Far From Easy Breezy
At the 34th America's Cup, the dynamic conditions of the San Francisco Bay--combined with the breakneck speed of the AC72s--leave only enough time for well-informed instinct.
On Their Own
ACEA/Photo Gilles Martin-Raget
"Once racing starts, the weather boat has to be off the course and we can't communicate with the race crew at all," said Luna Rossa Computational Fluid Dynamics Aero team leader Michael Richelson.
The massive uplift in performance from the new breed of flying AC72 catamarans compared to the America's Cup monohulls of old, combined with the introduction of short confined inshore racecourses in one of the trickiest venues on the planet, has presented a new set of challenges to the 34th America's Cup race crews and their technical support teams.
Amongst the thorniest of their issues is how to collect and process wind data in a form that's meaningful for the sailors as they hurtle through the mind-bogglingly complex winds and currents of San Francisco Bay at peak speeds of 40 knots and above.
To make things harder (although supposedly cheaper) the four teams are severely limited in where they can gather the weather information they need. According to Michael Richelsen, who heads up the Computational Fluid Dynamics Aero team at Italian challenger Luna Rossa, when it comes to weather data this edition of the Cup is actually less high-tech than previously.
"In the old days we had our own weather stations all over the course and Sail Vision technology and all sorts of other good stuff," he says. "Now, we send out a weather boat just before the race, but that is just to help with the call for the jib. Once racing starts the weather boat has to be off the course and we can't communicate with the race crew at all."
Oracle Team USA meteorologist Juan Vila says that limiting the syndicates to only using publicly available data levels the playing field to some extent but also puts a premium on how that data is interpreted and utilized. "There is quite a wide range of weather measurement devices around San Francisco Bay," he says. "Plus we get access to the wind data from the race management boats and information from the meteorological stations which have been put in place especially for this Cup."
On the morning of a typical Oracle Team USA training day, Vila goes through the weather forecast prepared by his colleague Chris Bedford with the sailing team before heading out on the water to monitor how accurate that forecast turns out to be. "San Francisco is a very complex place weather-wise," Vila explains. "There is a very shallow boundary layer -- just a few hundred metres deep -- and that means that local topography can have a very big effect on the wind patterns. Typically what we have found is that the wind models tend not to be as accurate as you might like and small features can have a big impact on the wind speed and direction."
Vila told me that OTUSA has been building their own weather models to try and predict what to expect when they defend the America's Cup this September. "For us it is very important to develop our local knowledge based on a wide range of different scenarios," he says. "You still use the general model as a guide, but on top of that you have to overlay your experience and the things that you have found out about the local conditions. So, we look at historical data and try to reconstruct those historic scenarios. Then you look at what you have learned about the local topography and how that affects what is modeled."
Over and above the considerable complexities of modeling the vagaries of San Francisco Bay weather, the teams have had to try to find a way of measuring and displaying accurate live wind data to the AC72 crews while they are racing. "When the boat is racing, the crew want to know the wind direction and speed," says Luna Rossa's Richelsen. "Primarily, what they use that for is in relation to target boat speeds which we have set beforehand based on VPP modeling. We base those programs on winds at a set height up the wing and because we are now using wind instruments on the bow we have to calibrate to allow for that height difference."
Luna Rossa, Emirates Team New Zealand, and most recently Artemis Racing have their wind instruments positioned on their bowsprits. Meanwhile, OTUSA are currently testing two measurement points -- one on the sprit and an second at the top of the wing. "The reason we have done that was just for testing purposes because we thought the wind measurements would be more accurate if we used more than one reference point," Vila explains. "Using more than one sensor has allowed us to correlate the wind as an average from the top of the wing to the bottom. Ultimately, for racing we might just change to one like everyone else."
All of the teams, it seems, have experimented with sophisticated wind measurement solutions before returning to an old-school vane and spinning cups setup. None were willing to disclose their particular manufacturers but Luna Rossa confirmed that they had customized a standard off-the-shelf hardware package. "We have put quite lot of of effort into making the wind measuring devices as accurate as possible, particularly because of the high performance of the boats," says Richelsen. "A small difference in angle can have a big effect in predicting true wind. The hardware components are standard, but we built our own mounting systems and developed our own wind sensor package - that's about all I can tell you."
However, despite all the effort and technology that has been deployed in the quest for accurate wind data for the sailors, it seems that the crews often rely on their own instincts and sense of feel as much as what it says on the on-board displays when they're in the heat of racing. Vila readily acknowledges this method. "The problem with using information on displays on the boats is that they are travelling so fast around the racecourse and the wind can change in a heartbeat," he says. "That means the conditions you are measuring at one instant can be very different the next second. So the sailors' feeling for the boat and their instincts are very important in the whole process."
Luna Rossa skipper Chris Draper said that he and his wing trimmer, fellow 49er Olympian Xabi Fernandez, often sailed the Italian AC72 like it was a "big 49er."
"With these AC72 beasts, the apparent wind is so high that it is definitely hard to get good wind data," Draper says. "The guys have been doing a great job of making that data as valuable as possible but they have got their work cut out for sure. We pretty much use speed over ground and heading and sack off (ignore) all the rest of the instruments.
"The true wind angle is there as a reference but the conversation between Xabi and I is mainly about the actual angles and how wide we are. The rest of it is just sailing it off the tell tales. We have found that our instincts and our visual stimulus are generally the reliable ones."