How it works: Hydroptère hydrofoil racing boat

Date:25 September 2013 Tags:, ,

A high-tech sailboat’s bold quest: setting a new record in the Los Angeles-to-Honolulu passage.

The experimental French trimaran Hydroptère has a footprint the size of a basketball court and a 30-metre carbon-fibre mast that can fly 560 square metres of sail – made variously of carbon, Kevlar, and Cuben fibre – yet there’s not much room aboard. There’s barely even a deck, just a scrim of netting that stretches between the narrow centre hull and two pontoons. I hunker down next to the bowsprit, which I’m told is the best place to soak up the action away from the spray that’s thrown by Hydroptère’s two underwater wings, one under each pontoon. Hydroptère is a hydrofoil: half plane, half boat, and it doesn’t just sail – it flies. With any luck, those wings will take Hydroptère across an ocean.

It requires a crew of four to man the boat. The co-skipper, Jacques Vincent, has his hands on the wheel. Two très célèbre sailors, Jean le Cam and Yves Parlier, share duties on the lines. Alain Thébault, the boat’s owner and designer, directs. I’m just a stowaway, along for a practice run in San Francisco Bay. Parlier turns the winch, the mainsheet snaps tight, and we start to accelerate, fast. We easily outrun the speedboat chasing us, and at 12 knots (about 22 km/h), the entire boat starts to climb up on its hydrofoils, no longer floating, but flying.

When it’s flying, Hydroptère has all the insectoid grace of a praying mantis, with elbows out and arms akimbo.  The trick to getting the boat speeding is reducing drag. So, when it flies, only the tips of the boat are in the soak: the end of the rudder and the lower halves of the two bladelike foils.

Getting Hydroptère aloft is simply a matter of catching a breeze; the foils do the rest, turning the force of water streaming around them into lift, just as an airplane’s wings do in air. But flying the boat can get a bit tricky. Even though there are sensors all over – a total of 30 – there are no readouts; the data are logged for post-sail analysis.

Off shore sailing is almost entirely dependent on gut instinct and good reflexes. The crew must meet every gust with fine-tuned sail-trim adjustments to keep the boat from getting overpowered and pushed into a heel strong enough to pop one of the foils out of the water. Flat is the hydrofoiling equivalent of an airplane losing a wing, and it can easily result in a capsize. There’s a “Jesus button” at the foot of the ship’s wheel that the skipper can stomp on at any time to depower the sails in a hurry.

As soon as we’re cruising, Thébault grabs me by the wrist, leads me amidships, where there is a joystick beside the captain’s chair. It controls the rudder, a vertical fin with a horizontal stabiliser that controls the boat’s pitch.

By tweaking the pitch of the boat, the skipper can adjust the angle of attack of the foils in front. The smaller the angle of attack, the less lift; more angle means more lift. So not only does the stick control the boat’s pitch, it also controls its altitude. “Like an airplane!” I exclaim. Thébault throws his arms wide and smiles even wider: “Like a magic carpet!”

Later that afternoon, Hydroptère hits 37,5 knots, setting a new sailboat speed record for San Francisco waters. The only boats in the bay that can beat it are the giant foiling catamarans – which hit speeds of around 40 knots – in the America’s Cup races. Hydroptère’s top sustained speed is 50 knots, a world record when the boat set it in 2009 (the record is now held by Sailrocket 2, which hit 65,45 in 2012). Hydroptère’s top speed ever – an instantaneous reading from GPS – was 56 knots, at which point it tripped over its foils and capsized. Flat’s comparable with crashing into a wall at 100 km/h on land – without seatbelts.

The Hydroptère crew spent most of the northern summer in Los Angeles, preparing to sail to Honolulu along one of the major transits in the world of off shore racing, the Transpac. The current record for the 4 120-km route is 115 hours, but by the time you read this Hydroptère may have shaved off as much as a day from that record. Conditions will have to be just right: calm seas, with the wind blowing between 20 and 25 knots the entire time, day and night. The rest is dependent on the skill of the sailors. Beyond just sailing, they must dodge ocean debris; the last time Hydroptère tried to conquer an ocean (the Atlantic, seven years ago), it hit a sea turtle the size of a VW Beetle and shattered a foil.

By the end of summer one of two things will have happened. Hydroptère will have made it to Hawaii, beating the Transpac record or, scenario two: Hydroptère itself will have been beaten and maybe even sunk, somewhere in the mid-Pacific. Either way, Hydroptère’s run to Honolulu is a swan song, a last sail. After flying, crashing and chasing records for nearly two decades, Hydroptère is near retirement. But Thébault promises he will sail again. He’s got Hydroptère 2 on the drawing board. It will foil just like the old bird – only faster.

Making a sailboat fly

Foils work like underwater wings. They must be angled in such a way that the water rushing towards them hits the bottom surface of the foils, pushing them up. The foils’ teardrop-like profile also creates a force: an upward suction courtesy of Bernoulli’s principle, which states that the faster any flow of air or water moves, the less pressure it exerts. The two forces add up, creating lift. When Hydroptère reaches about 12 knots, there’s enough lift to raise the hulls above the water’s surface.

1.     Mainsail

The boat resists the sideways force of the air that hits the sails, and the leftover forward force pushes the boat ahead.

2.     Winglet

When the boat heels, and only the vertical tip of the high-siding foil is still submerged, the lift the foil generates goes horizontal, stabilising the craft.

3.     Rudder

The end of the rudder is crossed with a horizontal stabiliser. By fine-tuning the angle of the rudder assembly, the altitude at which Hydroptère flies can be adjusted.

4.     Shock absorber

Each foil is mounted on a hinge, and between the hinge and the body of the boat is a strut that absorbs some of the force generated by the submerged foils.

5.     Foil

The hydrofoil wings bend inward at a 45-degree angle. This configuration keeps the boat flying when it leans into a turn.

Video: Watch as Hydroptère sets a new sailboat speed record for San Francisco waters

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