We Finally Know What Sank the U.S.S. San Diego During World War I

After six visits to the ship and sophisticated modeling, historians have concluded that a German mine sunk the cruiser off the coast of New York in 1918

USS San Diego
Naval History and Heritage Command

World War I was a triumph of logistics for the United States Navy. After formally entering the fight on April 6, 1917, the U.S. Navy helped escort and transport 2 million American doughboys to Europe, landing 10,000 a day by the summer of 1918. By the Armistice that November, just 431 Navy personnel had been killed in action, and only a tiny handful of ships were lost to the enemy.

One of the ships that did not see the war through was the U.S.S. San Diego, which sank about eight miles off the coast of Long Island in July 1918 after an explosion ripped through its hull. Though an early inquiry blamed a sea mine, questions lingered about what caused the ship’s demise. Could it have been a crew accident, a saboteur a torpedo from a U-boat that caused the ship to capsize? Now, reports Ian Stewart at NPR, a Navy investigation has determined once and for all that the ship did, indeed, hit a mine set by German submarine patrolling the area.

Of the more than 1,000 sailors aboard the San Diego, six were killed and six were injured as a result of the explosion; the rest were rescued by liferafts and passing ships. After the impact, the captain and crew tried to get the ship to shallow water so it could be salvaged, but they were unable to make it. The vessel overturned and sank about a half an hour after the initial blast. While the Navy court of inquiry concluded "the loss of the U. S. S. San Diego was due to an external explosion of a mine,” the fact that the explosion occurred aft of the ship’s widest point led to suspicions that the damage may have actually been caused by a torpedo.

To mark the centennial anniversary of the ship’s sinking, Eric Niiler at History.com reports that a team of investigators from 10 government agencies and academic institutions spent the past two years researching to find the conclusive answer. Using archival documents, as well as 3D scans of the shipwreck, the team was able to create sophisticated models of how the ship flooded and how the explosion impacted its hull. Ken Nahshon, research engineer at the Naval Surface Warfare Center Carderock Division, tells Niiler that the results are consistent with hitting a mine. The flooding model also shows that the design of the ship’s coal storage compartments probably led to its quick sinking, not mistakes by the crew.

The mine itself didn't do too much damage to the San Diego. But since the ship was preparing to sail across the Atlantic to Europe, its holds were fully packed with coal—it even had 150 tons of the stuff sitting on its open deck. When sea water rushed through the breached hull, the top-heavy ship went all wobbly. “The eye-opening part to us was that there was so many openings inside the ship,” Nahshon says.

Alexis Catsambis of the Naval History and Heritage Command’s Underwater Archeology Branch announced the conclusion at a panel discussion at the American Geophysical Union’s fall meeting.

“With this project we had an opportunity to set the story straight and by doing so, honor [the memory of the six men who died] and also validate the fact that the men onboard did everything right in the lead up to the attack as well as in the response,” Catsambis said in a press release.

The researchers also believe they know which U-boat set the mine. U-156 was one of Germany’s deadliest weapons, and during its 13 months of service, the U-boat sank 44 Allied ships and conducted the only known attack on U.S. soil during the war, launching torpedoes at tugboats docked in New Orleans. It had been spotted patrolling the area near the final resting place of the San Diego shortly before the ship went down. Ironically, it is presumed to have been sunk by American mines on its way back to Germany.

While the sinking of one ship is really a footnote in the annals of WWI, the impact of submarine warfare is not. As Stewart at NPR points out, Woodrow Wilson may have won re-election in 1916 promising to keep the U.S. out of the European conflict. But in early 1917, Germany re-authorized “unrestricted submarine warfare” attacking any and all ships aiding their enemies—including those from neutral nations. After Germany sank several U.S. merchant vessels, public sentiment turned quickly, and the U.S. entered the war, bringing a fresh influx of troops and supplies that turned the tide of the conflict onboard ships like the San Diego.

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