Before the sails can billow

IMAGINE the white sails of a 19th-century schooner unfurling and billowing in the wind against a blue sky, her sharp bow parting the waves. Weather permitting, that seafaring vision will be realized April 28 when Massachusetts launches its own ''tall ship,'' a 125-foot, two-masted Boston/Gloucester fishing schooner, to be christened the Spirit of Massachusetts.

The vessel is taking shape at the 184-year-old Charlestown Navy Yard. Saws and augers whine and hum continually at the building site, which abuts the imposing granite massif of Drydock (once used for repairing pre-World War I naval vessels). A wood-framed, plastic-shielded shelter surrounds the schooner. Three levels of scaffolding hoist the builders about 20 feet to work on the schooner's deck. Wood scraps litter the yard. The shipwrights, with the help of volunteers, are racing to meet their late April deadline. ''I'm under the gun right now,'' says master shipwright Andy Davis. ''We're really strapped. We're struggling.''

Mr. Davis, a Boston resident, is one of 18 shipwrights from various Atlanic-coast ports who have gathered at the old Navy Yard to have a hand in building this Fredonia-style schooner, a type originally designed in the late- 1800s by the renowned naval architect Edward Burgess. The shipwrights have been assisted by some 150 volunteer workers.

The shipbuilders' day begins at 7 a.m. Clad in plaid flannel shirts, woolen caps, brightly colored bandannas, and faded blue jeans covered with paint stains , they arrive for another nine hours of muscle-testing, exacting work. A six-day workweek is standard.

During the winter months the workers took breaks around a wood-burning stove in the wood shop, finding momentary relief, at least, from freezing temperatures. But cold weather is a blessing to some shipwrights. ''I think it's harder to work in the hot weather,'' says John Van Soest, a shipbuilder from Somerville, Mass. ''The sun drains motivation.''

Currently, the workers are completing the hull, decking, and deck structures such as the cabins. They're also putting in the plumbing and finishing the painting.

Since the masts and rigging are extremely heavy, they'll be added after the ship is launched. ''It's better to keep the boat weight light, because it will be lifted by two cranes'' during launching, says shipwright Danny Gould. ''The easier we make it for them, the better.''

''There aren't too many vessels this large being built these days,'' notes 21 -year-old shipbuilder Sarah Talbot. Ms. Talbot is in charge of painting the schooner. ''Two weeks before launching, we'll be working late trying to finish.'' But she has the payoff in sight: ''It will be enjoyable to see a beautiful ship on the water.''

Besides shipwrights, other specialized workers are contributing their skills - caulkers, blacksmiths, pipefitters, and riggers, to name some.

Shipwright Steve Hopkins sits on a stool in the wood shop with a piece of canvas on his thigh, carefully rolling strands of oakum, a form of hemp. A caulker, using caulking iron and mallet, will drive the ropelike strands of oakum between seams in the planking to make the vessel watertight.

Blacksmiths fabricate all the ironwork on the ship, some 100 pieces of hardware. Their work includes such crucial items as the ''chain plates,'' long iron straps that are fastened to the outside of the planking and come up to the rail cap. The shrouds - the standing rigging that holds the masts in place - are made fast to the chain plates.

THE job of fitting out the vessel's spars - masts, bowsprits, booms, and gaffs - falls to riggers like David Mullin and Ron Duprey. They're doing preliminary rigging now, making up the shrouds and stays that support the masts. The rigging wire rope has a diameter between one-half inch and 11/4 inches. The heaviest wire has a breaking strength of about 325 tons. It's used to make the bobstays which support the bowsprit.

Rigging demands precise measurement. ''Everything on a boat is so closely connected,'' Mr. Duprey explains. ''If a piece of wire is off by two inches, adds Mr. Mullin, ''it would be no good.''

The wood to build the Spirit of Massachusetts - about 130 tons of it - has come from around the country and the globe: longleaf yellow pine from Alabama for upper frames, some planking, and the deck beams; Douglas fir from Oregon for decking and masts; white oak from Massachusetts and Maryland for lower framing; and greenheart wood imported from Guyana for keel, stem, and sternpost.

The shipwrights explain that greenheart, which thrives in tropical rain forests, is ideal for the parts of the vessel that will always be under water, since the wood is naturally resistant to worms.

What does it take to build a large sailing vessel today?

''It takes dedication,'' says shipwright Gould. ''You've really got to believe in what you're doing.''

Gould, who has always liked boats, has done restoration work at the shipyard at Bath Marine Museum in Bath, Maine. He lists some of the problems in taking on the construction of a large wooden vessel like the Spirit of Massachusetts.

Foremost is materials. ''Good materials are hard to find,'' he says. ''They are things that are unique to the vessel. They have to be specially made, because each vessel is different. You can't just call up on the phone and order them.'' Examples of rare materials include bronze fittings like the pintles and gudgeons which support the rudder. ''Each piece is cast from wooden patterns,'' Gould says.

Despite the problems, Gould feels that traditional shipbuilding is far from being a lost art. ''As long as there are boats, there has to be someone to work on them,'' he comments.

To shipwright Steve Hopkins, shipbuilding is ''really a tremendous amount of repetition of relatively simple tasks like boring holes, rolling oakum, painting , and pounding spikes.'' He agrees with Gould that obtaining supplies is the biggest problem. ''A lot of supplies have to be imported,'' he said. ''And they take a long time to arrive.''

ASIDE from the April 28 launching ceremony, the shipwrights' work will be spectacularly diplayed June 2-7, when the Spirit of Massachusetts will be joined in Boston Harbor by an international fleet of tall ships from West Germany, Mexico, and Portugal, as well as other parts of the United States. The modern term ''tall ship,'' by the way, applies to any vessel whose mainmast is around 100 feet and whose deck measures 100 feet or more.

The gathering will commemorate the 200th anniversary of the opening of the China trade and Massachusetts' maritime heritage in general.

Like many of the other large sailing vessels, the Spirit of Massachusetts will become a training ship. During the summer she will accommodate 30 cadets and a crew of six on voyages to Halifax, Nova Scotia, or Bermuda and return to Boston.

Support for the schooner's construction has come from foundations, corporations, and private individuals. The Commonwealth of Massachusetts has also helped. To date, more than $750,000 has been contributed toward the $1.5 million goal.

The project is under the auspices of Boston's New England Historic Seaport, headquartered at Building 10, Charlestown Navy Yard, Boston.