returns to port after sinking HMS Royal Oak. The battlecruiser Scharnhorst is seen in the background.]] U-boat is the anglicized version of the German word , itself an abbreviation of Unterseeboot (undersea boat), and refers to military submarines operated by Germany in World War I and World War II. Although in theory U-boats could have been useful fleet weapons against enemy naval warships, in practice they were most effectively used in an economic-warfare role, enforcing a naval blockade against enemy shipping. The primary targets of the U-boat campaigns in both wars were the merchant convoys bringing supplies from Canada and the United States to Europe. Austria submarines of World War I were also known as "U-boats". The distinction between U-boat and submarine is common in English-language usage (where U-boat refers exclusively to the German vessels of the Wo