After the Revolution, the navy's restoration was slow, and only with the beginning of industrialisation in 1930 was a large shipbuilding program developed, but not accomplished before the beginning of the Great Patriotic War, the Soviet Union's portion of World War II. As a result, the Soviet Navy during World War II consisted of some old World War I-era ships, some modern pre-war built cruisers and destroyers, and a number of torpedo boats. Unfortunately, much of the Soviet fleet on the Baltic Sea was blocked in Leningrad and Kronshtadt by Finnish and German minefields during 1941–1944 and maimed by mines and air attacks, nevertheless numerous sorties by attack boats and submarines actions were conducted. On the Black Sea with the loss of the main naval base—Sevastopol, and effective actions of axis aviation as well as minefields the effectiveness of large surface ships was limited. The Northern Fleet, composed mostly of destroyers (World War I Novik-class and more modern Design 7 and 7U vessels), played a role in anti-aircraft and anti-submarine defence of allied convoys heading to Murmansk.