Romanian military industry during World War I was mainly focused on converting various fortification guns into field and anti-aircraft artillery. Up to 334 German 53 mm Fahrpanzer guns, 93 French 57 mm Hotchkiss guns, 66 Krupp 150 mm guns and dozens more 210 mm guns were mounted on Romanian-built carriages and transformed into mobile field artillery, with 45 Krupp 75 mm guns and 132 Hotchkiss 57 mm guns being transformed into anti-aircraft artillery. The Romanians also upgraded 120 German Krupp 105 mm howitzers, the result being the most effective field howitzer in Europe at that time. Romania even managed to design and build from scratch its own model of mortar, the 250 mm Negrei Model 1916. Other Romanian technological assets include the building of Vlaicu III, the world's first aircraft made of metal. The Romanian Navy possessed the largest warships on the Danube. They were a class of 4 river monitors, built locally at the Galați shipyard using parts manufactured in Austria-Hungary, and the first one launched was "Lascăr Catargiu", in 1907. The Romanian monitors displaced almost 700 tons, were armed with three 120 mm naval guns in 3 turrets, two 120 mm naval howitzers, four 47 mm anti-aircraft guns and two 6.5 machine guns. The monitors took part in the Battle of Turtucaia and the First Battle of Cobadin. The Romanian-designed Schneider 150 mm Model 1912 howitzer was considered one of the most modern field guns on the Western Front.