Motorists learned that this month when a faulty bridge pillar caused the shutdown of I-95. But the steel in the column was barely 40 years old, and partly shielded from the elements.

Imagine what happens to a 19th-century warship that hasn't been out of the water for 63 years.

It is Charlie Deroko's job to find out.

His title is marine surveyor, but you could call him a boat doctor - a doctor in a blue jumpsuit who spends much of his time crawling into tight spaces on 58-year-old hands and knees.

Deroko's patient is the Olympia, the grand old relic tied up at Penn's Landing, and he is spending the coming weeks probing the vessel's rusty innards with an ultrasound scanner, in preparation for a multimillion-dollar overhaul.

The goal is to find weak spots that will need to be fixed. It requires venturing into dozens of cramped, moldy compartments belowdecks that are never seen by the public - or by anyone, in some cases, since the ship was retired from commission 86 years ago.

The steel skin of the 344-foot cruiser, which is under the stewardship of the nearby Independence Seaport Museum, has lots of pitted spots below the waterline. The pits are typically a half-inch in diameter, and Deroko must pinpoint as many as he can.

"It's like finding a bunch of dimes in a half-acre field," said ship manager Jesse Lebovics, who is overseeing the work for the museum.

Commissioned in 1895, the Olympia is best-known as Admiral George Dewey's flagship at the Battle of Manila Bay in the Spanish-American War, where Dewey famously told his captain: "You may fire when you are ready, Gridley." The decisive U.S. victory in 1898 heralded the nation's emergence as a world power.

The Olympia later did patrol and escort duty during World War I, and in 1921 it delivered the body of the Unknown Soldier from France to Washington before retiring to Philadelphia.

"There are maybe half a dozen other ships of that importance in the whole world," said Ronald H. Spector, a history professor at George Washington University. "It's the last of its kind."

And Deroko, a Brooklyn resident who has spent his life in and around boats, wants to make sure it endures.

His target one day last week was a sealed chamber at one end of the forward torpedo room.

He put on his jumpsuit, white kneepads and a headlamp. He wore a notebook on a string around his neck and carried a small green device to measure oxygen levels. If metal has been rusting away for years inside a closed space, it sucks oxygen out of the air.

The chamber seemed OK, so in he went, wriggling through an oval opening about a foot and a half wide. Once inside, Deroko still had to scrunch himself into a sitting position to diagnose his patient. The space was perhaps three feet wide.

"This is like a hotel," he said, comparing it with tighter spaces he visited earlier in the week.

Deroko whacked away on various rivets with a hammer, pleased to discover that they responded with a nice, sharp ring. A faulty rivet would produce a dull sound.

Then it was time for the ultrasound. Wearing a protective mask, Deroko scraped away a small circle of the old lead-based paint with a rotating wire brush.

He squirted on some gel, much like what is spread on a pregnant woman's belly. This eliminates any air between the probe and the object being measured, ensuring efficient transfer of the ultrasonic energy.

The device measures the echo from ultrasonic waves that pass through the metal. The machine calculates the thickness of the material based on the known speed of sound traveling through steel.

The verdict: 0.419 inch. It had lost just 4 percent of its original thickness of 0.4375 inch.

In an adjacent storage room, however, nature had taken more of a toll, in a spot where water had drained in through a porthole. Deroko's measurements showed the steel had deteriorated down to 0.317 inch - 27.5 percent thinner than when the ship first set sail.

Decay can be especially severe in spots where dissimilar metals are in contact - a situation that promotes ion exchange, and thus corrosion. Rivets are typically made of different steel than the surrounding plates, for example.

The Olympia's sharp decline began once it was docked at the Navy Yard in 1922. Eventually all but forgotten, its condition grew so bad that Congress threatened to scrap it. A citizens group adopted the vessel in 1957, doing some repairs, but the biggest work remained.

The museum took over in 1996 and has since spent more than $4 million.

The nastiest job was shoveling out more than 30 tons of rust mixed with asbestos. Seven interior bulkheads also were restored, with a grant from the nonprofit group Save America's Treasures.

Deroko's survey will reveal just how much work is needed on the hull. After six weeks on the inside, he will bring in divers for two days to look at parts of the exterior that he couldn't reach.

The ship eventually will be dry-docked, ideally back at the Navy Yard, where problem areas will be patched with epoxy or covered with new steel plates. Lebovics, the ship's manager, estimates the total for hull repairs at $3.6 million.

Funds are an issue for most museums, and this one suffered an added insult. Its former president, John S. Carter, was sentenced to federal prison in November after bilking the institution of more than $1.5 million by prosecutors' estimates. But Lebovics is optimistic now that new management is in place.

"We're making big strides by getting people reinspired," Lebovics said.

He calls Deroko one of a handful of people qualified to tackle the Olympia.

Deroko was a gunner's mate in the Navy, stationed in the Mediterranean until 1972. He later worked on tugs and surveyed boats for New York's South Street Seaport Museum before starting his own survey business in 1998.

Soft-spoken, Deroko has an encyclopedic memory for maritime history. Asked how he developed the expertise, he answered:

"Practice and a lot of reading. There's no formal training for this type of work, actually."

As he and Lebovics clambered about the Olympia, they paused to admire its features. When launched in 1892, it had one foot in the past and one in the future. It was fitted with sails but was one of the first truly modern warships, with twin 9,000-horsepower engines.

And it was among the first to have a "turtleback," a stout protective shield that cuts across the inside of the vessel, sloping up on one side and down on the other, like a shell. In theory, if an enemy torpedo pierced the hull it would then be deflected away from the ship's engines, boiler and other vital systems.

The Olympia was built in San Francisco; the thick armor for its guns came from Pennsylvania's Bethlehem Steel.

One wacky feature: Just inside the vessel's exterior are coffer dams - compartments that were filled with corn husks or other dried vegetable matter in case the hull was penetrated. The theory was that the husks would expand after getting wet, sealing the leak.

"It wasn't perfect," Lebovics said. "They tried everything they could think of."

Standing outside for a breather, as the cars whizzed by on Columbus Boulevard, Deroko proclaimed the ship "magnificent."

Then it was back inside for more ultrasound. His patient was waiting.

at 215-854-2430 or tavril@phillynews.com.