrofoils. Photo: Thierry Martinez/Sea&Co

Whether you’ve seen the pictures or experienced the ghost-like whistle followed by the eerie vacuum that trails behind as they come slicing past, it is clear the Moth fleet has done much to publicise the thrill, grace and speed of sailing hydrofoils. The rapid growth of the class has also shown us that foiling is not just for pioneers and record breakers, but that despite several failed attempts in the past to bring it to the masses, this time there just might be a future in foiling.

Another reason why this particular class has exerted so much influence in sailing and spawned a new cycle of design is that Moths can race as a fleet round a course, foiling upwind and down. Although there are plenty of other boats that foil, most have a limited sailing repertoire and need specific conditions to fly. A Moth can fly on all points of sail and is capable of tacking and gybing too.

This has been one of the biggest steps forwards and has helped to boost recent interest from designers and sailors for other potential foiling projects.

It’s a drag

At high speed, drag and seakeeping become big issues aboard any vessel, particularly offshore monohulls, and while yacht design has made some big steps forward since the days of hauling half the ocean behind in a pair of breaking quarter waves, there are other issues holding back performance.

Razor-straight wakes ironed flat by beamy after sections are clearly a step forward. Yet the problem for monohulls is that, as they slam and crash their way into the waves, keeping the structure in one piece is a big challenge. Rising above the water’s surface not only reduces drag, but might help to reduce structural risks and make handling at speed easier.

Typical resistance curve showing the rapid reduction in drag once a vessel gets airborne

Typical resistance curve showing the rapid reduction in drag once a vessel gets airborne

Forty years ago it looked as though offshore foiling was about to make a breakthrough. David Keiper took the concept cruising in the 1970s with his trimaran Williwaw (see below), in which he clocked up a staggering 20,000 miles cruising the South Pacific. A decade later, French sailing legend Eric Tabarly broke the schooner Atlantic’s west to east transatlantic record set by Charlie Barr in his foiling trimaran Paul-Ricard.

Yet despite such feats, offshore foiling stalled. Tripping up at speed and keeping the boat in one piece when it touches back down have been the main concerns. And while neither of these two pioneers suffered such a fate, there are plenty of wince-inducing reminders of how it can all go pear-shaped.

But today foiling is more popular than ever. The French tri-foiler and former world record holder L’Hydroptère was the first sailing boat to break through the 50-knot barrier. Since then, a new world record has been set by Paul Larsen’s Sailrocket, another foiler, albeit an extreme one, which owes its success to a recent breakthrough in hydrodynamics.

L'Hydroptère, former speed record holder. Photo: Christophe Launay

L’Hydroptère, former speed record holder. Photo: Christophe Launay

Both share a longer-term view of taking foiling offshore. So does the British foiling cat project C-Fly with its canard configuration.

Meanwhile, the America’s Cup cats are trying to scorch around an inshore racecourse on foils that have no moving control surfaces. It’s a big ask of a high-speed boat and there have been spectacular crashes.

Then there’s SYZ&Co, the 35ft foiling cat on Lake Geneva, the new L’Hydroptè, as well as the hull-less Mirabaud LX. These are just a few of the better known foiling projects, but plenty of others are experimenting with hydrofoils in subtler, more discreet ways – and not just racers and record-breakers.

Foils not just for the few

Cruising catamaran manufacturers Catana launched a 59ft luxury cruising cat that has curved daggerboards that the builders claim produce half a tonne of lift at ten knots. The Dynamic Stability System (DSS), which uses a hydrofoil in the horizontal plane, doesn’t aim to raise the boat out of the water, but instead uses hydrodynamic lift to improve its performance – although the new Quant 23, claimed to be the first foiling keelboat, uses the DSS is a new and thrilling way.

Quant 23, claimed to be the first foiling keelboat

Quant 23, claimed to be the first foiling keelboat

And then there are those who seek to bring the thrills of hydrofoils to the average sailor. In quick succession there have been a couple of 36ft Infiniti multihulls, the Flying Phantom, the Gunboat foiling 40ft G4 and the Formula Whisper foiling catamaran, possibly the first foiler designed for club sailors. There has even been a first Foiling Week in July 2014, where a number of different foiling boats strutted their stuff.

Foiling is in vogue and capturing people’s imagination. But apart from the Moth sensation, why is foiling so popular now and where is this all leading?

Material advances

As is so often the case, the answer lies in the development of new materials and techniques. Weight and strength is at the heart of the issue and carbon has once again played a big part.

While foiling clearly appears to reduce the wave-making drag considerably, there is no free lunch. Lift and drag go hand in hand and what happens beneath the surface can sometimes wipe out any benefit above. For example, tilting a foil at a large angle of attack may create lift, but there will often be a large amount of drag too.

“You don’t fly for the beauty of it,” says aeronautical engineer Joseph Ozanne, Oracle’s lead wing designer for the two previous America’s Cup cycles. “People forget that to generate sufficient lift to raise an entire boat can mean introducing a lot of drag. Just because you’re flying doesn’t mean you’re suddenly more efficient or faster.”

Josephe Ozanne, who worked for Oracle duing the last two America's Cups

Josephe Ozanne, who worked for Oracle duing the last two America’s Cups

To test this, put your hand horizontally outside a car window while it’s going along and rotate it gradually. Initially, with a small angle of attack, your hand will want to rise, but as you twist your palm further, the force pulling it backwards increases significantly.

It’s the same with foils; you can generate lift at high angles of attack, but you won’t necessarily be going any quicker simply because you’ve lifted out of the water.

  1. 1. It’s a drag
  2. 2. 30kg all up
  3. 3. 'C', 'L' and 'S'
  4. 4. Page 4
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