Amid mounting losses to Allied warships, submarines and attack aircraft across the Pacific, the Japanese Navy was forced to enact an emergency submarine-building program during 1943-1944. This initiative produced the impressive I-201-class of fast ocean-going diesel-electric attack submarines. The class was to prove one of the better IJN submarine designs and feature revolutionary features matched only during the period by the groundbreaking Type XXI design of Germany (detailed elsewhere on this site). Of the 22 boats planned, only three were completed and none were sent on war patrols before the war ended in August of 1945. Only eight of the group were ever named / designated (I-201 to I-208).
I201-class hulls were given an electric-welded treatment and relatively lightweight diesel marine engines which helped to reduce displacement while providing for increased diving speeds. A rubber-based coating also aided in sound-reduction and powerful electric motors drove the boat when submerged to speeds nearing 20 knots - this at a time when most other submarines could manage submerged speeds of around 8-10 knots at most.
In the case of I-201 herself - the lead ship of the class - the boat exhibited a length of 259 feet, a beam of 19 feet and a draught of 18 feet. The German-originated MAN diesels outputted 2,750 horsepower and the electric motors combined for an impressive 5,000 horsepower. Both propulsion systems drove the same twin-screw arrangement astern. Surfaced speeds reached 16 knots with undersea speeds reaching nearly 20 knots. Range was out to 15,000 nautical miles.
The crew complement totaled fifty and the boat's endurance reached thirty days at sea before a resupply was in order. The boat's profile was very streamlined and well-rounded with the sail set at midships. The hull was characterized by bulging lower side walls running nearly the entire length of the boat. The aft planes were set low in the design while dive planes were fitted forward on the bow section.
Armament centered on 4 x 533mm (21") torpedo tubes at the bow seated in pairs, two launchers to a hull side. There were ten total reloads which limited the number of "spreads" the boat could put out. For surface work, I-201 was granted the services of 2 x 25mm Anti-Aircraft (AA) cannons, one mounted on a pedestal fore of the sail and the other in a similar mounting aft of the sail. Both 25mm mounts retracted into the hull when traveling to aid in streamlining the vessel.
I-201 was laid down on March 1st, 1944 and launched on July 22nd of that year, finally completed on February 2nd, 1945. I-202 followed with her keel laid down on May 1st, 1944 and being launched on September 2nd, 1944. She was completed on February 12th, 1945. I-203 was laid down on June 1st, 1944 and launched on September 20th, 1944. She was completed on May 29th, 1945.
I-204, I-205, and I-206 were launched in December 1944, February 1945 and March 1945 respectively but never completed. I-207 and I-208 had their construction begin in 1944 and 1945 respectively but were not launched before war's end.
With superb speed, diving capabilities and a healthy endurance window, I-201-class would have proven a fearsome opponent in the hands of a proper captain and capable crew - particularly in large numbers had they been available. However, the war in the Pacific ended before the vessel could be put to good use and the design was eventually claimed by conquering Americans who sent it back to Hawaii for extensive testing of its capabilities and systems.
I-201 was eventually sunk as a target off the coast of Oahu, Hawaii during 1946. I-202 was scuttled at the Goto Islands in April 1946. I-203 was sunk as a target, also off the Hawaiian coast in May of 1946. The hulk of I-201 was only recently discovered in 2009.