How German U-Boats Were Used in WWI—and Perfected in WWII

The most formidable naval weapons in both world wars, German submarines devastated trans-Atlantic shipping while sinking more than 6,000 merchant vessels and warships and killing tens of thousands of people. These U-boats (an abbreviation of Unterseeboot, the German word for “undersea boat”) prowled the oceans in search of prey and could attack ships 20 times their size from both above and below the surface with their deck guns and torpedoes.

Inside the dimly lit, claustrophobic submarines, sailors couldn’t shower or even change their clothes during patrols that could last two months at sea. Fifty men shared two toilets—one of which doubled as a food locker at the start of patrols. What’s more, the toilets couldn’t function when 80 feet or more below the surface because of the outside water pressure, according to The U-Boats by Douglas Botting.

U-boat crews inhaled a foul cocktail of bilge water, sweat and diesel fumes. Mildew blossomed on their shoes, and charts even rotted from the oppressive heat and dampness. “I feel like Jonah inside some huge shellfish whose vulnerable parts are sheathed in armor,” wrote German war correspondent Lothar-Günther Buchheim on patrol in 1941.

Submarines were still primitive naval weapons when Germany became the last major naval power to build one in 1906. By the start of World War I in 1914, however, Germany had caught the competition. Its 20 combat-ready U-boats were more sophisticated than other countries’ submarines and could travel 5,000 miles without refueling, allowing them to operate along the entire British coast.

U-Boats Come of Age in World War I

The U-boat fleet made its first strike on September 5, 1914, with an attack on a British light cruiser off the coast of Scotland that killed more than 250 sailors. Seventeen days later, U-9 sank three British armored cruisers within an hour, killing nearly 1,500. Despite these strikes, the Germans lost more U-boats than they sank during the first month of the war.

German submarines parked in harbor, circa 1914. Front row (left to right): U-22, U-20 (sank the Lusitania), U-19 and U-21. Back row (left to right): U-14, U-10 and U-12.

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In February 1915, Germany announced the start of unrestricted submarine warfare in which all vessels, even merchant ships from neutral countries, would be sunk without warning in a war zone around Great Britain. Many Britons had dismissed the idea that submarines would attack merchant ships. That included First Lord of the Admiralty Winston Churchill who wrote, “I do not believe this would ever be done by a civilized power.”

U-boats attacked not only food and oil supplies bound for the British Isles but passenger ships as well. On May 7, 1915, U-20 torpedoed the liner Lusitania off the coast of Ireland and killed nearly 1,200 passengers, including 128 Americans. Alarmed at the prospect of an American entry into the war, Germany eventually pledged to protect the safety of passengers before sinking unarmed ships.

The sinking of Lusitania by a German submarine off the Old Head of Kinsale, Ireland, 1915. The tragedy killed 128 American citizens, helping bring the United States into World War I.

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The Allies struggled to counter the U-boat threat. The Royal Navy camouflaged warships with paint jobs to pose as merchant vessels and piled haystacks to obscure guns. Some British patrols even carried canvas sacks and hammers that they could use to cover and smash U-boat periscope lenses.

After announcing the resumption of unrestricted submarine warfare against Allied and neutral ships on January 31, 1917, U-boats sank more than 500 vessels by the end of April. The U-boats nearly succeeded in defeating Great Britain, but attacks on American merchant ships played an important role in the United States entering the war.