Overview and historical setting
Five improved Japanese Kaidai IIIb fleet submarines completed from 1928 to 1930.
Kaidai IIIb belongs to the history of submarine warfare. The class was developed for Imperial Japanese Navy during the interwar period and the Second World War, and contemporary descriptions identify it as fleet submarine. This setting matters because the design should be read against the missions, technology, and fleet organization available to that navy at the time.
Development and program
The hull was lengthened and revised after IIIa trials identified stability, vibration, and speed shortcomings.
For Kaidai IIIb, the documented program can be summarized as construction activity is recorded from 1926 to 1930, 5 planned, 5 ordered, 5 completed, and 0 cancelled. Those figures separate the intended program from the ships that actually reached completion, a distinction that is especially important when requirements changed during a long construction cycle.
The recorded lineage links Kaidai IIIb to no explicitly identified predecessor. Lineage is treated as evidence of design relationship, not proof that equipment, protection, or operating doctrine remained unchanged from one class to the next.
Design and operational role
The technical character of Kaidai IIIb is described through diesel-electric propulsion and large submarine. These characteristics are most useful when connected to the missions assigned to the class, including fleet submarine and long range submarine; they explain what the navy expected the ships to do rather than serving as an isolated list of specifications.
Kaidai IIIb should not be treated as one permanently fixed configuration. Individual members could differ during construction and later change through refits, revised weapons, new sensors, altered boats or aircraft, and changes in crew or command facilities. The class page establishes the shared baseline, while dated ship evidence is needed for any claim about a particular hull.
Ships of the class
The documented roster for Kaidai IIIb contains 5 ships, and the roster is marked complete. The named members are IJN I-156, IJN I-157, IJN I-159, IJN I-60, and IJN I-63.
The roster gives Kaidai IIIb a measurable production history rather than treating the class as an abstract design. Comparing the member pages can reveal construction differences, refits, transfers, renamings, losses, and preservation outcomes that a class-wide summary cannot assign safely to every ship.
Across the member records, the currently documented outcomes are 3 scuttled and 2 sunk. This summary reflects the available ship records and should be revised when stronger evidence changes an individual vessel’s status.
Service history
Three surviving boats were renumbered into the I-150 series in 1942; I-60 and I-63 retained their original numbers until loss.
Service within Kaidai IIIb was not necessarily uniform. Ships built to the same design could enter different theaters, receive different modernizations, or leave service under different conditions. For that reason, an operation documented for one member is not presented as a class-wide experience unless the supporting source makes that connection.
Historical significance and evidence
Shows rapid iterative refinement in Japan’s interwar large-submarine program.
The significance of Kaidai IIIb is best evaluated by comparing its intended roles, the design features selected to meet them, the number of ships actually completed, and the different outcomes recorded across the class. That approach turns the program and roster into evidence about naval priorities rather than treating the class only as a set of measurements.
The evidence used for Kaidai IIIb includes Robert Gardiner (ed.), Conway's All the World's Fighting Ships, 1922–1946, Conway Maritime Press, 1980, Treaty Between the United States of America, the British Empire, France, Italy, and Japan, Limiting Naval Armament, U.S. Department of State, Office of the Historian, 1922, and Kaidai 3b submarines (I56), 1928–1930, Navypedia. These sources support the chronology and identification presented here; where they do not establish a ship-specific event or configuration, the profile says so rather than borrowing details from another vessel.
Ships in this class
Showing 5 recorded class members.
Sources and further reading
- Conway's All the World's Fighting Ships, 1922–1946Conway Maritime Press
- Treaty Between the United States of America, the British Empire, France, Italy, and Japan, Limiting Naval Armament ↗U.S. Department of State, Office of the Historian · accessed 2026-09-18
- Kaidai 3b submarines (I56), 1928–1930 ↗Navypedia · accessed 2026-09-18