Round one of racing for the right to compete in the 25th America's Cup this fall is over and the only man left with a secret is Australian Ben Lexcen.

Lexcen is a strange, bespectacled bird who squints, fidgets and sprinkles his conversations with bewildering flights of mumbling fancy. But he's designed two yachts for the 1983 cup that no one takes lightly.

Lexcen's Australia II and Challenge 12 dominated racing in the challengers' first series of trials last week. Australia wound up 11-1 and Challenge was 10-2. They lost one race apiece to each other. Only once, when Challenge 12 lost to the British Victory '83 by 43 seconds, did either lose to anyone else.

This is not unexpected. The Australians have a history of success in the challenge trials, having provided all the challengers since the Britons in 1964. What is discomforting to the Americans is that in Australia II, Lexcen has come up with a fairly radical innovation in 12-meter design. It is a mystery, it has proven to be fast and no one can judge yet how fast.

The innovation is a bulbous, winged protrusion affixed to the very bottom of the white yacht's keel. Lexcen keeps the bottom shrouded in plywood and canvas when Australia II is out of the water but spies have gained at least a glimpse.

Lexcen and others are convinced that the only way to wrest the cup from the Americans, who have never lost it in 132 years of racing, is by a radical design change. Could this be the year of a 12-meter design breakthrough, and could Lexcen's be it?

"It certainly looks like it works," said Johan Valentijn, designer of 1980 Cup champion, Dennis Conner's entry, Liberty, which fared only moderately in the American preliminary trials, finishing 6-5 against Courageous and Defender. Australia II is "a fast boat," Valentijn conceded. "Our own boats are very fast but this is very radical and it might work. I wouldn't put my money on anyone right now."

As Valentijn explains it, the winged protrusion gives Australia II extra weight where it needs it--as low as possible. With the leaden bulb way down below the waterline, he said, the Aussies were able to reduce the topside weight of the boat but not lose stability.

Valentijn said he tried to do something similar in designing Magic, one of Conner's three discards, but without the extra weight in the keel she was unstable and slow.

Two things concern the Americans, according to veteran cup watchers.

First, they can't tell exactly how fast either Australia II or the more conventional Challenge 12 is, because almost all their racing has been against other new foreign yachts. "We have nothing to compare them against," said Tom Blackaller, who skippers Defender.

Second, if the innovation proves over time to be a dramatic improvement, it will be too late for anyone to duplicate it. You can't just tack a bulb onto any old 12-meter and have it work, said Valentijn. "The boat has to be designed for it." Building a new boat takes four months. The races start in three.

Through it all, Lexcen is playing the gadfly. "If we win this thing, we'll take the bloody boat away without showing anyone the keel," he said today as he dashed excitedly around Australia II's workshed.

"Wouldn't that be great? We won't even get drunk after we win. We'll tell them we have to get back to Australia the next day. Go right home and go to bed, have them ship the cup over air freight."

Of the bulb itself, Lexcen said, "It's pretty wild when you see it. And you won't. It's the new generation. She's a mutant. Three-Mile Island got her. It glows in the dark. It's what every sailboat in the world will look like after this year."

He's amused by the stir he's causing. "We've even got Freedom (Conner's 1980 champion boat) over here getting her bottom chopped off," said Lexcen, pointing across the yard at Newport Offshore where Freedom was indeed shrouded in canvas and grinding, cutting noises emanated from her hull.

Valentijn insists Freedom's hull changes do not include a bulbous keel. He also dismissed reports that the British, third in the preliminary trials last week with an 8-4 record, are considering a bulbous keel for Victory '83.

The intrigue makes for a merry scene early in the cup game. The Australians have an armed guard at the head of their dock; the British have a combination lock at their gate; it's hard to find a crewman who will say on the record what he had for dinner and the serious racing hasn't even begun yet.

The seven foreign boats will resume their round-robin racing to select a challenger on Saturday. The American boats won't resume until July 16, when observation trials get under way.