America's Cup: AC75 simulation session an eye-opener for Luna Rossa's Jimmy Spithill

The AC75 concept will be a high speed vessel. Image / Supplied

Former America's Cup champion Jimmy Spithill knows his Luna Rossa squad have a lot of work to do to tame the beast that is the AC75.

Since the unveiling of the new 75-foot foiling monohull class rule, sailors have thought the AC75 would fly on the water, but be tough to sail.

And after watching a simulation at Luna Rossa's base, Spithill is now different.

Luna Rossa skipper Max Sirena told La Stampa Spithill was amazed by what the simulation showed, declaring to the team: "we'll s*** ourselves."

Spithill suggested the boats would be beasts when the class rule was first revealed, and Sirena warned the AC75 lacked the stability of the catamarans.

"This is a monohull without a keel, with appendages that weigh little. A hyper-powerful boat, very light, with little stability. There are fractions of a second that there is zero stability, if the helmsman and the trimmer cannot anticipate this zero moment, you fall back … go from 30-40 knots to zero, you have a g-force that shoots you in the air," he told La Stampa.

While Luna Rossa and Team New Zealand have been working with simulations on how the boats might sail, a number of teams have been out on the water on scaled down models.

Because the teams can't launch their first race boats until the end of this month, INEOS Team UK and American Magic have been out on the water trying to get their heads around how the full-scale vessel might sail.

American Magic skipper Terry Hutchinson told Sailing Scuttlebutt that sailing their test boat, named the Mule, had not come without issues.

"I can say the speed sensation on the Mule is impressive, and I suspect when we step up in size to the 75 it will have the same type of feel. You do get used to it while fully respecting how quickly a situation can get messy," Hutchinson said.

"We've made our share of mistakes like capsizing while getting off the tow the boat. What's impressive about the boat is you're way safer at 27 knots than you are at seven knots. The boat generates so much stability that you feel much better in full speed mode than you do when you're in displacement mode."