Washington State Ferries is devising an emergency system to reduce the chances of a Bremerton ferry being pulled to serve another run.

The provisional plan is intended to spread the impact of a vessel breakdown among other regional routes instead of relying primarily on Bremerton for backup boats, said Ray Deardorf, Washington State Ferries planning director.

The plan will be needed until next summer, when a new vessel will be completed, allowing for an adequate backup ferry in the event of an emergency. Until then, the ferry system is strapped for emergency options, which has aggravated commuters when a Bremerton ferry is diverted to another route.

“We really don’t have sufficient backup vessels in the system,” spokeswoman Joy Goldenberg said, adding that Washington State Ferries has only one available “half-boat,” the Hiyu, which carries up to 200 passengers and 34 vehicles.

With the revised emergency plan, the only scenario in which a Bremerton ferry would be diverted to another route would be if an Edmonds-Kingston vessel broke down. In that case, a Southworth-Fauntleroy ferry would fill in for the Bremerton-Seattle run.

If a Bremerton ferry breaks down, the primary plan would be to pull a passenger-only foot ferry owned by a private operator such as Victoria Express as a fill-in, Dearorf said. But in the summer, when there’s a demand for the private passenger-only trips, the Bremerton run would probably take a boat from the Southworth-Fauntleroy route.

In addition to the new vessel deploying next summer, another ferry will be completed in spring 2012, which ferry officials believe will further alleviate emergency strain on the system.