(NECN: Peter Howe, Boston) - After touring the cavernous wind blade testing center building on the Charlestown working port, U.S. Interior Secretary Sally Jewell Tuesday announced plans to open a huge swath of the ocean off Martha’s Vineyard and Nantucket for potentially thousands of wind turbine blades.

“The Atlantic here holds a great deal of promise,’’ Jewell said. “It's windy, and there's a lot of people who drive a lot of energy needs.’’

The new “Massachusetts Wind Energy Area” the Interior Department identified comprises 742,000 acres – nearly 1,160 square miles – starting about 12 miles south of the two resort islands. On the heels of wind leases proposed or awarded off Virginia, Delaware, and Rhode Island, Jewell said the new Massachusetts zone is “the largest wind energy area on the Atlantic Coast. It'll nearly double the federal offshore acreage available for commercial-scale wind-energy projects.’’

If you're having trouble imagining what 742,000 acres look like, it’s equal to a square 34 miles on each side – a zone that if it started in Boston would extend well north and south of Route 128 and west to Worcester and Leominster.

Despite Tuesday’s announcement of the zone that will be proposed to move forward for leasing, what’s far from clear: How soon these areas get leased, how many turbines get built, how many years from now, and how much electricity they yield. The zones are in theory big enough to support wind farms that could generate enough electricity to serve many hundreds of thousands of homes and small businesses, but in many places the ocean is 100 to 200 feet deep and erecting wind turbines on the sea floor or in floating arrays will be technologically challenging.

Massachusetts Governor Deval L. Patrick, who strongly supports offshore wind power, said: “These are commercial deals, and developers will decide what they think makes sense in terms of size and scale and scope.’’

In contrast to one of the key issues that has turned Cape Wind in Nantucket Sound into a 13-year saga of controversy and lawsuits, turbines in the newly identified offshore zone would be far off from the islands, and largely – but maybe not entirely – invisible from land.

Walter Cruickshank, acting director of the Interior Department’s Bureau of Ocean Energy Management, said the closest point to land is about 12 miles from Martha’s Vineyard and 13 miles from Nantucket, and a 400- or 450-foot-high turbine there and its blinking airplane crash protection lights might be visible from land.

“If a wind turbine was built right at that boundary, there would be days when it would be visible – again, depending on atmospheric conditions and how clear the sky was on any given day,’’ Cruickshank said. Developers would bid for rights to four different areas within the wind zone, and where they decided to actually erect turbines would be subject to environmental reviews and public comments.

With the controversial process of Cape Wind in mind, Jewell said the Interior Department has after first starting to identify potential offshore wind areas spent months listening to concerns from the Aquinnah Wampanaog Indian tribe, which considers many views over the waters and undersea areas as sacred, and other groups to remove thousands of acres from the wind zone lease area.

“We've been working with the Navy and with the tribes and with the commercial fishing industry and with the shipping industry and with the recreational boating industry,’’ Jewell said, “and the area that's picked, these 742,000 acres, we believe will have less conflict than other areas might.’’

With videographer John E. Stuart