DULUTH — A former U.S. Army Corps of Engineers tugboat has begun sinking into the Duluth Harbor.

The privately owned boat named Lake Superior, moored in a slip near Pier B off Railroad Street, developed a hole in a ballast tank, said U.S. Coast Guard Lt. Joseph McGinnis.

"It was listing over the weekend, but the ice held it up until it started melting," he said.

It was unclear how the hole developed. On Tuesday, the vessel's stern was submerged and the Coast Guard removed water with pumps, as its Marine Safety Unit in Duluth monitored the situation. McGinnis said the Coast's Guard's pollution responders are ready to mitigate any environmental hazards, with diesel and lube oil onboard the tug. The boat isn't leaking into the harbor and the vessel is in a safe location where it won't interfere with any other watercraft, he said.

According to its former owner, the Duluth Entertainment Convention Center (DECC), the 114-foot tug was built in San Francisco in 1943 for the U.S. Army and was first named the Emil H. Block. As an Army vessel, it worked in the South Pacific moving barges between the islands of the Philippines. It sank in 1950. Repairs were made, and it began work on Lake Superior for the Army Corps renamed the Lake Superior, pulling the dredge barge in the Duluth Harbor. It also did breakwall work in Two Harbors. The Army Corps retired the tug in 1995 and donated it to the DECC.

The DECC used the vessel as a museum until it was sold to Billington Contracting for $56,262 in 2007, said Lucie Amundsen, the DECC's communication director. Efforts to reach the owner of the company weren't successful.

An old listing for the tug touted its oak and mahogany features and brass fixtures.

The Lake Superior would be the second tug to sink in the Duluth Harbor in recent years. In 2009, the 85-foot tug Essayons sank at its slip. It was owned by Hobart Finn.