A jet-propelled ferry that can dart across the Gulf of Maine at more than 50 mph will get tourists and their cars to Nova Scotia and back in a hurry. But it's so fast that some people fear whales and lobster boats could get run down.

The 300-foot catamaran, which makes its maiden trip from Bar Harbor to the Nova Scotia Port of Yarmouth on Thursday, is the fastest car ferry on the continent, its promoters say.The Australian-built boat replaces a conventionally powered ferry, reducing the six-hour trip to 2 1/2 hours.

''You don't realize how fast you're going until you slow down,'' said Yvette Despres of St. Mary's Bay, Nova Scotia, who made the initial voyage from her home province to Maine on Wednesday on the $44 million vessel.

Even fully laden with 240 cars, four buses and 900 people, the ferry is capable of 50 mph. With a lighter load, it can whisk along at 58 mph as its four pumps shoot out enough water to fill eight Olympic-size swimming pools each second.

''It's like a giant Jet Ski,'' said John Surles of Bay Ferries Ltd., the Canadian company that operates the vessel. Similar ships run routes in South America, Australia and Europe.

The sight of the ferry racing across the ocean with white rooster tails shooting skyward may delight tourists. But Bar Harbor lobsterman Jon Carter said he is concerned about the safety of his fellow fishermen when the catamaran cuts its way through a thick Down East fog. He is even more worried about kayaks and other recreational craft that zigzag across the island-dotted waters all summer.

''If it takes the same route as the old ferry, we pretty much keep out of the way anyway,'' said Carter, a member of the town's harbor committee. But visitors who are unaware of the ferry's route may find it tricky to get out of the way of anything going so fast, he said.

Also, environmentalists say radar does not pick up whales and collisions are inevitable. Endangered right whales cross the ferry's route. Summertime sightings of minke and endangered humpback and finback whales also are common.

Bay Ferries said the propeller-less craft may be less of a threat to whales than conventional ships because only 12 feet is below the water's surface, compared with about 30 feet for other big vessels. It is also much easier to maneuver around objects and can stop more quickly, the ferry operator said.