Earlier tonight I a conversation with Giovanni Belgrano, the principle structural engineer for ETNZ.
I asked him about the "Holy Grail of the AC72" - foiling upwind.
Belgrano said that for slight periods ETNZ does foil upwind. But it is not consistent. "Sometimes it feels good. Very stable. And sometimes it's is very unstable."
Then he said something very surprising.
He said that foiling upwind is not important and not necessarily desirable, the reason being the narrowness of the course.
Belgrano reasoned that while foiling upwind you can't point as high and that boat will slide severely sideways.
That any gain you have in speed while foiling upwind is negated by causing you to possibly go out of bounds. And so as to not go out of bounds, you'll have to tack more often which will slow you down.
Belgrano continued, "If there was no out of bounds, we would certainly be foiling upwind."
As an aside, Belgrano also mentioned that ETNZ will soon break 3.5 times true wind speed.