Ernest Shackleton’s doomed Endurance wasn’t built to handle polar ice — and he knew it, new analysis finds

“She was doomed, no ship built by human hands could have withstood the strain.”

When polar explorer Ernest Shackleton wrote this journal entry on October 27, 1915, his dreams of reaching Antarctica had been famously crushed — along with his ship, HMS Endurance. The vessel had been ice-locked since January in the Weddell Sea, and by October’s end, the ice had torn off the rudder, ripped the keel, broken the deck beams in the engine room and punched holes in the sides of the boat.

The harrowing events that followed, with Shackleton successfully leading a daring mission to find help after the crew evacuated to an ice floe and the damaged ship sank, left perhaps the biggest impression on Antarctic exploration history. But new research is rewriting what’s known about the root problem that led to the vessel’s downfall.

Shackleton blamed the sinking of the Endurance on the destruction of its rudder; for more than a century, historians accepted this explanation. Now, an analysis suggests that structural weakness caused the Endurance to sink, and that Shackleton knew about his ship’s failings, according to Dr. Jukka Tuhkuri, a professor in the department of energy and mechanical engineering at Aalto University in Finland. Tuhkuri reported the findings Monday in the journal Polar Record.

Wooden vessels for polar exploration typically reinforced the hulls internally with diagonal support beams, which braced the sides of the ship against lateral compression from powerful sea ice, said Tuhkuri, who researches the mechanics of ice. While Endurance had a sturdy outer shell strengthened for collisions, it lacked internal diagonal beams that would have protected it from sea ice’s squeeze.

“It was not designed to take pressure,” Tuhkuri told CNN.

Even after the Endurance sank, Shackleton and others called it “the strongest wooden ship of its time,” Tuhkuri wrote. Yet Shackleton’s correspondence indicates he was well aware of the ship’s structural drawbacks and had misgivings about the vessel. Still, that wasn’t enough to stop him from buying it.

“I think that Endurance was the best possible ship he could get when he needed it and at a price he could afford,” Tuhkuri said.

Pack ice, a type of free-floating ice that drifts on the open ocean, surrounds Antarctica. Carried by currents and wind, densely compressed stretches of pack ice collide to form even bigger masses, and the force of their slow-motion collisions is considerable.

“When two ice floes collide with each other, they form what we call pressure ridges,” Tuhkuri said. “They’re like small mountains, and they can be tens of meters thick.”

A ship that became beset — caught between floes and immobilized — was at the mercy of the ice. For early 20th century polar explorers, being beset was an expected hazard that could last for months at a stretch, and the outcome was never certain, said Dr. Ross MacPhee, senior curator in residence at the American Museum of Natural History in New York City. MacPhee, who was not involved in the new research, has conducted five scientific expeditions to Antarctica and curated an exhibition on Shackleton’s Endurance voyage at the museum.

If a beset crew was fortunate, winds or currents would eventually shift, the ice would loosen its grip and the ship would break free. The German ship Deutschland, for example, was beset for eight months in the Weddell Sea in 1912 before it finally escaped the pack ice.

But if the crew were unlucky, the ice would squeeze their ship until it shattered, MacPhee told CNN.

“Most of the time you got away,” he said. “But sometimes you didn’t.”

Tuhkuri was part of the 2022 expedition that located the Endurance shipwreck; at the time, he was surprised to learn that no one had used structural engineering to evaluate why the Endurance sank.

The Endurance, Tuhkuri learned from historic records, was built for Arctic visits during the summer, not for navigating Antarctic pack ice. Originally called the Polaris, the ship was intended for tourism, specifically for wealthy travelers to hunt walruses and polar bears. But the owners decided to sell the ship before it made a single voyage, perhaps because they were short of funds or due to the looming threat of the First World War, Tuhkuri wrote in the new paper.

For Shackleton, who was probably eager to embark to Antarctica before war broke out, the chance to buy Polaris was too good to pass up. He bought the ship in early 1914 and sailed south several months later, on August 1.

“If he had waited for longer, maybe he would have lost the funding he had,” Tuhkuri said. “And then the war came. So maybe if he hadn’t started, he would have never gone.”

Shackleton’s correspondence with colleagues suggests he knew how important diagonal beams were for resisting compressive ice — in 1911, he advised German explorer Wilhelm Filchner to add diagonal beams to his ship, Deutschland, Tuhkuri found. That likely helped the Deutschland stay intact after it was beset in 1912. And from aboard the Endurance, Shackleton wrote to his wife, Emily, admitting that Endurance was “not as strong” as his previous ship, the Nimrod, adding “I would exchange her for the old Nimrod any day now.”

It’s difficult to say for sure why Shackleton set aside his own misgivings and prior knowledge to take such a huge risk, Tuhkuri said. But in those days, risk-taking was part and parcel of polar exploration — from 1818 to 1909, the death rate for such expeditions was about 6% to 9%.

“You have to hope for the best, do the best you can,” MacPhee said of the explorer mindset during Shackleton’s time. “Some decisions are not necessarily balanced decisions, because ambition very frequently trumps everything else.”

Rather than completing the first overland crossing in Antarctica, Shackleton instead made headlines for leading every one of his men to safety — a feat that also earned him a telegram of personal congratulations from the United Kingdom’s King George V. That heroic tale and uplifting outcome may shed light on why Shackleton’s explanation of the lost rudder as the instrument of Endurance’s destruction was accepted without question for so long, Tuhkuri said.

“When somebody is a legend, it narrows our view,” he said. “It’s easy to accept the story as it is.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine. She is the author of “Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control” (Hopkins Press).

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