PORTLAND — Bath Iron Works on Tuesday informed its largest union that it would lay off 39 pipefitters and nine machinists Jan. 21 to adjust for changes in the trades need to complete its current load of warship work.

After Jan. 21, the employees will get 30 days of additional work in other trades, BIW spokesman Matt Wickenheiser said. Pipe fitters will take on work in the insulation trade and machinists will take on work in the electrician trade through what are called short-term recalls, per the collective bargaining agreement between the union and shipyard.

Meanwhile, the shipyard is still hiring in other trades, including painters and electricians.

Jay Wadleigh, president of Local S6 of the machinists union, which represents about 3,500 shipyard employees, said the union tried to avoid layoffs by having pipefitters do work in the insulation department for the past 10 weeks.

“They’ve been burning up insulators’ hours by putting them in there,” Wadleigh said.

Wickenheiser said the company proposed additional reassignment within the shipyard’s trade classifications.

“We do have new needs in the shipyard and other trades where we would have preferred to apply these skilled manufacturing tradesmen and women as needed,” Wickenheiser said.

But the union, Wadleigh said, was concerned that would just delay any cuts that could fall on more senior members of the insulating department. Wadleigh said a work loan would have added about 42 people to the shipyard’s 79-person insulating department.

“That would really devastate that department long-term [because it] would eat up all of their hours,” Wadleigh said. “We foresee that when the loan ended, there would probably end up being a layoff in that department — they were basically trying to rob Peter to pay Paul.”

Wadleigh said he attributes part of the layoffs to the company “over-hiring” for pipefitters last year, contributing to loans to the insulating department for the past 10 weeks.

Wickenheiser couldn’t say when demand for pipefitters would rise again at the shipyard and declined to provide further information about the nature of the delay that led to less demand for pipefitters and electricians than expected.

Wadleigh said he expects that more demand in the pipefitting trade will return in the second quarter of this year.

Wadleigh said work commonly “ebbs and flows” through departments as ships progress through the yard. Wickenheiser said these types of layoffs in response to production cycles for the massive warships happen routinely.

The shipyard is completing two of three high-tech DDG-1000 destroyers and continues to build the DDG-51 Arleigh Burke class destroyers for the U.S. Navy.