There’s a saying that a boat is a hole in the water into which you throw money.

Just imagine what it’s like to maintain a floating old U.S. Navy battleship as a museum.

The USS Missouri Memorial Association, the nonprofit owner of the “Mighty Mo” in Pearl Harbor, has embarked on a $3 million project to repair some extensive corrosion high up on the superstructure and add some historical features to the 887-foot battlewagon.

It’s the biggest repair and preservation effort on the Missouri since it was drydocked in 2009-2010 for a $15.5 million top-to-bottom paint job.

That wasn’t as invasive and extreme as the work coming up, though, which involves “a lot of exterior steel repair — taking out a lot of bad metal, metal that has been corroding since the late ’80s, early ’90s, and replacing all that,” said Jason Morrison, vice president of facilities and engineering for the Missouri.

An estimated 17,000 pounds of steel will be replaced. Nearly 27,000 square feet of steel superstructure will be sandblasted, and 700 gallons of paint will be used.

It’s a big project, and one that will be completed while staying open to visitors and conducting most work after hours. It’s also part of the ongoing costs of running a battleship museum.

“It’s never done,” observed Morrison.

Ongoing is a project to replace all of the battleship’s 53,000 square feet of old teak deck. Morrison estimates that’s about 40 percent completed. The Missouri spends about $500,000 a year on that.

There’s a project to replace a scaffolding access ramp with a permanent one, $250,000 is earmarked this year for electrical work, and $300,000 will be spent this year for underwater sustainment, he said.

Paying for all of it are entrance fees from more than 600,000 people who visited the battleship last year — the best attendance year ever, said spokeswoman Jaclyn Hawse.

“I like the fact that they are preserving it,” said Chip LaFurney, 64, who was visiting last week from Bend, Ore. Josefa LaFurney, his 62-year-old wife, does too.

“I think everybody thinks of Pearl Harbor and this whole memorial as not just a memorial to those (who fought here),” but as a memorial to all who served and are serving, she said.

“The preservation of this ship is part of that — of course, I’m not writing the check,” she added with a laugh.

The Missouri was the last commissioned U.S. battleship, and Gen. Douglas MacArthur and Adm. Chester Nimitz were on its deck for the formal surrender of Japan on Sept. 2, 1945, in Tokyo Bay.

In Operation Desert Storm in early 1991, the Missouri launched 27 Tomahawk cruise missiles and hurled 305 16-inch shells from its huge deck guns at Iraqi forces.

The battleship was in Pearl Harbor in 1991 for the 50th anniversary of the Dec. 7, 1941, attack — its last operational mission — and on Jan. 29, 1999, opened as the Battleship Missouri Memorial opposite the sunken USS Arizona.

It’s one of four memorials and museums in Pearl Harbor along with the USS Arizona Memorial, Pacific Aviation Museum Pearl Harbor and USS Bowfin Submarine Museum and Park.

As part of the Missouri restoration, replicas of two SLQ-32 electronic warfare antennas, as well as a radome involved in the operation of remotely piloted aircraft, will be fabricated and installed.

The work involves the tallest portions of the ship from the 05 level, five floors above the main deck, and up, including the forward fire control tower, forward stack and forward mast.

The yardarms off the mast with lights and attachment points for lines and signal flags have supporting brackets that are severely rusted, and the yardarms will be cut off, removed, repaired and reinstalled, Morrison said.

Normally, visitors can climb to the 04 deck navigation bridge and 05 deck flying bridge, but both will be closed. Morrison said prime contractor Pacific Shipyards International will be working on the 05 level and above, and Missouri crews are taking advantage of the project to do similar restoration on the 04 level.

The need to replace some of the quarter-inch steel decking and bulkhead steel is clearly evident on levels where rust has eaten holes through the floor and walls.

Restoration work will be done primarily on weeknights when the Missouri is closed to museum traffic, officials said. Heavy industrial support equipment will be powered from one of the ship’s shore power bunkers rather than noisy portable diesel generators.

Morrison said plastic containment wrap around the scaffolding should also reduce noise.

“A lot of the major preservation work on the ship now and forever, we have to do it without closing our doors to visitors because we don’t get government money,” Morrison said. “We’re a nonprofit, so the only way we can fund preservation of the ship is through our operations and selling tickets. But we have to preserve the ship.”

The superstructure restoration is expected to be completed in September.