BOSTON — All he could see was a torpedo launcher and part of its steering mechanism protruding from the sand, but undersea explorer Robert Ballard knew what he had found: John F. Kennedy’s sunken World War II patrol boat.

Ballard found the famous PT-109 in May, 1,300 feet below the ocean surface and nearly 59 years after it was rammed and sunk by a Japanese destroyer in the South Pacific.

The oceanographer — who also located the Titanic in 1985 — helped host the Monday premiere screening of the movie about his PT-109 expedition at the John F. Kennedy Library and Museum. “National Geographic Explorer: The Search for Kennedy’s PT-109” airs Nov. 24 on cable network MSNBC.

Ballard said there was something special about finding PT-109, the wooden boat that became famous because of the 26-year-old skipper’s courageous effort to rescue his men.

“Over the years, I came to know some members of the Kennedy family … and we would joke, ‘We’re going to find that PT-109,” Ballard said.

“It’s critical to tell our oral history, and what this tells us is about what turned a young boy into a man who later became president of the United States,” he said.

Max Kennedy, a nephew of John F. Kennedy and member of Ballard’s expedition, and Richard Keresey, another PT boat captain who patrolled Blackett Strait with Kennedy the night his boat was rammed, also attended the advance screening of the film.

Kennedy said being there while his uncle’s boat was found sparked a flood of feelings.

“I thought about this instant which really was a significant, formative incident in President Kennedy’s life … and I also thought about my family and what it meant that my father’s brother was on this boat,” Kennedy said.

Kennedy’s boat was leading three other boats on patrol through the Blackett Strait in the Solomon Islands during the early morning hours of August 2, 1943, when the Japanese destroyer Amagiri hit the boat.

Although many historians had believed that PT-109 was split in two by the impact, Ballard said the sonar images captured by his expedition indicate the boat may have had only a small section sheared off or may have only received a glancing blow.

Two men died in the collision, and 11 survived.

The U.S. Navy, convinced that the entire crew had died, held a memorial service for the men.

Over the next few days, Kennedy swam from island to island looking for food and help for his crew. He hauled one of his injured men to an island by swimming with a strap from the man’s lifejacket in his teeth. Finally, on August 7, he carved a message into a coconut shell and gave it to native islanders to take to rescuers; help arrived two days later.

Kennedy was awarded the Navy and Marine Corps Medal for heroism, and also received the Purple Heart for his injuries.

A permanent display at the Kennedy museum includes the coconut shell and other artifacts, including the tattered American flag from PT-109, which had been sent home by a crew member just before the ship sank.

Keresey, who arrived on Ballard’s research vessel in the South Pacific on the day PT-109 was found, said he vividly recalled his own time on a PT boat when he saw the torpedo launcher.

“It was an emotional experience,” said Keresey, now 86. “It was really a thrill seeing that.”

The Kennedy family endorsed Ballard’s expedition — funded by National Geographic — after he agreed not to disturb the site out of respect for the two men who died. To confirm that the boat was PT-109, Ballard compared drawings of the boat’s torpedo launcher and steering mechanism. They matched.