Sen Taka Sho Type Medium Attack Submarine

The design for this class was developed using data from the experimental No. 71 to create a torpedo attack type with high underwater speed. It featured a streamlined hull, retractable deck fittings and weapons (including a snorkel), spring-loaded cover plates for limber holes, powerful electric motors, and a new high-capacity battery that enabled it to maintain full speed for up to 55 minutes followed by a cruise at 3 knots for up to 12 hours. The design was optimized for mass production with extensive use of prefabrication and an all-welded hull.

The design for this class was developed using data from the experimental No. 71 to create a torpedo attack type with high underwater speed. It featured a streamlined hull, retractable deck fittings and weapons (including a snorkel), spring-loaded cover plates for limber holes, powerful electric motors, and a new high-capacity battery that enabled it to maintain full speed for up to 55 minutes followed by a cruise at 3 knots for up to 12 hours. The design was optimized for mass production with extensive use of prefabrication and an all-welded hull.

The Japanese technological advantage did not wane during the course of the war. It diverged to three different branches: large, small, and fast. The Japanese used their skill to build the largest submarines in the world (Sen Toku Type) as well as some of the most capable small submarines (kaiten and other midgets). Most impressive of all of their designs, however, is the Sen Taka Sho Type medium attack submarine that had an acceptable cruising range coupled with outstanding underwater speed. Had more of these submarines reached operational status earlier, the American forces would have had a unique foe on their hands.

Even with the technological advantages of their designs, the Japanese submarines did suffer from a lack of resources that placed limits on the number of submarines that could be built and on the timeliness of the build process. Also, the Japanese did have a significant delay in developing and installing radar on their submarines. While it was a deficiency, it would not have had significant influence if the focus on operational security had been stronger.

The training of Japanese submarine crews was without equal. The submarines spent long periods of time out at sea constantly practicing elements of the plan for a decisive battle. The intense training periods had such a level of realism that three submarines were lost in prewar training accidents. The training was not without fault however. The overarching focus on the submarine role in the “decisive battle” limited the growth of submarine force capabilities. The overall training gave minimal consideration to key aspects of submarine operations: surveillance, commerce raiding, and sea control (area denial).

The Sen Taka I-201-class submarine took its name from an abbreviation of the Japanese words for “submarine” and “fast”, and it certainly lived up to that christening. It was the only World War II submarine that was easily a match for the German U-boat XXI and was superior to it in the three key areas of power, speed and weaponry.

The hull was fully welded and very carefully streamlined; no gun or other deck obstruction was allowed which might impair the under- water performance. Even the 25-mm (I-in) mount retracted into a streamlined housing in the conning tower. The whole design concentrated on underwater performance, and new electric motors were installed giving the vessels an underwater speed of 19 knots.

The maximum submerged depth achieved by the submarines was 110m (360 ft), the greatest depth achieved by a Japanese submarine. In many respects the vessels resembled the German Type XXI and when completed they were the first operational GUPPY type submarine in the world. Specially designed lightweight MAN diesels were used for surface propulsion, to keep displacement low. Only small bunkerage was pro- vided, and the surfaced radius of action was only 5800 nautical miles with an endurance of 25 days.

The Sen Taka stood out among the Japanese boats, which had something of a reputation for slow manoeuvrability and diving. The two 2,051 kW/2,750hp engines and streamlined welded hulls provided around 17 knots on the surface, but even more impressive was the coupling with heavy-duty battery cells supplying an impressive 3,728kW/5,000hp electric motor capable of achieving 19 knots – double the speed achieved by contemporary American designs. They were equipped with a snorkel, which allowed for underwater diesel operation while recharging batteries.

Eight boats were laid down, but only three were completed before the end of the war, and the commissioning came too late to see any operational activity. Construction employed full mass-pro- duction techniques, with the submarines assembled in section in factories, the completed sections being welded together on the slip. The whole operation from start to finish took on average only ten months. A total of 23 units were ordered from the Kure navy yard under the 1943 Programme, construction commencing in March 1944. A further 76 units were projected under the 1944 Programme, but the progress of the war and the decision to concentrate construction on suicide units led to the cancellation of I-209-122­ in 1945, and the l]nits in the 1944 Programme were never ordered at all. I-201 entered service on February 2, 1945, followed on February 12 by I-202 and on May 29 by I-203. I-204-208 were laid down but never completed and all the boats were surrendered at the end of the war.

The incoming Americans made sure they kept the Sen Taka secrets to themselves.

Two submarines, I-201 and I-203, were seized and inspected by the US Navy at the end of the hostilities. They were part of a group of four captured submarines, including the giant I-400 and I-401, which were sailed to Hawaii by US Navy technicians for further inspection.

On 26 March 1946, the US Navy decided to scuttle these captured Japanese submarines to prevent the technology from falling into the hands of the Soviet Union. On 5 April 1946, I-202 was scuttled in Japanese waters. On 21 May 1946, I-203 was torpedoed and sunk by submarine USS Caiman off the Hawaiian Islands. On 23 May 1946, I-201 was torpedoed and sunk by USS Queenfish.

Ha201 Sen-Taka-Sho fast attack submarine