In Newport, where a dozen boats will battle for the America's Cup this summer, the era of the throwaway yacht is emerging.

Magic, a brand new 12-meter built especially for this year's 25th Cup defense, sits on blocks on a side street on the way to Goat Island, a noncompetitive outcast. Another new boat called Spirit is nowhere around, and the British are preparing to discard one of their two new vessels, either Victory or Victory '83.

These boats are being dispatched before they even compete because of a new philosophy of yacht-building that's taking hold. The theory is that it's financially more sensible to design and build a 12-meter with a minimum of research than to tank-test extensively, come up with a design that should work and then have it fail at colossal expense.

"People are afraid of tank-testing results," says Sean Fawcett, who helped Dave Pedrick design Defender, one of the new U.S. contenders. "When you're dealing with a model one-eighth to one-sixteenth the size of the real thing, the results don't necessarily scale-up."

In this age of aluminum racing machines, boat-builders can bang out a 12-meter hull in 60 days at a cost of about $300,000. Throw in the price of basic design work, minimal tank-testing, winches, sails, spars and rigging, and a boat runs about $500,000, complete and ready to test in real racing.

By contrast, says Andy McGowan of Newport Offshore, where three new U.S. 12- meters were built for this year's Cup, it's easy to spend $400,000 just to tank-test a hull design in scale models.

For the $500,000, at least you get a boat, says McGowan. Even if it turns out to be too slow to compete, it's a great-looking yacht with 25 years of life left in it. The hull can be sold to some moneyed day-sailor and the rig scavenged for another effort. But for the $400,000 in testing, all you get is a drawerful of papers that say a certain hull should be fast, which may or may not be true.

Example: In 1974, tank tests led designer Britton Chance to a hull design he believed was the 12-meter shape of the future. Its stern was stepped behind the rudder like an upside-down staircase. The boat was called Mariner.

People started worrying about Mariner before she got her mast in place. The story goes that as the yacht was towed across Long Island Sound to get her rig, skipper- to-be Ted Turner and Chance lunched in the cockpit.

Turner reportedly flipped a banana peel over the stern, then looked back in horror 10 minutes later to find the peel still there, rolling and flipping in Mariner's wretched wake as the design of the future tried to drag Long Island Sound to Connecticut.

Meantime, Olin Stephens was designing Courageous and redesigning Intrepid, two of the great 12-meters of modern times, using results of his tests in the same tanks in Hoboken, New Jersey, that Chance had used.

The point, says McGowan, who worked for Stephens then, is that tank-testing is only as good as the people interpreting the results. "It is not an end-all," he says. "It does not eliminate the risk. You can spend $400,000 and still get a slow boat."

A chief spokesman for the new churn- 'em-out-and-see-if-they-work school of 12- meter design and construction is America's Cup champion Dennis Conner, who skippered Freedom to victory over Australia in the last Cup defense in 1980. His syndicate has built three 12-meters since, trying to improve on Freedom.

It was Conner who discarded Magic and Spirit. Last year, when it became apparent that neither of the new boats would do the job, Conner set about having a third yacht built.

He explains his seeming prodigality:

Simple, says Conner, "It's cheaper to build them and test them than to test them in the tank."

Not that economics has ever been the cutting edge in the 12-meter game, anyway. The non-profit amateur organizations of Conner and the other U.S. contender, the Defender/Courageous syndicate, are playing with tax-free money gifts from wealthy people and businesses. Current estimates are that the two U.S. syndicates will have burned up $10 million by the time the Cup races are over in mid-September.

Meantime, the seven challengers from five countries are expected to spend $30 million in their quest for the Cup that the U.S. has never lost.

With $40 million to play with, even the best kids are bound to throw a few toys away.