When the tide rushes into New York Harbor, a strange thing happens at the Gristedes supermarket on Roosevelt Island.

The freezers, cash registers - even the red neon "Bagels" sign - all hum with electricity from the rip coursing up the East River.

The power comes from six underwater turbines bolted to the river bottom by a little-known start-up called Verdant Power L.L.C. The turbines' seven-foot-long blades turn, silently and out of sight, until the flux crests. When the tide turns, the contraptions pivot 180 degrees, and make juice on the ebb.

Right now, Verdant's generators power the grocery store and charge a few hybrid buses on Roosevelt Island, which lies between Manhattan Island and the borough of Queens. The company is still testing its turbines, and doesn't collect a dime.

Tiny Verdant has a rich and famous partner, billionaire hedge-fund manager Paul Tudor Jones. His Tudor Investment Corp. has staked $15 million on Verdant, a company with 20 employees, zero profit - and a very aggressive competitor lurking just upstream.

From New York Harbor to San Francisco Bay, a new breed of green-energy entrepreneurs is warring over the tides, the next big thing in alternative power.

As oil and natural-gas prices climb, upstarts such as Verdant and rival Oceana Energy Co. are vying for investors' money and staking claims to America's coastal waterways.

Since 2005, the U.S. Federal Energy Regulatory Commission has granted Oceana, Verdant, and six other companies the exclusive right to study the tides in 22 locations, from the East River in New York to the Icy Passage in Alaska.

If the companies can figure out how to make projects pay there, they will get first dibs on federal licenses. The price of one of these potential golden tickets: $0.

The froth is flying. Washington-based Oceana has angered Verdant Power executives by seeking a site just north of Verdant's East River turbine farm. Oceana, chaired by William Nitze, son of the NATO architect Paul Nitze, has also irked the city of San Francisco by laying claim to the currents that swirl beneath the Golden Gate Bridge.

The company has applied for 13 study permits, and is trying to use its pull in Washington to steer taxpayer money its way. "They're claim-jumpers," Verdant Power cofounder William "Trey" Taylor said of his competitor.

Oceana cofounder John Topping says his company has played by the rules. He says the best tidal sites were there for the taking.

"Somebody is going to do it, and we want to be the somebody," said Topping, president of the Washington-based Climate Institute, which promotes awareness of global warming.

Tide-power proponents are trying to catch the wave of money that has washed over energy start-ups as the price of oil has tripled during the last five years. Costly OPEC crude oil and concern about global warming have ushered in a rich era for green power.

Venture capitalists have invested millions of dollars in solar-energy and geothermal start-ups. Money managers such as Tudor Jones are clamoring after companies that cook up gasoline substitutes from corn, sugar and soybeans. Alternative-energy stocks have surged the way dot-com shares did in 1999. The 43-stock WilderHill Clean Energy Index has rocketed since its August 2004 debut, rising 66 percent as of Monday.

So far, the reach of tide-power enthusiasts far exceeds their grasp. Taylor foresees a time when hundreds of Verdant turbines in the East River will power 8,000 homes in New York.

Because water is about 800 times denser than air, a tide mill can generate more energy than a much larger windmill. Unlike breezes, tides are predictable.

Tudor's cash is key. "It gets us through the valley of death," Taylor, 59, said. Verdant, founded in 2000, recently spent $2 million on fish sensors to prove that its turbines don't turn local striped bass into sashimi.