America's Cup: Emirates Team NZ gives Challengers legal warning shot
by Richard Gladwell Sail-World NZ 11 Sep 02:41 PDT
Emirates Team New Zealand - Sailing Day 16 - AC38 Recon - June 15, 2026 - Auckland, NZ © Sam Thom / America's Cup
America's Cup Challengers have been warned they are taking a legal risk over possible infringement of a sail design/engineering patent owned by Emirates Team New Zealand, relating to how AC75 sails are designed and manufactured.
Documents leaked to Sailing Illustrated, a twice-weekly live sailing webcast, include correspondence bearing the North Sails logo and a legal firm representing Emirates Team New Zealand, along with the patent document, and revealing that the team intended to enforce a patent it held over sail design.
North Technology has contested whether the patent is a unique development and has suggested a workaround - manufacturing the sails for the Challengers at its Sri Lankan facility, a country it says are outside the patent territories of the USA and Europe.
The heart of the October 2024 patent lies around the introduction of an elastic area in the sail just aft of the luff, which responds to luff and sheet tension to quickly alter the shape of the sail in a similar way that an airliner wing alters from a rounded high lift/high drag section for takeoff and landing to a faster/low drag shape to allow faster speed in flight.
Differing panel layouts were used to better distribute sail loads and reduce sail stretch and distortion in the 1986/87 America's Cup, and later became known as tri-radial construction. Other variations evolved with UK Sails' "tape drive". Those design iterations are quite different from the ETNZ patent 37yrs later.
ETNZ's AC75 concept is new. It uses very different materials, from the Fremantle era. A significant difference is that the AC75 has full-length battens, which are wound up to help induce mast bend. None of the prior art in the various America's Cup classes involved full-length battens.
Instead, 12Metres had backstays to control the mast bend and forestay tension, as did the first-generation AC75s.
Backstays were dropped from the 2024 version of the AC75 class rule, after Italian Challenger Luna Rossa tried to remove theirs in the 2021 America's Cup. Now the hydraulic mainsheet and cunningham systems are engineered to achieve the same rig effects in the now cat-rigged class.
The design elements are covered by the ETNZ patent, first lodged by designer Steve Collie in June 2023.
The loads in the foiling AC75 differ significantly from those in regular keelboats. Since the AC75's inception, Cup designers and engineers have grappled with the challenges posed by the huge range of apparent wind speeds under which the AC75 foiling monohull operates.
"You definitely need a lot of power when the breeze is light," explained sailmaker/designer/trimmer/skipper Glenn Ashby at the launch of Te Aihe, the first launched AC75, in 2021.
"Being able to get the sails through a really deep range from light winds to heavy, and also upwind to downwind, will be critical. The apparent windspeed upwind is phenomenal, and downwind while you are going fast, and you really need to get those sails through a massive shape and power range.
"That alone for the designers has been a huge challenge, and whoever does that well is going to put themselves in a great position for the Cup," the three-time America's Cup winner said prophetically.
Massive loads come from the hydraulic mainsheet control, which applies force to the leech of the mainsail skins to control the sail and applying high loads to manage forestay tension.
The dual cunningham hydraulic rams (see image above) apply substantial loads down the luff of the mainsail, which serves the multiple functions of quickly flattening the sail, controlling the position of the draft, and inducing pre-bend in the mast to alter sail shape.
The concept behind the modern twin-skin mainsail traces directly back to naval architect L. Francis Herreshoff who was granted a patent in 1927, which lapsed in 1944.
Eight years ago, Emirates Team NZ began testing the AC75 mainsail concept. The double-skinned mainsails, were originally thought to offer less power than the wingsails that were used in the 2010, 2013 and 2017 America's Cups, have proven to be much more powerful than early expectations, to the point where the AC75 has similar power to the wingsails.
“You have to go through and look at what you have - adjustable halyards, Cunninghams on each one, two outhauls, two mainsheets," North Sails designer Burns Fallow told Sail-World in an interview a year after the announcement of the AC75 class rule.
"It gets very complex very quickly. As we saw in the last two AC cycles (2013 and 2017) once the boats start foiling they get very dynamic, and you need to be able to respond quickly to changing conditions and circumstances - and that is all part of the equation.
“In some areas, you might combine controls, and others may be more independent," he added.
As it turns out, the teams are starting to consolidate their approach, aided and abetted by the design partnerships which have evolved between two groups of teams.
One item that is common to the French and likely the yet-to-be-seen Australian team is a clever but simple mainsail-control system devised by Emirates Team New Zealand, which clearly allows each mainsail skin to be trimmed independently. The effect is that the sail can be shaped like an aeroplane wing to optimise the sail shape for take-off, then flattened for lower drag and higher speed as soon as the AC75 is foiling. The mainsail skins are trimmed independently of each other to create a bird-wing-like cross-sectional shape.
With the advent of mechatronics, the sail shape can be controlled accurately and quickly by mechatronics working in concert through a series of actions between tacks, then adjusted further for an optimal exit in prevailing conditions. Flattening the sails during tacks reduces drag and increases bottom-of-tack speed.
In the ETNZ Patent granted on October 8, 2024, four days before the start of the America's Cup in Barcelona, the "Field of the Invention" is described in part, as: "The present invention is directed to a sail structure for a yacht which provides a modification of conventional sail design in order to retain the conventional mast and rigging design, while increasing the ability to modify the shape of the sail to react to different conditions."
Recon images of sections of the colour-coated mainsail, even when shown in high resolution, do not show the construction details. However, the tension being applied down the luff and from the clews can be seen with small areas of stress creases indicating the direction of applied tension.
In past America's Cups, patents used to restrict design poaching have had a mixed history.
After the announcement of the AC75 class concept, Emirates Team NZ pushed back on a Brazilian designer's claim that the AC75's Foil Cant system infringed a patented "Sail Booster System". The New Zealand team argued that the concept of canting ballast and dynamic foils drew on extensive public prior art. The issue went away after the 2021 Cup.
More memorable was the use of hull riblets by Stars & Stripes 87 to reduce hull drag. NASA developed the ribbed adhesive vinyl film for spacecraft, and the 3M Company turned it into a commercial product. While Stars & Stripes did not hold the patent, they did obtain an exclusive licence for the '87 America's Cup, before the use of hull films was ruled illegal in future regattas.
Some may feel that using patents to protect design innovation is unfair.
But that must be offset against the use of reconnaissance images combined with photogrammetry. In the past, a team spy could shoot a design innovation from a public area close to the launch site, with a long-lens camera and generate a computer design image.
For the last two Cups, a proper joint reconnaissance program has been in place which serves all teams. Plenty of excellent photography is still available, and it is getting better with advances in camera and lens technology.
Given that the America's Cup is historically a design contest, maybe design innovation should be protected to avoid a situation where the America's Cup becomes an espionage and design-replication exercise, with little original design thinking.