The supporters of turning a decommissioned aircraft carrier into a floating museum have asked for one more chance to sell their idea to the Portland City Council.

That’s what they should get, and no more. It’s time for the discussion of this flawed idea to end, and for the city to work on more pressing concerns on its waterfront.

The problem is not that a museum ship doesn’t belong in Portland Harbor. A 19th-century merchant ship would be a living example of the city’s maritime history. So would a Liberty Ship, like the ones built right in Portland Harbor during World War II.

The problem is this ship: It’s just too big. The John F. Kennedy would dominate the waterfront. It would be taller than the tallest building in town. It would reduce future options for new recreational and industrial uses of the waterfront, as well as limiting existing activity.

The cost would also be too big. The backers say that no public money would be required and that they would raise every dime – including the millions needed to dredge the harbor so that the JFK’s hull could stay off the bottom.

But this is not just a question of startup costs. Maintaining the vessel would be costly, and if the group that would run the ship runs out of money, the city would be in a tight spot, whether it was officially on the hook or not.

Portland councilors should be polite and listen to the JFK backers one more time. Then they should listen to their constituents and let this opportunity pass into the night.