Russell Coutts will skipper the tried and proven SUI64 for Swiss syndicate Alinghi in their semifinal clash with Chris Dickson's Oracle BMW Racing.
Alinghi have used that boat in every race so far, but there were hints they would introduce their new boat, SUI75, for the semifinal encounterwith Oracle.
The winner advances straight to the Louis Vuitton final, starting on January 11.
"SUI75 has been in the workshed for over a week," Coutts said. "We are doing some modifications to that boat. Those modifications I am quite excited about. They won't be ready for the semifinals, which means we will be using SUI64.
"We have been sailing the old boat SUI59 with SUI64."
Coutts, obviously, would not talk in detail about the changes made to SUI75, but said he was impressed with the work of their designers.
The Alinghi design team are co-ordinated by Grant Simmer (who was involved with 1983 America's Cup winner Australia II) and include naval architects Rolf Vrolijk and Manuel Ruiz de Elvira, who were responsible for the Spanish boat Bravo Espana in the 1999 regatta.
"Some of these changes were thought of in the last cup ... some of the ideas have been progressed through from there," Couuts said. He said the modifications would be "pretty evident," which indicates Alinghi may have remodelled their bow profile, opting for one of Vrolijk's famous banana-shaped bows.
"The game is changing at a very rapid pace this time," Coutts said.
"I would say the level of improvement in the boats is far beyond what we have seen in previous cups."
Alinghi and Oracle have had more than three weeks to prepare for the semifinals after winning their quarter-final matches against Prada and OneWorld.
At the end of the round-robin competition, Coutts said his team were firing at 80 per cent of their abilities.
And now? "That question will be answered very soon."
Oracle will wait until Sunday to announce whether they will stick with USA76 or switch to USA71.
The best-of-seven semifinals start on Monday.
OneWorld face Prada in the other semifinal.
The loser of that match will be eliminated and the winner will race the loser from the Oracle and Alinghi match to decide the other finalist.
