– When Duluth’s iconic William A. Irvin museum ship floated into a dry-dock slip for some repainting last month, and the water was drained away from its belly, some of the ship’s handlers were startled by what they saw:

Metal-eating bacteria had not only gorged on the massive vessel’s hull, but also the rivets holding it together. Though engineers expected to see that, the damage was more extensive than some Duluth ship managers had expected.

“At first, it was kind of scary,” said Chelly Townsend, executive director of the Duluth Entertainment Convention Center (DECC), which owns the 610-foot retired ore boat that has been moored in the harbor for 30 years. “At first we were concerned, and now we’re really assessing the whole thing.”

Experts expected extensive “pitting” on the Irvin from bacteria found in the harbor; they have seen it before on underwater sheet-metal pilings that stay in place for decades. They were somewhat surprised to see the damage to rivets, though, and are now getting cost estimates to figure out whether to repair the rivets now, or epoxy and paint them and keep an eye on them, officials said.

Officials expect to have a price and schedule together by the end of the week, said Chase Dewhirst, a marine civil engineer with AMI Consulting, which is monitoring the project.

Part of the decision will depend on whether a Minnesota Historical Society grant of $504,000 for the hull work will cover it all. The grant was for more than the expected $455,400 paint job, and the excess was going to go toward other ship maintenance projects, Townsend said.

Corroded rivets could be handled various ways, depending on the severity, Dewhirst said.

They might not require any special attention, they could require a coating, or if more than 50% of a rivet is corroded, it could require a structural repair.

Dewhirst declined to guess how many rivets might need repair, but even a small percentage could add up.

“Below the water line there’s about 95,000 rivets,” he said. “If we’re going to have to do a structural repair to 10 or 15% of them, it’s going to be extremely expensive.”

The Irwin has never leaked, he pointed out, and even if none of the rivets is repaired, the ship will survive a tow back to the Minnesota Slip. Officials would have to keep monitoring the boat for seepage and could make rivet repairs there as needed.

A bit more corrosion

The pitting in the Irvin may be more extensive than that in lake-going vessels typically brought into dry dock, officials said, because of bacteria that live in the harbor.

The bacteria, propelled in sunlight, cling to metal and form a rust nodule, Dewhirst explained. Ice in the winter will sometimes scrape off the nodule, exposing the steel and allowing the bacteria to cling to it again, starting the process all over, he said.

With a boat that has been sitting still in the harbor for 30 years, engineers expected corrosion.

Active vessels “are constantly in dry dock. They are constantly getting sandblasted and recoated,” Dewhirst said. The Irvin hasn’t been in dry dock since the late 1980s.

But the rivets, located on the ship’s bottom where there is no exposure to sunlight or scraping from ice, were a bit more corroded than anticipated, Dewhirst said.

Engineers believe stray electrical current from nearby boats or the ship itself may be playing a role in accelerating the corrosion.

To combat that, officials plan to ground the boat in a different way once it’s back into the slip, said Steve Rankila, the DECC’s director of property maintenance.

A tourist draw

Officials are eager to return the ship to its Duluth slip, as it has become a tourist destination and a symbol of the city’s industrial heritage.

As the flagship vessel of U.S. Steel’s Great Lakes fleet, the Irvin was launched in 1937 and “provided comfort and elegance to dignitaries and guests who traveled the Lakes,” according to the DECC. The boat ran for more than 40 years.

It became a tourist attraction in 1986, originally docked behind the convention center. But after a storm that year, it was moved to a slip between the DECC complex and tourist-popular Canal Park, where smaller boats are docked behind it.

About 55,000 people tour the ship each season.

Last fall, the Irvin was carefully moved out of the Minnesota Slip, with only about 15 inches of clearance through the opening of a blue pedestrian bridge that was blocking its way. Environmental cleanup workers needed the ship moved to reach sediment as part of a larger effort to remediate industrial pollution in the Duluth harbor.

Because it was already on the move, officials decided to have the boat dry-docked at Fraser Shipyards in Superior so its hull could be sandblasted and painted.

Delays kept the boat from being dry-docked until early last month.

“At first glance, we were very, very concerned. We wondered if it was going to be worth bringing it back,” Townsend, the DECC executive director, said. “We did have one person say we might have to scrap it because they thought it was that bad. ... But we’re pretty coolheaded here. We said, ‘Well, let’s get the facts first.’ And we don’t have all the facts completely. We’re still getting bids.”

Officials feel confident the ship will be back in place and open for tours by next May, Rankila said.