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A Look at the new AC90 Rule
by Britton Ward & Bruce Farr
(November 1, 2007) For the past few weeks we have been involved with the other members of the Desafio Espanol technical team in the review of the AC90 Class Rule and its many preliminary drafts. The AC90 Rule issued yesterday is the product of a series of meetings between the Defender and Challengers and embodies the experience and input of a large number of people. In the next few weeks the rule will be given close scrutiny by many more people and that spotlight will highlight areas of the rule that need further refinement and clarification. These additional refinements will be addressed through minor rule amendments by December 31st. These amendments will not change the fundamental concept of the AC90 Class which we feel reflects the need for a new, exciting, technologically advanced and exceptionally high performance yacht to contest the America’s Cup.
Version 5 class and the AC90 Rule
(image courtesy ACM)
Despite its quick gestation, the new AC90 Rule does a nice job of balancing the demands of design freedom and closely matched performance for good match racing. The complex trade-offs between length, sail area, stability and rated length of the past are gone in favor of a set of fixed limits – a 27.4m (90ft) overall length limit, an essentially fixed displacement, set upwind sail area of 475 m2 (5100 ft2) and rig dimensions [mast height of 37.4m (123 ft) above the sheer] and a maximum beam of 5.3m (17.4 ft) and draft of 6.5m (21.3 ft). At 23,000 kg (50,700 lbs) these boats are very light for their length, more in-line with a VO70 or an Open 60 than the former V5.0 AC Class. Combining the light displacement with a substantial sail area increase, particularly downwind where spinnaker area is unrestricted, and the result is a much more powered-up boat both upwind and down. These boats will be challenging to sail to their maximum potential, but promise some spectacular performance that will be very exciting to watch.
Determining the right amount of hull form stability will be one of the first major challenges all designers will face. Given the light displacement and the large sail plan you can expect them to be substantially more powerful hull shapes than the V5.0 ACC boats. These boats will be designed to maximize their sailing length so expect plumb or dreadnaught stems and potentially some level of transom immersion to be the norm. There are also only limited restrictions on the hull shape with limits only on transverse hollows leaving designers substantial latitude in shaping the hull. We expect there will also be plenty of variation in the appendage choices – single or dual aft rudders are allowed and will be tightly related to the style of hull form.
The boats will retain the big headed mainsails that have become common place but with a new non-overlapping headsail rig. Spinnaker poles are out, replaced with a fixed bow sprit that will be in for plenty of punishment in pre-starts and tight maneuvers.
There are also plenty of engineering challenges in this new rule. The drive to deepest possible VCG and lightest structural weight is present and may be even stronger than before adding to it the challenges of engineering such as a deep span fin that must lift from 6.5 to 4.7 meters in 5 minutes.
The new AC90 class embodies many of the latest advancements in yacht racing technology, and promises to produce a new fleet of boats with incredible performance potential. As designers we are very excited by the new rule and the challenge of developing the fastest AC90 solution for the next America’s Cup.
Britton Ward & Bruce Farr
Desafio Espanol America’s Cup Challenge / Farr Yacht Design
Farr Yacht Design, Ltd.
613 Third St, P.O. Box 4964
Annapolis, MD. 21403-0964, U.S.A.