COMMENT By Richard Gladwell, Sail-World.com/nz
May 13th marked 25 years since Team New Zealand, on their fourth attempt, won the America's Cup for the first time.
Since that victory, there have been three America's Cup classes used. New Zealand teams have won the America's Cup in two of those classes and only missed by a single point in the third - a remarkable achievement of management sailing and design competence.
The new class for the 36th America's Cup, the AC75 - a 69ft monohull with a 6ft bowsprit - presents a new set of challenges. In over 200 years of yacht racing, nobody has raced foiling yachts of this size and type.
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The AC75 is as much aircraft as it is a boat, with teams now quite openly referring to the canoe body of the yacht as a hull - when it is in conventional sailing mode, and as a fuselage when it is airborne and governed by a completely different set of physical criteria.
The next America's Cup will likely be determined by the lower metre and a half of the yacht/aircraft - or the foil arms, rudder and their attached wings and flaps - and how those are controlled.
On the AC75, the foiling system is very similar to an aircraft wing, comprising the wing itself, with controllable flaps on the trailing edge.
The crew use a "flight control" system to control the foils and rudder lift the boat from the water into much faster foiling mode. In testing and development, some teams sail their boat with foiling height controlled entirely by computer, but when racing, the rules require the flight control system to be manually operated by the crew, handling a series of functions and adjustments which control the flight of the AC75.
Get these settings out of kilter, and the AC75 can potentially respond uniquely and spectacularly - launching itself clear of the water at a takeoff angle sharper than that of a commercial airliner taking off from a runway.
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Te Aihe, Team New Zealand's AC75, was the first to give her aerobatic display just before Christmas, as 7500kg of boat and 11 crew leapt completely clear of the water.
Last week Italian challenger, Luna Rossa – sailing with reduced crew and electric winches - repeated the feat while training in Sardinia.
Neither team would comment on how they achieved this impressive feat of sailing physics, although the teams will know exactly what had happened to trigger the stunt.
A video of Te Aihe taken just before the incident showed the boat being spun through a sharp high-speed gybe, where the foil control systems could not keep pace with the rate of turn.
As the boat exited the sharp turn, the various adjustments were not synchronised, causing too much lift to be generated from the immersed main foil. At the same time a second wing on the rudder wing dramatically stalled and lost its lift.
Foiling is a function of equal and opposite forces from wind, water and the response of control surfaces. Get those out of kilter, and the results are quick, dramatic and spectacular.
On the two AC75s the effect was for the leeward side of the boat to be pushed high, while the stern sank, with the stalled rudder wing incapacitated and unable to generate a sufficient lifting force to complement that coming from the main foil.
Both crashed back into the water with all the elegance and grandeur of a humpback whale broaching.
The Italian and New Zealand incidents appear to have occurred under similar conditions - 10-14kts of wind and relatively smooth seas when the AC75 is at its most powerful.
In the simulator-driven world of the current America's Cup, the Italian and Kiwi teams will have gained valuable data to analyse, replicate and then develop a boat and foil management strategy to prevent a reoccurrence.
Spectators are in for a visual treat if there are similar aerobatic displays in the America's Cup racing, but in the meantime fans can expect more spectacular sights as the crews push the limits on the amazing AC75s.