On a big day, the waves at Castle’s break so far off Waikiki that surfers call the spot Steamer Lane because they have to wait near the shipping channel.

The waves come fast out of the Pacific before feeling the drag of shallower water there. That means a large swell can turn into a wave with a 25-foot face — even larger if you believe the stories told by surfers.

Hawaiians originally called this spot Kalehuawehe and for many, it is the holy grail of Oahu’s south shore. It doesn’t break very often but when it does, it has the power to sweep a surfer into legend.

That’s exactly what it did for the most famous surfer of all time: Duke Kahanamoku.

In 1917, Kahanamoku caught a wave at Castle’s — named after the kamaaina family whose three-story beachfront home was a prominent landmark — and surfed for more than a mile on a 16-foot-long, 114-pound redwood board without a skeg. He told a journalist years later that the wave could have been 30 feet high.

From Castle’s, Kahanamoku rode into Publics, located off the public baths by Kapiolani Park, then into Cunha’s, named after another family with an estate at the water’s edge. Connecting surf breaks, Kahanamoku kept going through to Queen’s and then to the edge of Canoes, where beachboys gave tourists rides in outrigger canoes.

Cheers erupted on the beach when Kahanamoku stepped ashore.

The ride is a significant part of Kahanamoku’s legacy, which will be celebrated in a Bishop Museum exhibit marking the birth of the waterman 125 years ago. From Sunday through Nov. 30 in the museum’s J.M. Long Gallery, visitors can learn about Kahanamoku’s Olympic swimming victories, his stint in Hollywood, the time he saved eight people from drowning when a boat capsized off Corona del Mar, Calif., and the grace and dignity that made him the state’s official ambassador of aloha.

Museum visitors also will learn how Kahanamoku surfed into history. There are photos, one of his boards and a video game linked to a small surfboard — custom-made by museum staff — that allows visitors to recreate that 1-mile ride.

The story of Kahanamoku’s big day at Castle’s was told (and retold) by beachboys in Hawaii, but it reached the masses in 1924 when American cartoonist Robert Ripley, who was a personal friend of the great surfer, illustrated it in his “Believe It or Not” syndicated feature.

Exact details are scarce, including a specific date for the epic ride. Ripley’s 1924 cartoon said the feat was done in 1916, but many say that is incorrect. The ride was described in a few books and, briefly, in a March 1965 interview Kahanamoku gave to Surfer magazine.

“It lives on as one of the great legends of surfing,” said Fred Hemmings, a former state senator and world champion surfer who knew Kahanamoku. “It’s become larger than life. It’s folklore now.”

When the 69-year-old Hemmings tells the story, which he first heard as a boy, he says the swell that broke at Castle’s arrived in May 1917.

“A very large swell came from the southern hemisphere off New Zealand,” he said. “Back then surfers didn’t know what generated the swell or how it got here but they certainly knew it was big.”

Hemmings believes Kahanamoku and his friends had never seen waves that big off Waikiki.

“I am sure Duke heard, deep in his conscience, the drums of ancient Hawaii beating and challenging him to ride the waves,” he said.

Kahanamoku was well-known internationally by 1917, having won a gold medal in the 100-meter freestyle at the 1912 Olympics in Stockholm and because he gave swimming and surfing demonstrations around the world. He was an early proponent of surfing, which he mastered while growing up on the shore at Waikiki.

But the wave Kahanamoku caught at Castle’s underscored his understanding of the ocean, Hemmings said.

“I tell people that the ocean was his mistress and he had a lifelong love affair with it,” he said. “He was the highest example of being an ocean man. He was the world’s fastest swimmer, the world’s best surfer, one of the world’s best canoe steersman.”

Not everyone agrees on the spot where Kahanamoku finished his ride. Some, including Hemmings, say it was at the Moana Surfrider hotel, not far from Canoes. Hawaii author and surfer John R.K. Clark believes Kahanamoku’s ride was a little shorter and that he came in near Queen’s.

When he wrote his book “Hawaiian Surfing,” Clark researched the event as well as the waves that break at Castle’s, which is mentioned dozens of times in Hawaiian language newspapers going back to 1834, he said.

“Traditionally a lot of the versions have Duke riding up to the beach and stepping onto the sand,” Clark said. “But if you go to the Surfer magazine of 1965 where he was interviewed, Duke said he fell right outside of the Waikiki Tavern, and the Waikiki Tavern is where the Duke statue is now. So in my mind he was coming in from Queen’s.”

Clark, a retired deputy fire chief at the Honolulu Fire Department, has surfed Castle’s when the wave faces were 15 feet and said it’s a challenge. But it is even harder to make a ride last as long as Kahanamoku’s, regardless of where he finished.

“It is not a perfectly peeling 1-mile wave,” Clark said. “There are steep sections and there are flats in between. The challenge is to get through the flats to the next section.”

Michael Wilson, responsible for designing the new exhibit at the Bishop Museum, said there was a period when some doubted Kahanamoku had made such a long ride.

“There was a long time afterward, in the 1930s through the 1950s, where people said it couldn’t be done,” Wilson said. “A lot of people said it couldn’t be done with the board he was riding and that the waves were not that big.”

But the discovery that others made similar rides had Wilson convinced otherwise. Many of those later rides were documented by Clark, who said the waves were ridden by big-wave surfer George Downing, a Hobie Cat sailor and a pair of paipo boarders. And the Kamehameha Day swell of 1995 brought waves with 25-foot faces to Castle’s.

Kahanamoku’s feat is totally believable, Wilson said.

“But for him to do that on a 16-foot, finless board was incredible,” he said. “I think it was the hardest wave he ever rode.”

One of the paipo boarders was Jim Growney, who said he caught a monster wave from Castle’s to the Moana with his friend John Waidelich in 1965. Their boards, wooden precursors to the ubiquitous bodyboards of today, were made out of 3/8-inch plywood. Growney, now 83, still has both of them and said he might take up the sport once more.

The two men caught a wave at Castle’s — farther out than Growney had ever seen it break — and they just kept going. Because they were prone, planing across the water’s surface, they were faster than surfboards of that time, Growney said.

“We only did it once,” said Growney, a retired public relations writer. “I estimated it could be even more like a mile and a half. But I don’t know. We were just two guys out there surfing who had this incredible experience.”

Hemmings, who caught his own huge wave at Castle’s in the summer of 1965, doesn’t believe the stories he’s heard about other surfers making it to the Moana.

When Hemmings told Kahanamoku that he couldn’t get farther than the Kapahulu Groin, Kahanamoku said no one could because the ocean bottom off Waikiki was changed by the construction of the Ala Wai Canal in 1928. The project cut off streams and altered drainage patterns, which allowed sandbars to build, negatively affecting wave lanes.

“There are people now, in subsequent years, who have claimed to have done it,” Hemmings said. “Maybe their imaginations were better than their ride.”

ILLUSTRATION BY MARTHA HERNANDEZ / MHERNANDEZ@STARADVERTISER.COM

ENLARGE PHOTO.

DUKE’S RIDE

In 1917, Duke Kahanamoku caught a wave at Castle’s that he estimated to be 25- to 30-feet high and rode it through several surf breaks — Publics, Cunha’s, Queen’s and, according to some versions, all the way to the Moana Surfrider near Canoes. On such a big day, the waves at each break would crest farther out than normal, which would have given Kahanamoku a straighter line.