Harry Burkhardt isn't sure what it was. He just knows he encountered "something strange" last month in the boiler room of the USS Olympia.

"I felt two hands grab my arms - and they were icy cold," said the longtime ship volunteer, who described turning around and finding no one there.

The same month, a couple of visitors told Chris Hall, a visitor-services worker, that they saw the translucent form of a man appear.

And in July, visitor Melissa Miller said she saw a "full-body apparition manifest" in the engine room, then vanish.

The unusual happenings aboard the Olympia - last surviving warship from the Spanish-American War of 1898 - have drawn the attention of paranormal investigators, including SyFy's Ghost Hunters, which came to the Penn's Landing attraction this month to check out the reports.

"I'm a complete rationalist," said Jesse Lebovics, manager of the Olympia and submarine Becuna for the Independence Seaport Museum. "I can explain most of it.

"But [the ship] certainly has a colorful enough history that I think if something were to be around, it makes sense it would be around the Olympia," Lebovics said.

Believers have produced digital recordings and photos to help back up their claims, while skeptics remain unconvinced.

"If you can't verify - and replicate it - scientifically, then it's unproven," said Ted Goertzel, a sociology professor at Rutgers University in Camden. "Photos are blurry and recordings filled with static, but people perceive something there. It's like making something out of the clouds in the sky."

What is verifiable are the many deaths reported on the ship during its long years of service, which included the Battle of Manila Bay in 1898, when Navy Commodore George Dewey stood on the bridge and uttered the famous words, "You may fire when you are ready, Gridley."

Most casualties occurred in peacetime. One sailor was crushed by the barrel of a big gun during a firing exercise, another lost his leg when an anchor cable snapped, and still another fell 40 feet from the deck into the engine room.

The Olympia, once a source of national pride, spent World War I in the Atlantic Ocean and later brought home the remains of the Unknown Soldier from France in 1921.

But the deteriorating vessel faces an uncertain future because of the millions of dollars needed to maintain and restore it. Museum officials have considered the possibility of purposely sinking it to turn it into an artificial reef off Cape May, if a new benefactor cannot be found.

As Olympia's fate has been mulled, unusual reports have ticked up.

The visitors who reported seeing an apparition last month "acted like they had a big secret," said Hall, the museum worker. "They seemed a little embarrassed and didn't want to go further. It was a little unsettling for them."

Among the most active spots on the ship are the engine and boiler rooms, where Burkhardt has been restoring equipment, right down to the spit-and-polish shine of brass parts.

After being "grabbed" in the boiler room over the Labor Day weekend, Burkhardt heard "a female voice, saying, 'I think he's OK,' followed by a female laugh," said the merchant marine captain, 53, who lives in South Philadelphia.

One of the more chilling encounters came in the winter of 2008. Burkhardt's son, Kevin, then 12, was working with another volunteer in the engine room when he said they saw a dark form dart across a doorway.

"I looked at the other volunteer, and I couldn't explain his face," said the 14-year-old Burkhardt. "He looked lost. We both ran out."

The same day they saw the reflection of the figure in a pane of plexiglass, turned around, and saw nothing.

Such reports attracted Melissa Miller, founder and investigator with Ghost Research and Investigation of Pennsylvania (GRIP), which is associated with TAPS, the group on Ghost Hunters.

In July, Miller, 38, of Birmingham, Huntingdon County, said she saw an apparition of a man appear. "It came out of a corridor, turned right, walked [to the rear of the ship], and dissipated into the darkness," she said.

"I could see what he was wearing - boots, slacks, a button-down shirt, and short-brimmed hat."

In the boiler room the same day, Miller said she sensed a presence, turned on her recorder, and asked, "Is there someone here?"

When she listened to the recording later, she found a voice - electronic voice phenomena, or EVP. "It was what you'd expect from a sailor. He said, 'I think I'm in love,' " she recalled.

Skeptics such as Goertzel see ghost hunting as a harmless hobby. "It's like other fads - alien abductions and flying saucers," he said. "It's new and exciting, then it dies out - and that is what will happen with this."

After volunteering on Olympia for 16 years, Burkhardt has probably had more unusual experiences than anyone else. He is president of the Cruiser Olympia Historical Society, which is trying to raise money to restore and acquire the ship.

"We were sitting here in the crew's berth deck [in June] and asked, 'Do you mind us being here?' I got a flat-out, 'Get out!' " Burkhardt said. "You don't hear it at the time. You find it later when you play back the [digital] recording."

He has also picked up other voice recordings, including one that told him: "Save the ship!"

But visual experiences are always the most unnerving.

In August, a visitor was standing next to the pilothouse, taking photos of the adjacent Moshulu, when he saw a man in a white dress naval uniform holding onto the wheel, Burkhardt said. Then, the figure was gone.

Another visitor reported a similar run-in last month, Burkhardt said. An 8-year-old girl saw a man inside one of the locked officer's staterooms and hollered to her mother. When she looked again, he had disappeared.

"The mother reported it to the lady at the ticket booth," Burkhardt said.

The volunteer has researched the deaths of many of the ship's sailors, including Navy coxswain John Johnson, who was killed when the 6,000-pound barrel of one of the guns jumped from its carriage and landed on him during a firing trial off San Diego in 1895.

One of Johnson's friends wrote a poem to memorialize the sailor:

No chance for a prayer,The gun "caught him against the bulkhead right here," said Lebovics, pointing to the spot. "We have a drawing by one his fellow crew members showing the gun on top of him."

Not a word of farewell,

To his shipmates around

That loved him so well . . .

His frame, crushed and quivering,

Was borne to the deck;

"No hope," said our surgeon,

"It has broken his neck."

Though well-versed in the history and lore of Olympia, a Philadelphia waterfront fixture for 50 years, Lebovics has never experienced anything out of the ordinary.

"There are a lot of thumps and bumps, noises and movement" that are all explainable, he said. "But when you turn the lights out and walk through the ship, it is definitely creepy."

Contact staff writer Edward Colimore at 856-779-3833 or ecolimore@phillynews.com.