The surprise in early America's Cup racing among U.S. contenders this summer comes from an old boat and a young crew.

Ten-year-old Courageous, twice cup champion but an utter failure her last time out in 1980, has risen from the ashes with a bone in her teeth.

"The only thing old about Courageous," ebullient skipper John Kolius said today, "is the name."

The sleek white and pea-green yacht was reborn 18 months ago when she was selected as the stalking horse in the Defender/Courageous syndicate's bid to win the right to defend the cup this fall. Last week the stalking horse managed on several occasions to outrun not only her stablemate, but the opposition as well.

She opened preliminary trials to pick a U.S. defender by defeating defending champion Dennis Conner and his new yacht, Liberty, twice straight.

By the close of the 10-day trials with Liberty and Defender, Courageous' record was 6-5, the same as Liberty's and a little better than stablemate Defender's.

What happened? "We've worked very hard to modernize the boat," said Kolius, at 32 the youngest American skipper and a yachtsman more at home in a day-sailor than an 11-man cup contender. "Everything is new: the sails, the winches, the rig, the hull shape, the keel."

Kolius said the old boat was first updated, then modified to improve handling. "We had boat speed but we wanted to improve her turning ability." So they chopped 1,200 pounds of lead out of the top of the keel with a chain saw and refitted it at the bottom. "Now she turns so fast we can get ourselves into as much trouble as we can get out of," said Kolius. "We might have overcooked it."

It's a mistake to put excessive emphasis on results of June trials in the summer-long America's Cup idyll. Early racing is traditionally a testing time. Observation trials in July and final trials in August will show who reacted best. Courageous, Liberty, Defender and even Conner's No. 2 boat, Freedom, will all undergo modifications this week to make them faster.

But it is at the same time impossible to overlook the fact that, in 10 days, Defender and Courageous handed Conner more losses than he accrued over an entire summer of sailing during the last cup, when he lost only four trial races with Freedom. It bodes for a much more competitive summer.

"They've obviously closed the gap," Conner said today from California, where he was minding his drapery business while his boats were being worked on. "All three American boats are basically pretty even now."

Nor did Courageous' speed surprise him. "She's the breeding stock of all the modern 12 meters," he said, "and aluminum is a wonderful substance. You can just keep upgrading." Her poor showing in 1980 wasn't her fault, he said. "She wasn't sailed to her full potential. She's always been a fast boat."

But she had a long way to come. When Kolius took her over last winter in California, he said skipper Tom Blackaller of Defender "embarrassed the hell out of us (in practice racing) all December and most of January. I felt like I was taking the Evelyn Wood speed-sailing course.

"It was like they said, 'Okay, Kolius, here's your boat. You've got three months to learn not to embarrass yourself."

Judging by June's results, he and his crew of mostly young ex-dinghy racers have gone that far, at least.

The crew of Defender, on the other hand, has a little to worry about. After hammering away with Courageous for a year, the new, blue 12-meter still can't put the old-timer away. But Blackaller and tactician Gary Jobson insisted today the mediocre showing in the preliminary trials was a result of last-minute adjustments that cut down speed.

Jobson said the Defender crew discovered two weeks before the trials that the yacht's measurements were out of whack. The only quick adjustment they could make to get her legal was to cut 30 square feet of sail area out of her mainsail.

The effect, he said, "was that the boat stopped and I didn't sleep for three days." In yard work this week the measurements will be put right, Jobson said, and the sail area and speed restored.

That will give the Americans three fast boats in which to whack away at each other, marking a pleasant change from 1980's one-sided Conner romp and a fitting revival for fine old Courageous.