The longest ship on the Great Lakes was set free on Saturday evening after spending close to four hours aground in the Duluth harbor near Bayfront Festival Park.

The 1,013.5-foot Paul R. Tregurtha was wrested loose by two tugboats at about 7 p.m., said Adele Yorde, spokeswoman for the Duluth Seaway Port Authority.

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It had been departing with a full load of coal when it failed to negotiate the turn toward the Aerial Lift Bridge, she said. “It was making the turn to go under the lift bridge, but it didn’t turn,” she said. “It went straight toward Bayfront Park.”

The incident occurred about 3:20 p.m., the U.S. Coast Guard said.

There were no known injuries and no signs of pollution, the Coast Guard said.

Ohio-based Interlake Steamship Co. owns the Tregurtha, which can carry as much as 71,000 net tons of coal, according to its website.

The stranded ship drew a crowd of spectators, many of whom were attending the Duluth Balloon Festival.

No information was available on the cause of the mishap, said Lt. Judson Coleman of the Coast Guard’s Marine Safety Unit in Duluth.

The ship’s position probably did not impede other harbor traffic, Yorde said. Lift bridge operations weren’t affected.

Interlake Steamship spokesman Tom Wynne said the Tregurtha last was grounded two years ago, in the St. Mary’s River. The ship has gotten stuck only twice in the eight years he has worked for the company, Wynne added.