A forum bringing together those for and against the controversial Cape Wind electricity project drew more than 120 people to the Katharine Cornell Theatre on Thursday night and generated far more light than heat.

The forum, organized under the auspices of the Vineyard Haven library lecture and workshop series, was intended to establish a factual basis for further discussion of the project rather than encourage debate, and by that measure can be counted a signal success.

Baseline information was presented, myths were debunked and audience involvement was more about questions than opinions.

Moderator Judy Crawford began with a recitation of the agreed facts.

The proposed power generation project, America’s first offshore wind farm would, if approved, see 130 turbines, 1,800 to 2,700 feet apart, installed over 25 square miles on Horseshoe Shoal, in federal waters in the center of Nantucket Sound.

The field would be about nine miles from Edgartown and 9.3 miles from Oak Bluffs.

There would be one electric service platform 200 by 100 feet and 100 feet tall, connected by cables buried at least six feet into the seabed to all the turbines and then to shore at Yarmouth.

At full capacity the project would produce 420 megawatts at full power, with average production of 182 megawatts, or about 75 per cent of total 230 megawatt average demand on the Cape and Islands.

Ms. Crawford also outlined the complex regulatory picture. Cape Wind is being reviewed by 17 state, federal and local government agencies.

The former lead permitting agency, the Army Corps of Engineers, produced a 3,800 page environmental impact statement before responsibility passed to the Minerals Management Service in 2005. The MMS will produce a new draft EIS in November.

In 2005 the Massachusetts Energy Facility Siting Board approved the plan and in March this year the Massachusetts Energy and Environmental Security agency determined the EIS was adequate.

The Cape Cod Commission is reviewing a portion of the project and is expected to make a determination next month. The final MMS determination will occur in 2008.

Then the floor was turned over to speakers for and against the project, Mark Rogers from Cape Wind and Audra Parker from the Alliance to protect Nantucket Sound, the major opposition group. Each spoke for 15 minutes before taking questions from the audience.

In both their presentations and their responses to questioners, they traded point and counterpoint on various issues of concern, including the cost of the power, the effects on navigation, endorsement or criticisms by various interested parties, the economic and employment effects, impacts on the ecology and fishing and safety of the project.

Ms. Parker said the project will endanger birds on a major migratory route. Mr. Rogers said studies elsewhere in America indicated the bird kill rate was about one in 10,000 birds and an eight-year study in Denmark showed low instances of bird fatalities. Mass Audubon, he said, was “moving in the direction” of supporting the project.

Ms. Parker said the pylons presented a hazard to shipping. Mr. Rogers said in Europe there had never been a collision.

She said a study done by the proponents had shown that in the event of a rupture, 40,000 gallons of transformer oil stored in the service platform could reach the shoreline in less than five hours. The risk would be highest on the Vineyard in spring, when the winds were predominantly north to east.

He said the possibility of such a rupture was remote and that the environmental risk posed by a leak of the light transformer oil was far lower than that associated with the deliveries to oil-fired power stations on the Cape, such as the 2003 disaster which dumped 100,000 gals of heavy oil into Buzzards Bay.

Ms. Parker said commercial fishermen opposed the project as a potential threat to their livelihoods. Mr. Rogers said they would still be able to fish the area, that the pylons could act as artificial reefs, and in any case the affected area now contributes only five per cent of the total catch in the federal area of Nantucket Sound.

Ms. Parker cited a study done by Cape Wind opponents which found tourist numbers could decline by two per cent, at a cost of some 2,000 jobs on the Cape and Islands.

Mr. Rogers said the project would create 600 to 1,000 jobs in the two-year build-out, and 35 to 55 permanent direct jobs in operation and maintenance. He said there was no example in the world of a wind farm harming tourism and that they had actually encouraged tourists in some European resort areas. Ms. Parker said Europeans had different attitudes to the environment and aesthetics.

She suggested the power generated by Cape Wind was likely to be expensive. The only other offshore wind project contemplated in America, now abandoned, off Long Island, estimated costs at 29 cents per kilowatt hour, compared with 13 cents from other sources. The average cost in Denmark also was 29 cents. Mr. Rogers could not give figures on the cost of power from the project, but suggested the market would be attracted by the fact that generation costs would be constant over the 20 to 25 year life of the project, while other fuel costs would likely rise.

She said property values could fall; he said there was no evidence of that elsewhere.

Ms. Parker claimed the private proponents of the project would benefit from some $1 billion in subsidies over the life of the project. Mr. Rogers said the only guaranteed subsidy was up to $29 million a year for 10 years in reduced federal taxes through a federal production tax credit, although there would likely also be an as-yet unquantified state subsidy as well.

She advocated wind generation from deep water sites; he said the project would not be financially viable in deep water, further from shore, and would be at greater risk from storms in the open ocean.

Ms. Parker said aquatic life, including gray and harbor seals, turtles and shellfish could be endangered.

Mr. Rogers said the two greatest environmental threats to the region were sea level rise due to global warming and nitrogen loading, of which up to 40 per cent came from the burning of fossil fuels.

Ms. Parker cited opposition from organizations including professional fishermen, both major ferry lines operating in the Sound, Barnstable airport and various boards of selectmen and chambers of commerce. Mr. Rogers cited support from Sierra Club Greenpeace, Conservation Law Foundation, Union of Concerned Scientists, Natural Resources defense Council.

Certain assertions went uncontested, such as the one by Mr. Rogers that Cape Wind would produce as much electricity as a power plant burning 500,000 tons of coal or 113 million gallons of oil, or Ms. Parker’s that interference with the radar on boats and possibly aircraft could be affected.

Some facts cannot help but be subjectively assessed, such as this one: If a person stands in Ocean Park in Oak Bluffs and holds up his arm, with finger and thumb about a quarter-inch apart, that is the apparent size of the turbines would be, nine miles away on the horizon.