SAN FRANCISCO -- Using the latest developments in environmentally friendly technology, a new generation of ferries is being built to will use a hybrid combination of traditional diesel, wind and solar power to make their daily commute treks across the San Francisco Bay.

A model of the 600 passenger "solar" sailor was unveiled in San Francisco on Thursday. It works on the same principal as a hybrid car using a battery-powered electric motor and sail for low speeds and the switching to a more traditional but highly advanced diesel engine for high speed.

"Up to about 8 knots, it uses an electric battery and supplementing that with solar," said Robert Dane of Solar Sailor Holdings. "Once you want to go faster than that, you do have to burn fossil fuels… Using the hybrid technology, you would save -- depending on the run -- somewhere between 50 to 60 percent of the fuel."

At night, the ferry will be plugged-in to shore power to assure adequate battery power during the day.

A smaller version of the "solar" sailor has been making daily treks across the Sydney Harbor for the last five years. The Bay Area version will have an even more sophisticated system to take advantage of the often windswept bay.

Some Bay Area environmentalists often critical of the current fleet of smoky, diesel-powered ferries say this new version makes even more sense as fossil fuel prices climb.

"It's a huge difference in terms of emissions and the amount of fuel that you're gonna be using," said Teri Shore of the Bluewater Network.

At about $5 million each, a "solar" sailor will cost $1.5 million more than one of the current generation of ferries.

"With the current price of marine diesel, the payback period for that $1.5 million extra cost is now about three and a half years," Dane said.

Two of the new "solar" sailors have been ordered to ferry tourists to Alcatraz and could be in service by 2008.