Ferry officials say they’re closer to finding the source of vibration in their newest boat and may have found a way to work around it so the boat can begin carrying passengers.

Until more sea trials are conducted, however, officials still won’t know when they can put the Chetzemoka into service.

The $76.5 million vessel, built for the Port Townsend-Keystone run, vibrates at top speed and while slowing down. The boat was due to begin service Aug. 29 but its debut has been postponed.

The ferry’s diesel engine appears to be turning the propeller so fast that it’s vaporizing the water around it and spinning in a pocket of air, causing it to vibrate, said George Capacci, deputy chief of operations and construction for the ferry system.

More air is then sucked in, creating a tornadolike effect, Capacci said.

When the boat slows down, the spinning propeller slows down too much. The propeller and the engine then “chase each other back and forth,” Capacci said.

Other components of the drive train, such as engine mounts, couplings and the gearbox, were ruled out as causing the vibration in sea trials last week.

It’s still not known why the engine turns the propeller too fast. To get the ferry up and running while that riddle is solved, officials believe they can upgrade the vessel’s software to handle more functions electronically, including accelerating and slowing down.

This way, boat captains won’t have to worry about whether they’re going to trigger the vibration, Capacci said. They can instead concentrate on maneuvering the 64-vehicle Chetzemoka in the shallow harbor at Keystone.

“The ship driver can focus on the matters at hand, including properly entering the harbor and steering the boat,” he said.

The boat is safe and likely wouldn’t be pushed to top speeds on the 35-minute trip, said David Moseley, assistant secretary of transportation in charge of the ferry system. The software measure would be a precaution that would allow the boat to get into service.

“We want to make sure we’re operating in an abundance of caution and safety,” Moseley said.

More sea trials are scheduled for next week.

The vessel is one of three scheduled to be built by Todd Pacific Shipyards for the state. The new boats are supposed to replace the Steel Electrics, which were pulled from service two days before Thanksgiving in November 2007 because of corrosion in their hulls.

Those boats — the Klickitat, Quinault, Illahee and Nisqually — were more than 80 years old.

Since January 2008, the state has leased the Steilacoom II, a small ferry from Pierce County Transit, to carry passengers between Port Townsend and Keystone on Whidbey Island.

The state still has not formally accepted delivery of the Chetzemoka from Todd Shipyards, Gov. Chris Gregoire said in a press briefing Thursday morning. It likely won’t accept the vessel until officials know it can be put into service.

“All I can say is thank goodness we caught it,” she said of the vibration.

Finding glitches such as the vibration during sea trials is common for the first boat in a series, Moseley said.

“We’ll certainly learn some lessons from this,” he said.

Jerry Cornfield contributed to this report.