Shackleton knew his doomed ship wasn’t the strongest before sailing

It has been 110 years since Endurance, often described as the strongest wooden ship ever built, sank after becoming trapped in sea ice near Antarctica. But a reassessment of the evidence reveals that Endurance was actually far weaker than other polar ships of the time – and also suggests that expedition leader Ernest Shackleton was aware of its shortcomings.

Shackleton had planned to trek across Antarctica from the Weddell Sea to the Ross Sea, visiting the South Pole on the way. But Endurance never made it to Antarctica’s icy shore. In 1915, it became stuck in ice in the Weddell Sea and sank – although all crew members survived the disaster using the ship’s lifeboats.

Jukka Tuhkuri at Aalto University, Finland, was involved in the Endurance22 expedition that discovered the wreck of the ship on the seafloor in 2022. Tuhkuri began to wonder why such a sturdy ship sank. But as he explored the history of polar vessels built at the time, he realised there was a simple explanation: Endurance wasn’t particularly strong.

In the late 19th and early 20th centuries, a handful of boats were constructed to cope with sea ice. Some were more oval in shape than a standard ship, with a shallower keel. Both features make it harder for sea ice to get a strong purchase on the sides of a vessel, causing the ice to slide beneath the hull instead. Inside the ships, meanwhile, the lower decks gave the hull greater rigidity, because they crossed from the left to the right side of the ship along its full length, creating box-like structures within the ship that strengthened it.

Endurance had none of these features. It was a relatively long ship with a tall keel. Tuhkuri calculated that, as a result of this design, some of the other polar ships of the time could withstand between 1.7 and 2.7 times greater compressional load than Endurance. What’s more, the ship’s engine was so large that the lower deck could run along only part of the length of the ship, ending at the engine room and creating a weak spot in the hull where there was no reinforcing box-like structure.

When Tuhkuri examined Shackleton’s correspondence, he discovered that the explorer knew about these problems. In a letter to his wife shortly before he set sail for Antarctica, Shackleton confided that Nimrod, a ship he had used during an earlier Antarctic expedition, was stronger. He continued with the expedition anyway. “He was ready to take the risk,” says Tuhkuri.

Predictably, Endurance couldn’t cope with the crushing pressures of the sea ice. The boat was squeezed and bent, and eventually its keel was torn away to leave a gaping hole below the water line.

But by then, the myth that Endurance was the world’s strongest wooden ship had emerged, with its origin possibly being an article in The Times, according to Tuhkuri, and Shackleton perpetuated the idea. It is unclear why he did so, but it is a detail that Tuhkuri says adds colour to the story of Shackleton’s ill-fated expedition. “Endurance was a strong and heroic ship in a poetic sense,” he says. “In an engineering sense, unfortunately, it was not.”