Surcouf had the largest guns of any cruiser submarine.
A cruiser submarine was a very large submarine designed to remain at sea for extended periods in areas distant from base facilities. Their role was analogous to surface cruisers; cruising distant waters, commerce raiding, and otherwise operating independently. When operating in a fleet, cruisers and cruiser submarines were expected to scout for and screen the battle fleet. Cruiser submarines were successful for a brief period of World War I, but were less successful than smaller submarines during World War II. Large submarines remained vulnerable to damage from defensively equipped merchant ships (DEMS), were slow to dive if found by aircraft, offered a large sonar echo surface, and were less able to defensively maneuver during depth charge attacks.
The First World War combat experience of these submarines encouraged all major navies to build submarine cruiser prototypes between the world wars. Nevertheless, their cost discouraged most from proceeding with 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.
The Imperial Japanese Navy’s focus on forcing a decisive fleet action encouraged development of a wide variety of submarine cruisers, (known as Junsen (巡潜, "Cruiser Submarines")) including the J Type of the 1920s, and the A, B, and C types in the 1940s. Germany decided against building projected 3,140-ton type XI U-boats with an aircraft hangar and four 5-inch (13cm) guns. Long-range submarines with less impressive deck guns, including Type IXD2 U-boats and United States Navyfleet submarines, evolved through the Second World War; and may be identified as cruiser submarines in comparison to submarines designed for shorter patrols over lesser distances.
