The nonprofit that owns the SS United States announced Monday that it has received an anonymous $250,000 donation, but would not divulge its plans for the rusting ocean liner, which has spent 18 years docked in South Philadelphia.

The SS United States Conservancy said funds from the anonymous donation will go toward planning an onboard museum, which will require restoration of key areas of the celebrated ship.

Last December, the conservancy reached what it called a "preliminary development agreement," but offered scant details, saying only that the vessel would leave Philadelphia.

Previous plans called for moving the ship to New York.

Dan McSweeney, a conservancy board member and managing director of the SS United States Redevelopment Project, said a nondisclosure agreement prevented him from elaborating.

"What we can say is the ship is currently under option for development," McSweeney said. "We're very enthusiastic about the opportunities in that agreement."

The project, according to the conservancy, is at an "early and delicate phase." Some have not been optimistic about repurposing the historic vessel, given the staggering cost attached to refurbishing the largest passenger ship ever built in this country. It measures 990 feet from bow to stern and was capable of reaching 44.7 knots, or 51.4 m.p.h.

It still holds the trans-Atlantic speed record, which occurred on the ship's maiden voyage in 1952. It was taken from service in 1969 and towed to Philadelphia in 1996 with its interior stripped.

The caretakers of the vessel have experienced difficulties raising the money to keep the SS United States afloat.

Last July, a $120,000 donation from Washington businessman Jim Pollin saved the conservancy from selling one of the ship's massive propellers to raise cash.

Pollin pledged to match an additional $120,000 in contributions. The conservancy said Pollin's investment has yielded "more than a half-million dollars for America's flagship."

But the vessel, docked at Pier 82, near Columbus Boulevard, costs $60,000 to $80,000 a month to maintain. The unnamed developer is fronting some of the maintenance costs, a conservancy spokesman said Monday.

The estimated cost of the ship's redevelopment plan is $300 million, McSweeney said last August.

Norwegian Cruise Line, the ship's previous owner, offered it for sale in 2009. Philadelphia philanthropist H.F. "Gerry" Lenfest, now owner and publisher of The Inquirer, intervened in 2010. He said he would donate up to $5.8 million to save the ship.

That allowed the conservancy to purchase the vessel. But Lenfest said last summer he would not contribute more money to the project.