[I was unaware of the small size of the German U-Boat fleet in WW1. Their successes were amazing. I like the German U-boat concept a lot. Remember this is the war when Hitler was only a corporal. But during WW2, the Germans focused a lot of energy on their U-boats. That is a separate topic. Jan]

U-boat, German U-boot, abbreviation of Unterseeboot, (“undersea boat”), a German submarine. The destruction of enemy shipping by German U-boats was a spectacular feature of both World Wars I and II.

World War I

Germany was the first country to employ submarines in war as substitutes for surface commerce raiders. At the outset of World War I, German U-boats, though numbering only 38, achieved notable successes against British warships; but because of the reactions of neutral powers (especially the United States) Germany hesitated before adopting unrestricted U-boat warfare against merchant ships. The decision to do so in February 1917 was largely responsible for the entry of the United States into the war. The U-boat campaign then became a race between German sinkings of merchant ships and the building of ships, mainly in the United States, to replace them. In April 1917, 430 Allied and neutral ships totaling 852,000 tons were sunk, and it seemed likely that the German gamble would succeed. However, the introduction of convoys, the arrival of numerous U.S. destroyers, and the vast output of American shipyards turned the tables. By the end of the war Germany had built 334 U-boats and had 226 under construction. The peak U-boat strength of 140 was reached in October 1917, but there were never more than about 60 at sea at one time. In 1914–18 the destruction—more than 10,000,000 tons—caused by the U-boats was especially remarkable in view of the small size (less than l,000 tons), frailty, and vulnerability of the craft.

Source: https://www.britannica.com/technology/U-boat

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