Defending SailGP champions Team Australia have capsized their foiling 50-foot catamaran on San Francisco Bay, doing major damage to its high-tech wing sail three days before the global league's $US1 million, winner-take-all season title race.

The capsize by skipper Tom Slingsby's crew came three days after another title contender, fellow Aussie skipper Jimmy Spithill's Team USA, capsized while training off Alcatraz Island.

Australia, the season points leader, and the United States have already qualified for Sunday's $US1 million championship race, the finale of the United States Grand Prix. The third finalist will be determined during fleet racing on Saturday and Sunday.

The Australian boat - nicknamed The Flying Roo for the yellow kangaroo on the wing sail - pitch-poled, with the right hull digging into the water and carrying the catamaran over onto its side.

The port hull was suspended in the air, with some of the crew in the cockpit. The team said there were no injuries.

Slingsby said the capsize came at the end of a training session, when a gust hit the boat while it was doing a tack bear-away manoeuvre. He wasn't sure what went wrong, but called it an unforced error.

"We've done 20 of those manoeuvres in the last few days with no issues. It was all going well until it wasn't," Slingsby said. "We're a little unsure but obviously upset that we've done so much damage to the boat."

Slingsby said several of the wing sail's frames were broken and the flaps were "quite damaged."

The wing sails look and perform like an airplane wing, helping power the F50 cats to close to freeway speeds as they skim above the waves on hydrofoils.

"It's disappointing. It's not the preparation we wanted and we don't want to create work for anyone," Slingsby said.

"Unfortunately there's going to be a lot of people having late nights working through the night to try to get us ready.

"It's not what we want but we know there's amazing people who are going to help us get back on the water."

Slingsby thinks the Aussies will be OK to race in Saturday's three fleet races. However, he said there could be water damage to various systems that won't be apparent until the boat is racing, so he hopes to be able to practice before Saturday's racing.