Structural Weakness Doomed HMS Endurance

Polar explorer Ernest Shackleton wrote in his journal entry on 27 October 1915 that, "She was doomed, no ship built by human hands could have withstood the strain."

He was referring to 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 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."