The navies of the world now moved to focus their attention on developing boats to serve in the roles of reconnaissance, offensive patrols against enemy warships, and cooperation with the battlefleet— all missions that had been envisaged prior to the outbreak of World War I. They were motivated in particular by the accomplishments of the Imperial German Navy’s submarine arm; the success of Allied countermeasures in the later stages of the conflict and the impending emergence of superior detection equipment; and a general revulsion against the very idea of unrestricted submarine warfare against merchant shipping. Navies in general sought to produce faster, stronger, more powerfully armed, and longer- range boats to fulfill these tasks. The improved submarine types shared many features. Functionally they were submersibles, rather than true submarines; their designs were optimized for operation on the surface, with only limited capabilities submerged. Underwater, they relied for propulsion on electric motors fed by large storage batteries; on the surface, they used diesel engines for propulsion and to recharge the batteries. While they incorporated substantial batteries of torpedo tubes and reload torpedoes, they also carried guns for use against surface or aerial targets. Their operational range was a function of their bunkerage for diesel fuel, while their submerged radius of action was limited by battery cell capacity. Maximum submerged speed usually was not much more than half of their surface speed, and maintaining high submerged speeds was impossible for any length of time without totally draining the batteries and forcing the submarine to the surface. Consequently, most navies conceived of operating their submarines primarily as stealthy surface vessels with the ability to submerge for evasion or escape prior to or after an attack.

German submarine designs exerted a major influence, either directly or indirectly, on most of the world’s submarine development in the years between the two world wars—except in Britain and, to a lesser extent, the Soviet Union. All the major navies of the victorious Allies—Britain, France, Italy, Japan, and the United States—received examples of the latest German U-boats under the terms of the Armistice and the Treaty of Versailles. They intently examined and analyzed these German craft to determine the applicability and suitability of their features for incorporation into their own types and, in several instances, commissioned former German submarines into their own services to acquire operational experience in their use. Both Italian and French designers were very much influenced by studying and operating examples of the later Mittel-U and UB-III types prior to developing their first new postwar boats. The big U-cruisers had even more impact. The first French oceangoing submarines, the Requin class, benefited substantially from their designers’ study of U-cruisers. The big U.S. Navy fleet boats owed a great debt to the German boats (including even their diesel engines, in some cases), and German engineers were intimately involved in the development of the early Japanese kaidai and junsen types.

German design influence spread to lesser fleets too, largely through the activities of the Ingenieurskantoor voor Scheepsbouw (IvS). The IvS was established in July 1922 at Den Haag in The Netherlands by a consortium of the Krupp and Vulcan shipbuilding yards to circumvent the Versailles Treaty’s prohibition on submarine design and construction. The engineering staff was led by Hans Techel, who had headed Krupp’s submarine design team since 1907, and the firm also received clandestine financial support from the German Navy, which was desirous of maintaining German submarine design expertise despite the treaty. IvS engineers produced submarine designs that were constructed for Turkey, Finland, the Soviet Union, Spain, and Sweden, and also served as prototypes for the German Navy’s Type IIA coastal, Type IA long-range, and Type VII oceangoing U-boats.

During the period between the two world wars, the world’s major navies constructed small series of submarines that served the dual purpose of meeting immediate operational needs and also providing data for the development of improved vessels. By the mid-1930s most of these navies had evolved one or two basic types apiece that were well matched to their operational requirements, had attained substantial design maturity, and were suited to large-scale series production. In general, and with very few exceptions, the vast majority of the submarines that served during World War II were of these standardized types, modified in the light of operational experience, rather than radically new designs.

The U.S. Navy took the process of type standardization the furthest, entering World War II with a single basic design that was improved but never replaced during the course of the conflict. These “fleet boats” emerged through the crystallization and synthesis of a series of designs produced to meet requirements for fleet submarines to accompany the battlefleet, cruiser submarines for long-distance raiding, and patrol submarines for offensive operations in the Pacific. Nine vessels, essentially experimental prototypes, in five classes were produced between 1921 and 1934, ranging in size from 1,100 to 2,700 tons standard (defined by the Washington Treaty as fully-equipped except for crew and fuel) on the surface. Overall these submarines were not very successful, suffering problems with their diesel machinery, diving ability, and general reliability, but they provided valuable experience and data for an improved design.

The new series that began with the Porpoise class of 1934 were of 1,310 to 1,475 tons standard on the surface. They introduced diesel-electric reduction drive, which proved vastly more reliable than previous arrangements. Surface cruising range was 11,000 miles at 10 knots, and they had a patrol endurance of 75 days. They could operate for up to 48 hours submerged at 2 knots and had a safe operating depth limit of 250 feet. A battery of 6 to 8 torpedo tubes with 16 to 24 torpedoes was fitted, along with a light deck gun. Between 1934 and 1940, 38 submarines of this group were constructed, and they formed the backbone of the U.S. submarine force when the United States entered World War II.

The Gato class that followed became the first mass-produced wartime class. Displacement rose to 1,526 tons, the torpedo tube battery increased to 10 tubes, and safe depth increased to 300 feet. They were followed by the very similar Balao class, which featured a deeper safe operating depth of 400 feet, accomplished by substituting high-tensile steel for the mild steel used in earlier boats. The Tench class introduced diesel-electric direct drive that brought about a very significant reduction in noise and internal machinery space, leading to the addition of four reload torpedoes to the outfit. A total of 221 submarines from these three classes were completed during or immediately after World War II. Significant wartime modifications included reducing superstructure, adding radar, and enhancing the gun armament by fitting 4- or 5-inch deck guns and adding multiple light antiaircraft weapons.

Japan also constructed very large submarines, intended to operate primarily as integral components of the battlefleet. The kaidai type design was based on a large German cruiser submarine from World War I; the type evolved into a series of 24 boats in five classes, constructed between 1921 and 1935. These vessels displaced between 1,390 and 1,635 tons standard, had operating ranges of between 10,000 and 14,000 miles at 10 knots, carried a battery of 6 to 8 tubes with 14 to 16 torpedoes, could operate submerged for 36 hours at 2 knots, and had a safe operating depth of between 200 and 250 feet. Japan, with considerable assistance from German engineers, also developed very large cruiser submarines of the junsen type between 1924 and 1938. These eight huge vessels had standard displacements of between 1,970 and 2,231 tons and an operational range of 24,000 miles at 10 knots; they could dive safely to 300 feet. In 1939, Japan essentially standardized its large submarine type with a vessel design displacing about 2,100 tons capable of cruising for 14,000 miles at 16 knots, or 24,000 miles at 10 knots, and diving to 330 feet.

Three models were produced: a headquarters type emphasizing communications and command facilities, an attack type emphasizing torpedo armament, and a scouting type that added hangar space and a catapult for a small reconnaissance floatplane. Some 46 of these large submarines were constructed, as well as three others that brought together the facilities of all three types into the sen-toku type, a single monster hull displacing 3,530 tons standard. Japan also constructed 10 final examples of the kaidai type early in World War II. Unlike the United States, Japan also developed and constructed a series of medium submarines intended for coastal work. In addition, considerable effort was expended on midget submarines—small boats with two-man crews intended for stealthy attacks on ports and roadsteads after they had been transported close to the scene of operation by larger submarines. Finally, late in World War II, Japan was developing submarines with high underwater performance, but these never entered service.

German submarines were developed clandestinely, inasmuch as the Versailles Treaty prohibited them in the German Navy. Design work, both at IvS and by the Blohm und Voss firm, continued for foreign navies with production undertaken in the customer’s yards under German supervision. These boats also served as prototypes for domestic production, which made it possible for the first new German submarine, the U-1, to be completed on 29 June 1935, only five weeks after the repudiation of the Versailles Treaty.

The overwhelming majority of the 1,150 U-boats commissioned between 1935 and 1945 belonged to two groups: the so-called 500- ton Type VII medium boats, and the 740-ton Type IX long-range submarines. The Type VIIC actually displaced between 760 and 1,000 tons on the surface, had a cruising range of 6,500 to 10,000 miles at 12 knots on the surface and 80 miles at 4 knots submerged. They had a battery of 5 torpedo tubes with 14 torpedoes, an 88mm deck gun, and ever-increasing numbers of light antiaircraft weapons. Almost 700 of these boats in all of their variants entered service during World War II. The Type XIC actually displaced 1,120 tons; it had a cruising range of 11,000 miles at 12 knots on the surface and 63 miles at 4 knots submerged. They had a battery of 6 torpedo tubes with 22 torpedoes, a 105mm deck gun, and ever-increasing numbers of light antiaircraft weapons. Almost 200 of this type and its variants were commissioned.

Germany also commissioned a number of other important types during World War II. Among the most important were the Type X minelayers and the Type XIV supply boats. Both types operated as resuppliers for the operational boats during the Battle of the Atlantic, providing fuel, provisions, medical supplies, reload torpedoes, and even medical care and replacement crew members. Consequently they became prime targets for Allied antisubmarine forces, and few survived. The other major vessels were the radical Type XXI and Type XXIII boats, designed for high submerged speed and extended underwater operation. Revolutionary streamlined hull shapes, greatly increased battery space, and the installation of snorkels allowed these boats to operate at submerged speeds that made them very difficult targets for Allied antisubmarine forces. Confused production priorities, however, and the general shortage of materials late in the war prevented more than a very few from putting to sea operationally.