We can think of a lot of good reasons why Portland shouldn’t bring an aircraft carrier into its harbor to serve as a floating museum and convention center. But we can’t think of a better one than was expressed this week by City Councilor Dory Waxman.

“It’s just too big,” she said.

With a mast as tall as the top of Munjoy Hill, as long as 3 football fields and enough deck space to build an apartment complex, the decommissioned aircraft carrier John F. Kennedy would dwarf anything in the harbor, blocking views of the city from the water and the water from the land.

And that’s just the part that you can see. Despite Portland’s vaunted deep-water access, it’s not quite deep enough, at least in the place proposed for the J.F.K. Backers say that they would have to move 100,000 cubic yards of mud from the berthing area to make room for the ship, which draws 33.5 feet of water.

Like other aspects of this project, the budget for dredging is fuzzy. Supporters estimate it would cost between $1 million and $35 million, depending on what kind of contamination they find in the mud.

There is so much room between those numbers that they barely qualify as an “estimate,” but let’s be assured that the mud in the bottom of what has been a busy industrial harbor for a few centuries is going to be on the expensive side to dispose of under modern environmental standards.

And what would they do with the ship if they could overcome all those hurdles? How about making it a convention center to attract groups of visitors to the city?

A convention center is a great idea, and one that has had a lot of support in this community for a number of years.

But no one has been able to find a way to make one work economically. Putting this function inside a “building” that would need to have its bottom painted every so often doesn’t sound like a formula that would make the math work any better.

The people who back the J.F.K. in Portland are passionate, but there just aren’t enough of them to give this project a real shot. The Greater Portland Regional Chamber of Commerce is supportive, but only if backers can resolve the financial, environmental and location issues. That’s a lot of “ifs.”

The City Council should thank the supporters for their time, and then vote down this proposal. There are a lot of smaller things on the city’s agenda that require their attention.

 This editorial was amended at 12:26 p.m., Thursday, September 16, 2010 to correct the comparison of the length of the USS JFK to a number of football fields.