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AC35: It's all about the foils

Friday, June 2, 2017

AS YOU LIKELY KNOW, each team is allowed to design and use two sets of foils, and that most have a set for light air (what the teams call their "high lift" foils) and a set for medium to heavy air ("high speed" foils). There's a "crossover" range in the middle at which both sets of foils work reasonably well in case their weather teams get the forecast a bit wrong.

 

The physics of foils – whether airplane wings, sails, or AC50 daggerfoils – is the same. In a nutshell, use a thicker foil if you want higher lift, e.g., to get the boat up out of the water in lighter wind when the boat is moving slower through the water. Thicker foils, however, also produce more drag and don't let the boat go as fast at it might in breeze with thinner foils. It's a delicate, er, balance between more lift and more drag, or less of both.

 

When there is more wind driving the boat faster, the foils are moving through the water at higher speeds and are more efficient, so they don't need to be as thick to the lift the boat. So the teams have thinner "high speed" foils that do the job lifting the boat in breezier conditions while producing less drag, e.g., a jet plane wing instead of a prop plane wing. Length is also an issue, and for hydrodynamic reasons the teams generally use high-lift foils that are longer than their high -speed foils.

 

So here's the foil game. Figure out the likely wind and sea conditions for the race course in Bermuda in May/June, then design and engineer right the foils for those general conditions, decide at what wind speed to design the crossover between the two sets, and then get the weather forecast right on the day and make the right choice of foils. All are crucial to a team's success. You also have to get the control systems right, and then learn how and when to adjust the foils (both vertically and horizontally) to keep the boat foiling – as much, as fast and as smoothly as possible, in different wind conditions, both in a straight line and through tacks and jibes.

 

And what about two-race days when it might be light in the first race and breezier in the second or vice-versa? The rules prohibit the teams from changing foils during the race day.

 

And one other wrinkle – the teams are not required to have identical foils port and starboard. So it's not that they are limited to two sets, per se, but to no more than four foils. Notably, OTUSA has been experimenting with different foils port and starboard given that the race courses are not 50-50 as to time sailing on port and starboard tacks, the wind speed is not the same all around the track, and in some races you are entering the pre-start box on port tack and in others on starboard. Remember, too, that the crucial first reach to the bear-away mark is always on starboard tack. Hmmm.

 

The better teams have complex computer modeling programs that take into account the course configuration, the expected wind conditions around the track, whether your assigned entry into the pre-start will be on port tack or starboard, what foils your opponent is expected to use and what you think their crossover points are, whether the wind will increase or decrease during the race and over the day (for the second and maybe third race of the day), and more. The computer program then helps the team decide which foils to put on the boat each race-day morning. 

 

Wednesday evening Artemis skipper Nathan Outteridge said they had on their light air (high lift, but also higher drag) foils when it turned out to be breezier than they expected. The draggier foils lost them both races, he said, and it has put them in the basement of the Qualifier scoreboard – unexpectedly so for most pundits – along with FRA and JPN. 

 

Here's a brief video that OTUSA put out Wednesday, proclaiming to the world (and other teams) that they were just now starting to use their light air (high lift) foils as the the lighter-wind days of June roll around. Tom Slingsby allowed as how he thought OTUSA's foils perhaps have a lower crossover than the other teams, implying that they have a good set of "AP" (all purpose) foils with a wider range usable on more of the race days. Again, hmmm. Now why would they go public with any of that, and why now? To try to psych out the other teams – "We're at the top of the leaderboard and we still haven't used our other foils? Or was it a ruse to say that they were introducing new foils now when, in fact (I don't know) they had already been using them?  After 37 years in this game, one learns not to take anything in the AC at face value, especially when it involves a top team.

 

Regardless, and unlike previous Cups, you hear precious little talk about hull shapes, sail shapes, sail selection (although there are different-size jibs for different conditions), tactics, sail handling and crew work outside of the helm and his trimmer. Because it's all about the foils.

 

Copyright © 2017 T F Ehman Jr – All rights reserved.

 

 

 

 

 

 

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