A weakness in the ship's rudder has long been called the fatal flaw in Endurance's design that caused the ship to be crushed by sea ice and sink to the bottom of the Weddell Sea in late 1915.
Researchers at Aalto University in Finland contend in an analysis that the polar ship was not lost because ice tore the rudder away before its keel ripped off, but instead was crushed because it lacked internal beams to hold its hull in place while moving through compression ice.
"Even simple structural analysis shows that the ship was not designed for the compressive pack ice conditions that eventually sank it," Jukka Tuhkuri, who wrote the study, said in a university press release.
"The danger of moving ice and compressive loads -- and how to design a ship for such conditions -- was well understood before the ship sailed south. So we really have to wonder why Shackleton chose a vessel that was not strengthened for compressive ice," said Tuhkuri, sea ice structure researcher and professor of solid mechanics at Aalto.
Tuhkuri and 14 other researchers found the Endurance's wreck in 2022 during the "Endurance22" mission, leading him to conduct a structural analysis of the wreck, as well as Shackleton's diaries, correspondence and other crew communications, to figure out why it sank.
Shackleton left Grytviken in South Georgia in December 1914, sailing south toward Antarctica aboard Endurance, planning to cross the Weddell Sea and reach the continent at Vahsel Bay, according to the analysis.
Once they docked, the expedition was due to cross Antarctica to meet another waiting ship and travel to New Zealand. Endurance did not make it to the continent, however, because in January 1915 it got stuck in ice in the Weddell and it started to drift northward.
By October, the ship had been so compressed by ice and was leaking so badly the crew abandoned it and set up camp on sea ice. By mid-November, the ship sank and the crew drifted North on ice floes and then lifeboats, before reaching Elephant Island in April 1916.
According to the analysis, there were three types of polar ships in the late 1800s and early 1900s: wooden ships based on traditional whaling vessels, wooden expedition ships built for pack ice conditions and icebreaking ships made from steel.
Endurance, built in 1911 and launched in 1912, had three decks, with the lower deck, where the machine room -- for the ships' steam engine and boiler -- was located, having just one deck beam that spanned the breadth of the ship among other discontinuous beams on the deck.
The ship was mostly constructed of oak and pine, with connections of beams and frames strengthened with knees -- spruce on the main deck and iron on lower decks -- and the bow was strengthened with iron.
The boat, which was designed and built for polar tourism and had never made a polar voyage before Shackleton's, lacked beams to give it strength in compression ice.
According to Tuhkuri, Shackleton knew this, even going so far as to mention it in a letter to his wife that noted he had recommended additional beams for another ship that got stuck in compression ice but survived.
Based on the analysis, the rudder and keel being torn off the ship significantly contributed to the ship sinking, but they were torn off because the Endurance was being crushed by ice.
Still, Tuhkuri noted that the reason Shackleton took the risk, on a ship he had questions about from the start, remains a mystery.
"We can speculate about financial pressure or time constraints, but the truth is we may never know why Shackleton made the choices that he made," he said. "At least now we have more concrete findings to flesh out the stories."
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