One of the city's two commuter ferries will likely be out of service for three weeks or more while a faulty engine is replaced, officials said.

The 149-passenger Rachel Marie was taken out of service Thursday — the second time mechanical problems have docked the vessel in six weeks.

City Transportation Director Melvin Kaku said mechanics found that the crankshaft in one of the vessel's two diesel engines was damaged.

Hornblower Marine Services, which operates the ferries, will pay for a new engine it has ordered from the Mainland, he said. The engine should arrive here sometime next week.

It was not immediately clear how long final repairs would take, but the vessel will probably be out of service until at least December, Kaku said.

The problems mean the 6 a.m. sailings from Kalaeloa and 5:20 p.m. departures from Aloha Tower will be replaced by shuttle buses.

The 7:05 a.m. departures from Aloha Tower and 4 p.m. trips from Kalaeloa have also been canceled.

The Rachel Marie was taken out of service for two days in September after an engine began sputtering. It went back in service after a new control unit was flown in from the Mainland.

Earlier that month, officials told U.S. Coast Guard inspectors they planned to replace or rebuild the Rachel Marie's engines within six months.

More than 10,000 passengers have sailed on the ferries since the service, dubbed TheBoat, began operations in September. Some trips have carried more than 100 passengers, but others have been nearly empty.

The two vessels are twin 72-foot aluminum catamarans.

The ferry service and five linked bus routes are expected to cost $6 million for one year. Federal grants are providing $5 million.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.