The giant Bath Iron Works shipyard came to a halt at 2:30 p.m. on June 29, 1984, as the voice of company President William E. Haggett boomed over the loudspeakers.

For Maine's largest employer and one of the nation's leading Navy shipbuilders to endure, he said, "concessions" by its 7,000 employes were crucial. Pay and benefits would have to be reduced.

The unionized shipyard workers, moved by his unprecedented plea, voted overwhelmingly to reopen their contract.

But a year later, those talks broke down and Local 6 of the Industrial Union of Marine and Shipbuilding Workers, AFL-CIO, voted by 3,500 to 25 to reject what its members considered overly severe concessions. On June 30, the union's contract expired; now the shipyard is silent again, idled by a bitter strike that both sides predict could be long and costly.

The strike reflects the dilemmas that confront labor and management in shipbuilding and other heavy industries: Increased foreign competition and the collapse of commercial shipbuilding are forcing superheated rivalries among the surviving American firms, and the resulting drive to reduce labor costs is often pitting unions against companies with added ferocity.

Here in this historic New England seaport, where seagoing vessels have been built since 1607, the strike has been particularly painful and puzzling to both sides. BIW and the union have prided themselves on a decades-old partnership that made Bath one of the nation's most profitable and highly regarded yards. The sign on the gate beneath a 394-foot crane proudly proclaims: "Through These Gates Pass the Best Shipbuilders in the World."

Just three months ago, the Bath yard rang with cheers when the Navy announced that BIW had won a $322 million contract to build the first of the new DDG51 class of destroyers. Bath officials predicted that the victory could mean as many as 15 more destroyer orders, bringing more than $4 billion of work here and virtually ensuring the yard's health through the 1990s.

But the joy was short-lived. Bath's winning bid was based on the presumption that the company would receive the concessions subsequently rejected by Local 6. The new Navy project is on hold, pending talks due to resume today.

BIW, a subsidiary of the Congoleum Corp., has proposed a three-year wage freeze, a cut in health insurance benefits that could cost employes up to $1,500 a year, a $3-an-hour cut in wages for new hires, and -- perhaps the most controversial proposal -- changes in rules that would allow workers to "cross trades" and temporarily replace those in other trades.

"We must have these concessions. We just cannot continue paying the highest wages and benefits on the East and Gulf coasts and expect to stay competitive," Haggett said in an interview. " . . . But we have had a very difficult time convincing our union membership."

Bath pays an average of $11.47 an hour, compared with $10.70 by its chief competitor, the Ingalls Shipbuilding division of Litton Industries in Pascagoula, Miss. Ingalls has received concessions from its union through 1987, and Haggett said Bath must follow suit or lose its ability to underbid competitors.

But to the men and women who perform difficult and often dangerous jobs, whose labors have helped BIW obtain an estimated $75 million in bonuses for early delivery of Navy ships, the company's argument rings hollow. Bath has not been in financial trouble, they note, and the privately held company will not open its books to union scrutiny to justify the need for concessions.

"Our union has done great work for them. We earn higher wages than Ingalls, but we do ships much quicker and we do better work" despite the frigid Maine climate, said union President Ray Ladd.

Ladd said union members were willing to accept the pay freeze, but now feel betrayed because the package of concessions went far beyond initial expectations.

Ainsley McPhee, 49, who has worked here 25 years and whose son, Todd, also works for BIW, said he is prepared for a lengthy strike. McPhee said he feels "stabbed in the back" by a company whose corporate advertising had long applauded its employes' productivity.

"Our people are really proud of their work," the pipe fitter said. "There is still a funny feeling when you see your ship launched. You smile and think, 'I was part of that ship.' You never let your supervisor know that, but you're filled with pride. And we are more proud, because we build 'em faster and better . . . . Now they want to take back what we have earned."

Veteran welder John Severy agreed. "You're working in the bilge of the ship, with ice around your feet, breathing in smoke and welding fumes, while you're shaking with cold," he said. "We earn our money. We are the ones who build these ships."

BIW has run a series of advertisements since 1981 promoting President Reagan's defense buildup and touting the company's achievements in winning work and delivering high quality. Some company officials now believe that those ads have backfired.

"I think part of this is our fault," said shipyard spokesman James McGregor. "We read our own press clippings. We milked it for all we could, and we have publicized our achievements. I think we went on too long with our best-in-the-world, billion-dollar-backlog advertising."

That image, he said, has made it more difficult for employes to accept the need for concessions.