Prediction: The United States will retain the America's Cup this year in races beginning Tuesday, but only after staving off the stiffest challenge since 1934, when Harold S. Vanderbilt's Rainbow lost twice to British Endeavour in the best-of-seven cup series.

Twelve-meter yachts never made sharp turns. With their massive weight and long, curved keels, the boats that race for the America's Cup always have been slugs when called on to change direction.

Then came Australia II, with its radical, innovative winged keel that can turn the yacht "like a Caterpillar tractor," as one observer put it. "One second it's going one way; the next, bang, it's off on another."

The crew members of the British Victory '83 watched Australia II spin through a 90-degree right-hand turn in the starting sequence before their final trial race last week and they're still talking about it. The British tried to follow suit but "it was embarrassing," said a crewman. By the end of their slow, sweeping tack they were three lengths behind; the race was over before it began.

To seal the verdict, the Australians quickly joined in a tacking duel with the British as the two boats sped upwind. Each turn through the wind put Australia II farther ahead. After six tacks the British gave up. Australia II rounded the first turn 1 minute 20 seconds in the lead.

How can the Americans, with a conventional 12-meter yacht very similar to Victory '83, counteract the Australian advantage to win for the 25th straight time since cup races began?

"There are ways," said Dennis Conner, current cup champion and skipper of U.S. defender Liberty. "I don't want to go into all of them, but for one thing it seems obvious that once a boat like that (Australia II) is stopped (during the starting sequence) it will be harder to get her going again. That's one thing we'll try to do.

"For another, there is no rule that says you have to join in a tacking duel with the other boat. If you're ahead, you can go off on your own. It's a little riskier, but when you sail off three things can happen: You can gain, stay the same or lose ground. Two of those things are good."

"Patience," said John Kolius, who had the same problem last winter when his straight-line speedster Courageous was racing nimble Defender. "It cuts down on your options, but sometimes having a highly maneuverable boat can get you in more trouble than you can get out of."

That final trial race with Australia II was an eye-opener for the British. Australian skipper John Bertrand "was subdued in using his turning ability earlier," said a Victory '83 crewman, "but the last race he was amazing.

"Dennis Conner has obviously worked out a theory for combating that advantage," added the crewman, "but the first time he sees Australia turn, he will be awed."

Yet around Newport for the last week knowledgeable cup watchers, even some from the Australian camp, have been saying Liberty should be favored.

They believe the Australians have liabilities as well as advantages, and that the veteran Conner is as good at exploiting his opponents' liabilities as anyone in yachting.

Liability 1: Downwind speed. Australia II's performance is suspect on the race legs where the wind is behind the boat, presumably because on that tack the wings slow her. Victory '83 consistently gained time on her white rival on these legs. Liberty is a downwind flyer.

Liability 2: Rough seas. The five-foot-wide wings on the base of Australia II help her point closer to the wind in a calm, but when the ocean roils, as it should in mid-September in New England, the wings may act like a sea anchor, all but stopping the boat as she pitches back and forth.

Liability 3: Experience. This is Bertrand's third trip to Newport, but Conner has sailed here every decent summer day four of the last five years. He is a master at the starting line, where many races are won and lost, and has uncanny skill predicting wind and weather changes on the race course. Few expect that once he gains a lead he will give it up.

Liberty and Conner have no known liabilities. She is a proven contender in light, moderate and heavy breezes, and is strongest in the middle ranges most likely to occur during September racing.

The Australians maintain they designed Australia II for moderate to heavy air, but results from the summer of racing indicate she sails away from her foes in light airs but must battle for an advantage in any breeze above 13 or 14 knots.

It's been a rare season in Newport, where natives normally expect eight days per summer above 90 degrees. This year they had 25. Along with the heat came long spells of gentle southerly breezes of 10 to 12 knots, which may have helped Australia II to her 48-6 won-lost record in the trials.

If September is like July and August, with gentle air and calm seas on race days, the Americans may have to bid goodbye to the cup at last.

But if it blows 15 in the afternoons, as the National Weather Service in Boston says it should, Conner should win.

Final? Four-two in favor of the Americans.

Just like 1934.