Washington State Ferries is canceling the Keystone to Port Townsend ferry route for one weekend.

The route’s only boat, the Steilacoom II, will be pulled from service Saturday and Sunday, Jan. 9 and 10, so the ferry can undergo a safety inspection mandated by the United States Coast Guard.

During the two-day period, motorists heading to and from the Olympic Peninsula will have to take different routes. For most, that means a lengthy trip that includes voyages on the Clinton and Edmonds ferries.

When the Steilacoom II went out for scheduled service in the past, passenger-only ferry service was sometimes offered.

Laura Johnson, spokesperson for Washington State Ferries, said it would have been too expensive to bring in a passenger-only ferry for just two days.

The Steilacoom II has been removed from service for as long as two weeks to undergo the required inspections and maintenance work. Normally, that work is done at the ferry system’s Eagle Harbor facility on Bainbridge Island. To expedite the process to one weekend, workers will travel to Port Townsend and do it there while the ferry is docked, Johnson said.

The Steilacoom II, which is owned by Pierce County, has had several instances where the vessel broke down, which prompted cancellations of ferry runs, most recently on Dec. 19 when the ferry was pulled while workers repaired damage to the vessel’s generator. The ferry returned to service Monday morning.

With the exception of cancellations because of tidal conditions, the Coast Guard’s mandated inspection is the last scheduled interruption on the Port Townsend to Keystone route before the new ferry, known as the Chetzamocha, is built and ready to service the route sometime in 2010.