Helped by a strong, steady breeze and short windy speeches, the East's first large windmill was dedicated here today.

The sleek white-and-orange wind turbine on its 100-foot tower looks as much like a traditional Dutch windmill as evening pumps look like wooden shoes.

When the swithc was thrown, releasing the turbine's brake, the wind was blowing at 25 miles per hours across Block Island. It less than a minute the 125-foot-span blades were spinning at their regulated maximum of 40 rpms. At that rotation, the wing tips travel 178 mph.

In another minute, the blades had stopped - the windmill was only putting on a performace for today's official dedication. Although the $2.3 million turbine is ready to begin producing electricity for the tiny Block Island Power Co., it won't go into operation until September.

The reason for the delay is that the spinning aluminum blades would interfere at times with TV reception, already weak here. The Department of Energy, which paide for the windmill, also is paying about $700,000 for a cable TV system that won't be installed until Septmber.

When it goes into operation, the turbine will produce the maximum of 200 kilowatts, which could save the Block Island utility more than $30,000 worth of fuel each year, according to Henry DuPont, the utility's wind power team manager.

The wind turbine will supply 5 to 15 percent of the island's electricity. In the winter, when Block Island's winds are strongest and its electricity needs lowest, the turbine could produce almost half the power needed by reidents. Jack Corey, president of the Chamber of Commerce, said 459 people were recorded at this year's Groundhog Day census, an annual event that takes place in a Block Island bar.

Summer residents and tourists raise the island's population as high as 17,000 on peak holiday weekends, Corey said.

Block Island natives call their electric rates the highest in the nation. DuPont says they are 20 percent higher than those in New York City, costing the utility's 800 customers an average of 17 cents per kilowatt hour, twive as high as rates on the Rhode Island mainland 12 miles away.

Block Island now uses a series of inefficien diesel generators, and fuel has to be brought by barge. But the turbine won't bring cuts in electric bills, DuPont said. It will save fuel, but rising costs of diesel fuel will keep island rates high, although a little lower than they might otherwise have been.

Block Island's turbine is the third dedicated by DOE and the National Aeronautics and Space Administration. All are considered experiments, as the government and manufacturers work with rapidly developing wind technology.

"You have to remember that four or five years ago if you walked into a utility president's office and started talking windmills he would fall on the floor laughing," said Louis Divone, chief of DOE's wind systems branch.

The federal government got into windmills in 1973, about six months before the Arab oil exmbargo. The year's program cost $200,000. This year the government budgeted $60.7 million for wind programs.

As space-age modern as Block Island's wind turbine is, it will never go into commercial production because better models since have been developed. Divone said one more cycle of improvement to increase efficiency and lower costs is needed before wind turbines will begin to become attractive to utilities.

The Energy Department would be delighted if wind generated 2 to 4 percent of the nation's electricity by the year 2000, Divone said. Although that sounds small, each percentage point represents a capital investment of $20 billion to $30 billion.

About 300 people turned out to watch Rhode Island Gov. J. Joseph Garrahy, Sen. Claiborne Pell (D-R.I.), Block Island Town Council leader Herbert Whitman and DOE and NASA officials push the red lever that started the turbine.

They applauded warmly and one young man shouted: "No nukes."

Residents said there has been almost no local opposition to the windmill, which exploits the winds, averagin 17 mph, that make Block Island pleasantly cool in summer and cruelly cold in winter.

Public acceptance is one of the keys to how fast wind troubles sprout on the American landscape. In order to generate significant amounts of electricity there will have to be wind farms - groupds of turbines - at windy sites and not everyone may be receptive as the people here.

The TV interference problem may vanish if metal blades are replaced by wooden or fiberglass ones on newer models.

Another potential problem, the hazard that DOE jargon calls "airtborne fauna," appears not to be too serious. Federally funded testes have found that birds can see and avoid the blades, and when migrating - Block Island is the Atlantic flyway - fly higher than the turbines.

The turbines also are quieter than diesel generators. During today's test only a soft squeak like a distant mouse could be heard.

The two other federal turbines are in operation at Culebra, Puerto Rico, and Clayton, N.M.

Clayton, population 3,000, offers Block Island a glimpse of potential things to come. After 18 months of living with the turbine there, a Clayton motel has been renamed the Windmill and souvenirs of the turbine are being sold. "When people in Clayton see the blades aren't turning they call the local radio station to complain," an Energy Department official told the crowd.

"First we need a local radio station," a Block Island native said. CAPTION: Picture, Experimental wind turbine rises above homes on Block Island, R.I., ready to help produce the island's power. Department of Energy