Public opinion is beginning to heat up on the Cape and Islands over a proposal for a private alternative energy project that envisions a giant offshore wind farm anchored across some 28 square miles of Nantucket Sound.

The Martha's Vineyard Commission will host a public hearing on the project on Tuesday, Jan. 15, in the lower level of the Old Whaling Church in Edgartown. The hearing begins at 6:30 p.m.

The purpose of the hearing is to gather public comment, although the commission has no formal jurisdiction over the project.

Cape Wind Associates, a privately funded consortium, wants to build a $500 million offshore wind farm on Horseshoe Shoal, an area in Nantucket Sound about five miles off Hyannis and seven miles northeast of Nantucket. The wind farm proposal calls for anchoring 170 electricity-generating turbines across some 28 square miles of the shoal.

Each turbine would be 426 feet tall - taller than the Sagamore Bridge. The turbines would be visible from 20 miles away.

If it is approved, the project would be the first offshore wind farm in the United States. The project is reportedly modeled after wind farms in Germany, Denmark and Spain.

The project is still a long way from reality - and sentiment against the project is already growing on the Cape and Islands, especially among environmental advocates and fishermen.

Proponents of the project say the wind farm would generate 420 megawatts at peak output - roughly two-thirds the output of the Pilgrim nuclear power plant in Plymouth, and enough electricity to power half a million homes.

Because the project is a private enterprise, it is still unclear how much the Cape and Islands would benefit from the project, either in terms of being able to tap into a source of clean, renewable energy, or in terms of saving money.

Opponents of the project have set up a website that includes a computer-simulated photograph of what the wind farm will look like, but the proponents of the project say the illustration is out of scale and misleading. The sponsors of the project do acknowledge that the wind park will be visible from the Cape and Islands.

Because Horseshoe Shoal is under the jurisdiction of the federal government, the project will ultimately need approval from the Army Corps of Engineers. Larry Rosenberg, chief of public affairs for New England for the Army Corps, said yesterday that the public process for the project has not even officially started yet. He said the project proponents have filed an application with the Army Corps, and an environmental impact statement will be required. The review process will take place at both the state and federal level, he said. "What we are trying to do is align all the processes," he said. "But there will be plenty of time for public comment and we welcome all the public participation in this," he added.

The Cape Cod Commission held a public hearing on the project in December.

Cape and Islands Sen. Robert O'Leary and Rep. Eric T. Turkington have both expressed strong skepticism about the project.

The backers of the project include Energy Management Inc., a Boston group that has developed other power projects; Wind Management LLC, a Yarmouth group that has developed wind projects, and Environmental Science Services Inc., a Wellesley consulting firm.

The project proponents say Horseshoe Shoal is an ideal location because of its steady wind, shallow depth and proximity to the Cape Cod power grid. The proponents say that the project will promote energy independence in the region through the use of "green" energy; they also say that it will create jobs on Cape Cod. They say the design for the project is environmentally sensitive and will not harm the fisheries or impede navigation.

Critics are not so sure.

"The proposal to ruin 28 square miles of a beautiful natural resource is crazy. . . . In a good year I can make up to 60 per cent of my income in the waters surrounding Horseshoe Shoals," wrote Raoul Ross, a Barnstable fisherman, in a recent letter to the Cape Cod Times.

Horseshoe Shoal is known as a rich fishing ground for both commercial and recreational fishermen.

"Although many of us have told them that this project would scar our gorgeous natural scenery, impinge on boaters' recreational freedom and eliminate our commercial [dragger] fishermen's yields in these productive fishing grounds, the developers keep pushing forward," wrote Wayne Kurker, president of the Hyannis Marina, in a letter urging the public to get involved in commenting on the project.

"Cape Wind Associates has developed the project to ensure that the wind park coexists successfully with the current watershed uses of Horseshoe Shoal," wrote project manager Chris Sherman in a response to Mr. Kurker's letter.