 These distant patrols enjoyed unique immunity to the defensive convoy measures which limited successful submarine attacks in the vicinity of the British Isles.The First World War combat experience of these submarines encouraged all major navies to build submarine cruiser prototypes between the world wars, but their cost discouraged most from further production.Developments were further limited by the London Naval Treaty of 1930, under which each signatory was permitted to possess no more than three large submarines, each above 2,000 tons (2,032 metric tons) but not exceeding 2,800 tons (2,845 metric tons) standard displacement, with guns not exceeding 6.1 in (150 mm) in caliber.