An O'ahu commuter ferry that's been out of commission for more than two weeks may be back in action by Monday.

Mechanics are scheduled to install a new diesel engine in the 72-foot catamaran's port hull today, after earlier tuning up the vessel's starboard engine.

The 149-passenger Rachel Marie is one of two ferries that began operating in September between Kalaeloa and Aloha Tower, in a city program dubbed TheBoat.

The crippled ferry has been idle since Nov. 8, when operators discovered damage to the port engine's crankshaft.

The second boat has continued operations, but two of the program's six daily trips in each direction have been canceled until the Rachel Marie is repaired.

The vessel has been removed from the water, and workers cut open its aluminum hull to remove the old engine and prepare to install the new one.

If all goes well today, the hull will be welded back together this week, and mechanics will complete final engine tuning. The vessel will then be placed back in the water for propeller-shaft adjustments and a Coast Guard inspection, according to ferry project manager Darin Mar.

The ferries recently had carried an average of 250 to 300 passengers per day, according to city spokesman Bill Brennan. Since the Rachel Marie broke down, the other boat, the Melissa Ann, has carried fewer than 175 per day, he said.

The Rachel Marie has had trouble since the ferry program began. The boat's maiden voyage in Hawaiian waters was canceled in September after the port engine began sputtering. The vessel began service a few days later after a faulty master control unit was replaced.

More than 12,000 passengers have ridden the ferries since September. Some crossings have carried more than 100, but others have been nearly empty.

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

Though the ferries have been popular with some commuters, some City Council members and others have questioned whether the program is an inefficient boondoggle that will sink when the federal grants are exhausted.

The city's ferry contractor, Hornblower Marine Services, is paying for the Rachel Marie's repairs, and will credit the city for the lost service, officials said.

Until the Rachel Marie resumes operations, the 6 a.m. departures 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. runs from Kalaeloa also have been canceled.

Reach Johnny Brannon at jbrannon@honoluluadvertiser.com.