"The speeds we could have… it'll be a beast of a boat. It's out there," Spithill said.
"This one will be another level."
"Will it work? That's the question. The America's Cup has always been at the leading edge of boats. Look at what's happened over the last decade. The AC72 kind of reminds me of this boat in that it is very, very powerful and never been done before."
"It's extreme, it's expensive, from first take, and what we are seeing, it's unstable, and it's going to be very, very physical."
The first of the new America's Cup boats are only due to be launched from March 31 next year but a video which emerged on social media last month has already given fans a sneak peek to the radical design on water.
The video posted to Facebook by London Corinthian Sailing Club appeared to show INEOS TEAM UK's first prototype.
The smaller QUANT 28 seen in the video looked stable in flight mode although it was not revealed whether the foiling is under manual or full computer control.
Sail-World reported that the prototype was sailing downwind at speeds of 30
kts in 12kts of wind, and upwind at 20kts while performing foiling tacks and gybes with relative ease.
