Bath Iron Works’ second Zumwalt-class stealth destroyer, the $7.5 billion USS Michael Monsoor, needs a $20 million engine replaced because inspectors found damage following the ship’s naval acceptance trials.

The Monsoor must have a main turbine engine replaced before the ship can sail to San Diego for its combat system activation, according to a U.S. Naval Sea Systems Command spokeswoman. The engine was made by Rolls-Royce and installed at BIW in Bath, where the ship is docked.

“In February 2018, a post-cleaning inspection of one of the DDG 1001’s two main turbine generators revealed damage to the rotor blades of the generator’s MT30 engine,” said Navy spokeswoman Colleen O’Rourke in a statement. “After the damage was identified, and out of an abundance of caution, the Navy decided to remove the engine in its entirety to ensure a successful and safe transit of the ship to her San Diego home port.”

The Navy is preparing to remove and replace the engine with a spare, she said. Upon removal, the engine will be inspected to determine the root cause of the damage, then it will be refurbished.

The Navy is working closely with Bath Iron Works and Rolls-Royce to get the damaged engine replaced before the ship leaves Bath this fall and sails to San Diego to begin having its weapons systems installed next year, according to the U.S. Naval Institute news service USNI News.

The Navy will pay for the engine replacement but could seek to recoup those costs from Rolls-Royce if it is determined that the engine was defective, the news service reported. The engine model installed on the Zumwalt-class ships is not new and has been used previously on the Navy’s Freedom-class littoral combat ships.

Both the Monsoor and its predecessor, the USS Zumwalt, have had a variety of mechanical problems.

Crew members on the Zumwalt found a seawater leak in a lubrication system for one of the ship’s propeller shafts in September 2016, less than a month after the stealth destroyer left Bath.

In November 2016, the ship had another mechanical breakdown, requiring that it be towed to a port in the Panama Canal. It was transiting the canal when it lost propulsion in one of its two drive shafts, and crew members noticed water coming into bearings connecting the shaft with its electric motor.

The Monsoor had to cut short its first sea trials because of an electrical system failure the day after it left BIW in December 2017. The failure during the ship’s builder trials prevented workers from testing propulsion and electrical systems at full power. The ship returned to the shipyard under its own power, and it returned to sea after the problem was resolved.

Loren Thompson, chief operating officer of Arlington, Virginia-based think tank the Lexington Institute and CEO of Source Associates, a for-profit consulting firm, said it’s not unusual for naval acceptance trials to reveal a significant problem with a new ship.

“When the Navy sends ships to sea for trials, it drives them pretty hard to determine that they are capable of everything they were designed for,” he said. “I’m still surprised (about the engine damage), but it is, after all, the whole point of the exercise to test all of the ship’s capabilities.”

Thompson said it’s possible that the engine was delivered to BIW already damaged, but it will be up to the Navy to determine what happened.

“Normally when Bath (Iron Works) intalls an engine, it’s not involved with the innards of the engine,” he said. “So it’s possible the engine itself had defects on delivery.”

The Zumwalt-class destroyers are the largest and most technologically advanced destroyers built for the Navy. They are designed to appear as much smaller vessels on radar thanks to their smooth exterior surfaces, “tumblehome” hull and other design features.

While they are significantly larger than Arleigh Burke-class destroyers, automation allows the Zumwalts to operate with half the crew of the other class. They also are regarded as technological showcases for future ships in terms of their all-electric powerhouse and weaponry.

Thompson said he does not regard the number of problems suffered by the Zumwalts to be excessive or unusual, because they are the first two ships of a new class that is significantly different and more advanced than anything that came before.

“The other thing I would say is that Bath still has the reputation for world-class workmanship,” he said. “It’s just regarded as being more capable than the Gulf Coast shipyards. And therefore I don’t think this is really affecting Bath’s reputation.”

The bigger issue for the Zumwalt class is the fact that the ships don’t have a clearly defined purpose, Thompson said.

“The Zumwalt class was originally supposed to provide dozens of next-generation destroyers to the Navy, but the Navy changed its mind about what it wanted, and that left only three fairly expensive ships,” he said. “It’s not so much that the class of ships has a bad reputation as it is that it’s sort of lost its place in the naval force posture, because there’s only going to be three of them when the work’s done.”

“I mean, they used to talk about building over 30 of these ships, and they were going to be the centerpiece of surface warfare for the Navy,” Thompson said. “But now there’s three ships in the class, and they’re looking for special missions the ships can accomplish because they’re just not like other destroyers.”

The third and final Zumwalt-class destroyer on order, the USS Lyndon B. Johnson, is under construction at BIW. The total cost of the three-ship program is estimated at $22 billion.

A crowd watches the christening ceremony for the USS Michael Monsoor (DDG 1001) on June 18, 2016, at Bath Iron Works. (Joel Page/Portland Press Herald)