On a day when Dennis Conner and Freedom were officially chosen to defend the America's Cup, as expected, a daring secret plan by the Australian challengers was uncovered.

In a huge, decrepit fishing shed on the back wharves of Newport, the Aussies have worked in total secrecy the last month to construct a mast that is designed to greatly increase boat speed in a bid to take away the Cup.

American boats have not lost the Cup since they captured it in 1851.

The mast is a standard aluminum spar, except that the topmost 20-foot section is fashioned from a fiberglass substance that bends about double what aluminum will stand.

The effect of the bend is to increase sail area by about 200 square feet, providing a critical advantage over conventional rigging, particularly in light winds.

The breakthrough in design was initially hatched by British challengers on their boat Lionheart early this summer. They sprung their secret on a stunned yachting community two months ago. In one race in light airs, they domolished the Australian boat, winning by more than nine minutes.

But the British had trouble with their rig. Though knowledgeable yachtsman recognized that the new mast advanced 12-meter design significantly, the British waited so long to spring the surprise that they lacked time to appropriately tune their sails to the whippy spar.

They were ousted from the Cup Challenge early this week when they lost their best-of-seven semifinal series to France III.

Evidently, the thrashing by the British gave the Australians an idea they felt they had to pursue.

A month ago, they rented a tumbledown shed on Waites Wharf, away from the mainstream of downtown Newport, and moved a spare mast into it under cover of darkness, according to a source with intimate knowledge of the scene.

For a month, they worked in secrecy. Slowly, bits and snippets of information leaked, but no one had seen the secret spar until today, when two reporters on a search mission found it by peering through a dusty window.

Later, they were invited inside to see the mast by a source who asked to remain anonymous.

It is clearly a copy of the British design, although the Australians have painted the top section with aluminum-silver paint so that it looks like a one-piece unit.

The mast lies on sawhorses in the dim shed, which is full of fishing nets. An indication of the rush nature of the job is a crude model of a boat made of a slab of pine with the tip of a cheap fishing rod standing amidships, representing the mast.

The fishing rod is rigged to the plant in boat fashion, and evidently by pulling on a string representing the backstay the fabricators could imitate the mast bend they were seeking.

A source said the Australians flew a mast-builder in from their native land across the globe to fashion the fiberglass piece.

The Australians evidently hoped to keep the mast a secret until they had reached the final Cup series with the American defender, then spring it on their opponents. Australian syndicate director Warren Jones was incensed when approached on the subject.

"You didn't see anything," he said. "You don't know what you're talking about. If you want to help us, do this: Don't write your story."

Confirmation on the nature of the effort came from British syndicate manager Tony Boyden, who said the Australians have asked for technical assistance on the mast program from the British. So far, there has been no cooperation. Boyden said, but a meeting is scheduled for Saturday to discuss the proposal.

The bendy mast would make Australia "unbeatable in certain circumstances," Boyden said. "It's a bit risky at this stage," he added, "but it could tip the balance, especially if they have our help."

The British unveiled their secret two weeks after they arrived here in June, having kept it carefully under wraps. When they stepped the spar, a great hue and cry developed, with the New York Yacht Club, which runs the America's Cup races, protesting that the mast was illegal because it allowed more sail area.

An international jury dismissed the protest, ruling in favor of the British.

Boyden indicated that the Australians apparently intended to make their way to the final Cup series against an American defender without letting on about their secret weapon. They still must face France III in a best-of-seven final series to select the challenger. Those raced begin Saturday, and Australia evidently hoped to win four races with the conventional spar.

The Australians apparently hoped to disarm Dennis Conner by unveiling their bendy mast when they meet for the grandest prize in all yachting, in a series scheduled to start Sept. 16.