In April 2015, when the AC62 Class Rule was dropped in favor of the 48 footers of the America’s Cup Class, the “mostly one design” nature of the new rule focused designers’ attention on a very small number of areas: daggerboards, rudders, control systems and fairings. Virtually everything else is specified by the rule – hull shapes, cockpit dimensions, crossbeams and longeron and the wing planform and section.

Even before the appearance of the America’s Cup Class Rule, the AC62 Class Rule had already eliminated certain design decisions, with the wing as a notable example. Remember that in San Francisco in 2013 Emirates Team New Zealand used a wing with an internal spar surrounded by a flexible skin for leading element twist, while Oracle Team USA opted for a “D” shaped section for the spar with no adjustment of the aerodynamic surface of the leading element. ETNZ had experimented with a tab on the leading element but abandoned it early in their testing, while OTUSA stayed with a tab. Changing the tab setting mid-way through the match was an important factor in their comeback. That change opened the slot, moving the wing center of effort aft, which helped eliminate their lee helm problem. The AC62 rule forbade both leading element twist and the tab. The new ACC rule is even tighter, with the planform and the section of the wing specified to tolerances of six millimetres. On the other hand, the control systems for the wings will remain an important area for design attention. OTUSA’s AC72 wing was self tacking while ETNZ used hydraulic rams to tack their wing. The cause of ETNZ’s near capsize in Race 8 was reported to be the result of not having enough hydraulic pressure to pop the wing through the tack.  Wing control systems proved crucial between races in San Francisco, too. OTUSA was able to mode their wing quickly, changing settings after the first race of the day, to allow for the building sea breeze in San Francisco.

If most of the CFD work for wing design has been eliminated, that will be more than offset by the demands of designing the daggerboards and rudders. At the high end of the wind range, boatspeed will exceed 40 knots and cavitation will be an important consideration. At lower wind speeds the boats may struggle to get up on the foils. In all conditions, the transitions during manoeuvres will be crucial. Trading off lift and drag at high and low wind speeds will keep the designers busy. The protocol limits the number of daggerboards the teams can build to a maximum of eight lower sections, assuming two-piece boards. Will any teams opt for different boards for light air and windy days? How difficult would it be for the sailors to master using different sets of boards?

OTUSA, Artemis Racing and Land Rover BAR have all enlisted technical partners with strong control systems experience. The systems will need to manage the energy generated by only four grinders and distribute hydraulic pressure to wing and daggerboard controls. A training incident in Spring 2013 showed OTUSA the need to set priorities for pressure distribution – Jimmy Spithill got no response to pushing the buttons to adjust board rake because pressure had just been bled off for a wing trim adjustment. This led to a frightening moment and visions of the October capsize. Systems need to be simultaneously sophisticated and simple. Control  functions are needed for wing shape, traveler trim, and for board rake, cant and height. Physical demands on the grinders and mental demands from frequent manoeuvres will dictate that system operation be clear and reliable.

Fairings are getting attention from some of the best aerodynamicists in the world. Adrian Newey brings his experience from Formula 1 to BAR. OTUSA’s strong in-house designers are complemented by Airbus. Artemis picked up top designers from both OTUSA and ETNZ. In the 2013 America’s Cup we saw OTUSA get a big advantage by increasing their effective wing span and reducing induced drag with their pod understructure. ETNZ’s AC72 sprouted the “pie warmer” spoilers on the rear cross beam between the Louis Vuitton Cup and the beginning of the America’s Cup Match. Both teams added solid end plates under their jibs just before the match. All teams had fairings on the foot of their wings to seal them tightly to the deck. For the racing in 2017 we can expect to see a lot of attention on reducing parasitic drag. This may provide the most visible differences in the boats, beyond the branding and cosmetics.

At the beginning of the design cycle for the AC72’s, no one was publicly predicting foiling or the speeds the boats would eventually reach. A tighter design space and the experience from San Francisco seem to preclude any big surprises in this cycle. Aerodynamic fairings and daggerboard shapes will give us subtle differences to see. But the most important differences among the boats may be completely invisible, hidden in the control systems.