Early German submarine engine room. German naval officers in the engine room of a kerosene-burning German submarine in 1911. German submarines were called U-boats, and they played a major role in the First World War (1914-1918). They spent most of their time on the surface due to ventilation problems from the fuel they used. In the years before the First World War, German submarines developed new engine technologies combining diesel fuel and electricity. The diesel engines were used on the surface, and batteries were used while submerged and attacking. During the First World War, the German U-boats were used to attack merchant shipping in an attempt to starve the Allies into surrender. Nearly 5000 ships were sunk by the U-boats during the war.