The response by the IJN was to produce the Ha-201 (Type STS) class boats - a small and fast submarine group intended for operations in coastal water depths under the program name of Project Number S61. She was of a basic shape with little external details, consisting of a conventional, highly streamlined tubular hull design with bulged sides, a flattened surface deck, a blunt nose assembly and tapered stern. Her running length measured nearly 174 feet. The conning tower was set amidships while the propeller unit was held extremely aft of the cruciform rudder arrangement. The Ha-201 was deliberately built for speed and whatever offensive firepower she could fit, requiring just 22 to 26 crew and fielding only a pair of bow-facing 533mm (21") torpedo tubes. Power was supplied by a conventional arrangement of an intermediate diesel engine of 400bhp when on the surface and battery-powered electric motor of 1,250shp when submerged. Power was outputted to a single shaft fitting producing a top surface speed of 12 knots and a submerged speed of 14 knots. Range was out to 3,000 nautical miles on the surface and 105 nautical miles submerged. An integrated snorkel installation allowed the vessel to operate at periscope depth while still being capable of attack under cover and recharging her batteries and oxygen supply. Point defense was to be handled by just a single 7.7mm anti-aircraft machine gun for no deck gun of higher caliber was afforded the crew.
In all respects, the Ha-201 was in many ways the same class of fighting boat as the German XXI and XXIII series of U-boats half a world away. Japanese development stemmed from data collected in trials before the war began, this accomplished by the experimental "Vessel No. 71" submarine test bed of 1938. In practice, the Ha-201 class would have been of some tactical use but, more or less, operationally limited on a grander scope. Her two torpedo tubes were all her offensive capability (with access to four total torpedo reloads), requiring the vessel to operate in packs and release her "fish" at rather uncomfortable ranges to ensure they would find their mark. By this time in the war, the Allies were enjoying a good deal of air and sea superiority meaning that any Japanese naval vessel - merchant or military - was a prime target from air or sea. The Ha-201 class was also afforded onboard supplies for just 15 days of at-sea operation, vastly limiting her reach before needing resupply.
Some 79 Ha-201 vessels were contracted for construction across five naval shipbuilding yards by Sasebo Naval Arsenal, Mitsubishi Heavy Industries and Kawasaki Shipbuilding Corporation and these vessels would see final assembly from pre-fabricated portions built elsewhere to help curb production times. This endeavor proved rather ambitious considering the constantly changing nature of the war - in particular against Japanese hopes for a more prolonged conflict. Resources began to run scarce or earmarked for other production requirements as needed. The daily Allied bombing campaigns that proved so fruitful in Europe were also making their mark on the Japanese mainland with equal fervor, disrupting much infrastructure and production goals. Couple this Allied success with the successes being encountered across the Pacific Theater and the hope for a 90-strong Ha-201 fleet soon waned.
In the end, only 9 Ha-201 submarines were completed to the point that they were deemed sea worthy though none of these were ever sent out to sea in an operational sortie. Another 28 keels were laid by this time but these were only half-completed boats by the time of the Japanese surrender on August 15th, 1945. The existing examples were subsequently captured and evaluated by the Americans, some technological advances incorporated into the new generation of USN submarines (along with technology garnered from evaluation of German U-boats in Europe).
One Ha-201 class submarine was completed "after" the war on August 16th, 1945, bringing the full sea-ready Ha-201 complement to 10. All were decommissioned and retired, scrapped or scuttled before 1950. Ha-201 submarines were tested to operational depths of 330 feet.
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