The latest victim is the USS Freedom, which had seawater leak into one of its two main diesel propulsion systems on July 11, according to a Navy press release.
The Navy said seawater entered the engine oil lube system through a leak in a seawater pump's mechanical seal.
The leak occurred when the Freedom was participating in the 26-nation Rim of the Pacific exercises, the Navy said.
The ship returned to its San Diego home port for seawater decontamination on July 13 and then returned to the exercises under power of its gas turbine engines, rather than the main diesel propulsion systems, the Navy said.
The extent of the damage -- rust inside the engine -- wasn't discovered until an inspection on August 3, the Navy said.
"Based on initial assessments from the inspection, Freedom's #2 (diesel engine) will need to be removed and rebuilt or replaced.
The cost and timeline for the repair of the engine are unknown at this time," the Navy release said.
The Navy has launched an investigation to "determine the definitive cause of the casualty and examine all relevant elements of training and supervision."
$23 million to fix
The news on the Freedom's breakdown comes as its sister ship, the USS Fort Worth, makes a slow journey from Singapore across the Pacific to San Diego so it can undergo repairs after a mechanical problem in suffered in January.
The damage came when the four-year-old Forth Worth tried to operate its propulsion system without enough oil its combining gears, the hardware that transfers power from the ship's diesel and gas turbine engines to its water-jet propulsion system.
"The casualty occurred due to an apparent failure to follow procedures during an operational test of the port and starboard main propulsion diesel engines," said the Pacific Fleet in Pearl Harbor, Hawaii, in a statement issued earlier this year.
The Navy put the cost to US taxpayers for that incident at $23 million.
The Fort Worth is calling in Cebu, Philippines, this week after leaving Singapore on August 22.
The incident cost the then-commander of the Fort Worth, Cmdr. Michael L. Atwell, his job in late March when the Navy announced Atwell was being reassigned to LCS squadron duties in San Diego.
"Sufficient findings of facts emerged during the investigation to warrant the relief of the commanding officer," the Pacific Fleet said in a statement at the time.
The mechanical mishap on the Fort Worth came shortly after a similar mistake on its sister ship, the USS Milwaukee, which broke down in the Atlantic Ocean on December 10, less than a month after it was commissioned.
The ship had to be towed 40 miles to Joint Expeditionary Base Little Creek-Fort Story in Virginia.
The Navy said at the time that metallic debris was found in filter systems in the ship, causing a loss of pressure in lubricant to gears that transfer power from the ship's diesel and gas turbine engines to its water jet propulsion system.
Navy officials later explained what happened in an email to CNN.
The Milwaukee "is designed to operate with gas turbine and diesel engines, which can operate in tandem or independently," Navy Lt. Rebecca Haggard said.
"In the case of Milwaukee, when switching from one system to the other, a clutch failed to disengage as designed. Instead, the clutch remained spinning and some of the clutch gears were damaged."
The Milwaukee has since undergone repairs.
Despite the problems with some of its newest warships, Navy officials are steadfast in their support of the program.
"As with any new venture, the LCS program has had its ups, downs, and learning moments. Things do not always go as planned. That is no surprise," Adm. Scott H. Swift, commander of the US Pacific Fleet, wrote in the U.S. Naval Institute's Proceedings magazine in July.
He pointed out early criticisms of platforms such as the Oliver Hazard Perry-class frigates, Spruance-class destroyers and Ticonderoga-class cruisers that went on to have long Navy careers.
"I am convinced the LCS/frigate program will be similarly vindicated," Swift wrote.
The Navy's littoral combat ships come in two variants: the monohull and the trimaran, with three hulls.
The Freedom, Fort Worth and Milwaukee are monohulls. With a draft of between 14 and 15 feet and a speed of 40 knots, the ships are designed to operate in littoral environments, or shallower coastal areas.