Jack Griffin: the AC72 design battle.

Image credit: Carlo Borlenghi/Luna Rossa

| This article appeared in the June 2013 edition of Sail Racing Magazine. |

Ever since the yacht America arrived in Cowes in 1851 with her sharply raked masts, concave bow and tightly woven egyptian cotton sails, design and technology have been key to winning the cup that bears America’s name. As the technology battle for the 34th America’s Cup rages, let’s take a look at challenges facing the designers and how each team has responded. 

The tight box rule for the AC72 wing sail catamaran sets very narrow ranges for length, beam and wing sail dimensions, but the devil is in the details. The wing sail originally got the most attention in the media but focus has now shifted to the daggerboards and hydrofoiling.

Oracle Team USA shows a lot of attention to reducing aerodynamic drag. The fine shape of the hulls and details like the faired in understructure and cockpits for the crew will make this boat aerodynamically slippery, especially upwind. Their first boat pitchpoled on its eighth sailing day, destroying the wing and badly damaging the hulls. Before the capsize they looked unstable when foiling. The platform appeared very flexible, with obvious racking. The repairs to boat one made her noticeably stiffer and she showed much more stability when up on the foils. OTUSA’s boat two launched in late April and immediately showed off her ability for stable foiling. 

Emirates Team New Zealand focused on hydrofoiling early in their design process, testing control systems on a pair of Morelli & Melvin SL33 cats before scaling up on their AC72. Their AC72 platform looks very stiff, with diagonal beams. Within days of launch they were foiling stably and soon reporting speeds over 40 knots. Their second boat has some obvious aerodynamic developments and lots of hidden refinements to control systems and structures. ETNZ will soon complete their move to San Francisco and plans to begin sailing boat two on San Francisco Bay on 23 May.

Luna Rossa bought the design of ETNZ’s first boat, as permitted under the Protocol. They are the only team that will not build two boats. Like ETNZ, they were foiling confidently during their training in Auckland. Some visible elements of Luna Rossa’s ongoing development were obvious after their move to San Francisco. Not surprisingly, Luna Rossa has the most striking visual design with a mirrored silver finish on the hulls and wing.

Artemis Racing’s fatal capsize killed olympic gold medalist Andrew “Bart” Simpson. The catastrophic damage to their first boat ironically came on what was planned to be the final day of sailing for that boat. Artemis originally planned not to foil, believing that the hydro drag penalty during transitions and upwind would be greater than the gains from straightline speed when foiling downwind. They quickly changed plans after initial brushes with OTUSA showed they were well off the pace. They announced that boat two would be capable of “full foiling.” Modifications to boat one were made to increase lift to about 80% of displacement and enhance their “skimming” ability.

ETNZ and Artemis have each built two boats, but will only sail one at a time. OTUSA has two full crews and will use both boats for inhouse race training during the Louis Vuitton Cup. Luna Rossa will not build a second  boat and so will have no backup.

AC72 wing sail design

Moving to wing sails takes almost all sailors into such new territory that we need new thought processes and even new vocabularies. All four teams’ wings have two “elements” with “slotted flaps.” The leading element may have a “tab” that provides a smoother shape and controls the size of the slot when the flaps are deflected to introduce camber. The leading element incorporates the structure that serves as the mast. The flaps have multiple “segments” that enable twisting the wing shape to control the power.  

All four teams have developed wings with two "elements" - the forward (main) element incorporates the "mast" structure and the trailing element functions like the flap on an airplane wing. The flap is divided into segments. By varying the angle of deflection of each segment you can "twist off" the upper part of the wing and depower it. You need to be able to depower, since the Protocol calls for racing in as little as 5 knots of breeze and as much as 33 (although the race committee can call off racing if they deem it unsafe). The angle and strength of the wind vary by over 10 knots from the surface of the water to the top of the wing, 40m (130 ft) higher - another reason to change the shape along the span of the wing.

The flaps on OTUSA, ETNZ and Luna Rossa all have four segments; Artemis has six. 

It appears that New Zealand and Luna Rossa (who shared the same design in the first phase of development) can also twist their wing’s leading element.

The chord is the width of the wing from leading edge to trailing edge. All the teams now have wings with 40% to 50% chord flaps, although Artemis  took a different approach with their first wing, with a wider forward element and narrower flaps – about 20% to 25% chord. Their second wing is much closer to the other teams' with wider flaps.

Let's take a closer look at Oracle Team USA's wing. You can see the tab on the trailing edge of the main element and the slot between the main element and the flap.This is the second wing they have built. The first was destroyed following their pitchpole in October 2012. Each team is allowed to build three wings (or, more accurately, six wing sections, since the rules require the wing to be built in two sections along its 40m length).

Foils & foiling

Oracle Team USA looked very unstable when they first tried to foil. They modified their boat after their pitchpole capsize in October 2012 and now are foiling stably.  

Emirates Team New Zealand mastered foiling first. There have been reports that they can gybe while staying up on the foils. Artemis Racing planned not to foil but after a speed test with OTUSA, they sent their boat back to the shed for modifications. Their second boat will fly on its foils. Luna Rossa bought the design for ETNZ's first boat, so they started foiling within days of launching their boat.

Foil shapes on AC72's

Foil shapes are getting a lot of attention.  The Protocol for the 34th America's Cup limits teams to 10 AC72 daggerboards (paragraph 29.7). All four teams launched their first AC72 with the same shape boards on port and starboard. Now they test different shapes in each hull - so they will be able to test more designs.

Ergonomics & crew choreography

Three AC72 designs. Three ergonomic approaches for layout and crew choreography. The 2013 America's Cup in San Francisco may be decided by crew work - the short, tight courses will mean a lot of maneuvers. 

Consistent with their attention to aerodynamics, OTUSA has their grinders face fore and aft, standing in cockpits, to reduce windage. There are four grinding pedestals on each hull. One of the grinding stations is aft of the helm. Advantage: less aero drag. Disadvantage: harder for the crew to get in and out of the cockpits.

Luna Rossa bought their design from ETNZ (yes, this is legal under the rules), so both AC72 designs have the same layout, with four grinding pedestals on each hull, facing side to side in a very open layout with no cockpits. All four pedestals are forward of the helmsman. Advantages: The open layout makes it easy for the crew to move from one hull to the other during maneuvers. Disadvantage: more aero drag from crew standing sideways. Both teams have lowered the grinding pedestals to reduce drag by having the grinders kneeling

Unlike the other three teams, Artemis Racing’s boat 1 did not have four grinding pedestals on each hull - only two. Their boat had a central cockpit with two more pedestals and six winches. They removed the central cockpit in before their capsize. The central cockpit allowed them to save the weight of two pedestals on the boat but it also reduced righting moment when the crew were not on the windward hull. It will be interesting to see the layout on their boat 2.

 

Ongoing development

Boat development will continue right up to the America's Cup Match. You can follow along by subscribing to the Cup Experience Newsletter or by visiting the AC72 design pages at 

http://www.cupexperience.com/sail-racing-magazine 

About the author

Jack Griffin - Cup Experience

Jack Griffin is a self-described America's Cup geek, who takes pride in helping others understand and enjoy the battles of the tycoons, technologists and sailors in this quirky and fascinating event.

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