What weapon did the Germans use to fight the British blockade

The Blockade of Germany, or the Blockade of Europe, occurred from 1914 to 1919. It was a prolonged naval operation conducted by the Allied Powers, especially Great Britain, during and after World War I to restrict the maritime supply of goods to the Central Powers, which included Germany, Austria-Hungary, and Turkey.

A wish to retaliate and to break Britain's command of the seas motivated Germany to launch its campaign of unrestricted submarine warfare in 1917. The result was to make the blockade even more complete, by provoking the United States to join the Allies.