Like many others who had been closely involved with a team that competed in the 32nd America’s Cup in Valencia in 2007, I returned to the Spanish city to watch the 33rd Cup match, this time as an outside observer. At SAIL’s request I had run some Velocity Prediction Programs (VPPs) on both boats and the results of that analysis are in the current issue of SAIL.
Both boats' rigs had changed since I had analyzed the VPP data and concluded that USA would win. USA, of course, had gone from its soft rig to a very tall wing sail, and Alinghi 5 had also made some minor modifications to its sailplan. Here, in no particular order, some more observations about the boats and their crews.
The reason USA was sporting very long amas was that the 90-foot waterline length that BMW Oracle had stipulated in the challenge document was measured on the hull when ama was just touching the water surface. But when the boat was sailing the hull would come out of the water. In contrast A5’s scow-like hulls appeared to me to produce a shorter waterline length under sail.
USA’s rig was powerful in all conditions while A5 looked to be under-powered most of the time. A5's crew also seemed to make a number of poor headsail choices, particularly when they used a small jib on the first reaching leg of the second race.
It was critical to maintaining precise control of the wing on USA and the skill of the crew kept the trimaran sailing steadily and always flying a hull and the tall wing provided plenty of power in winds that varied between 5 and 10 knots. USA also carried a jib upwind except for a brief period in the first race where, I believe, they deliberately furled the jib in order to point higher.
Although this probably wasn’t necessary, the wing’s upper sections were designed and built so they could be adjusted to provide different twist and camber distributions for higher wind speeds. I have been told that that the upper sections of the wing were built so they could have a reverse camber, a feature would effectively lower the sail’s center of effort in higher wind speeds. The long span of the wing, when combined with the seal at the bottom of the sail (see SAIL, February) added even more power to the sailplan.
It also seemed that USA could always outpoint A5, and this suggests to me that the USA design team produced a boat with extremely low drag. I don't know if the USA crew also ejected additives along the hull to reduce drag further. In any case when low drag was combined with the sophisticated aero efficiencies of the wing sail, the outcome was inevitable.
Because the two races were sailed in relatively light winds, we will never know what the potential of A5 would have been in stronger winds, but I doubt that its relative performance would have improved. That said, at some point (although it would have to be a pretty fresh breeze), the wing’s higher center of effort would put a cap on the advantage it enjoyed at lower windspeeds.
A5's crew also appeared uncertain about how to trim their boat. In the first race it seemed they had major problems sheeting in the genoa, and also the smaller jib. This apparent problem may have been why they set a low-hoist jib in the second race.