Design
By late 1942 it had become apparent to the IJN that conventional submarines were unable to survive the new ASW techniques coming into service, such as radar, HF/DF, sonar, and new depth charge projectors. New submarines were required, with a higher underwater than surface speed, quick-diving capability, quiet underwater running, and a high underwater operational radius.
The IJN General Staff made an official request for high-speed submarines in October 1943 and among the ships planned in 1944 to be constructed in 1945 were 23 "underwater high speed submarines" (Sen taka) temporarily designated "Ships No. 4501–4523".
The General Staff's final requirements were stated in Order No. 295 dated 29 October 1943 to the Navy Technical Department. These included an underwater speed of 25 kn (46 km/h; 29 mph) which was reduced to 20 kn (37 km/h; 23 mph) for practical reasons. Nevertheless, they were the fastest operational submarines of World War II, outpacing even the German Type XXI.
To meet the requirement for high underwater speed the designers had to:
- Adopt a single-hull structure
- Locate the main ballast tank higher than previous submarines to give a higher center of gravity and improve dynamic stability
- Give the pressure hull and casing a highly streamlined form
- Make the conning tower as small as possible
- Replace fixed deck guns with retractable mounts housed in shuttered recesses when submerged.
- Use steel plates for the upper deck rather than wood
- Install underwater charging system (snorkel)
- Large horizontal control surfaces were fitted at the stern instead of the more usual bow-mounted dive planes; this improved directional stability and may have decreased turbulence-induced drag.
The I-201 class bore little resemblance to earlier I-boats, which were optimized for long range and high surfaced speed. By contrast, the I-201 emphasized submerged performance. It featured powerful electric motors, streamlined all-welded hulls, and a large capacity battery consisting of 4,192 cells. The maximum underwater speed of 19 knots (35 km/h) was double that of contemporary American designs. The I-201s, like other Japanese submarines of the period, were also equipped with a crude snorkel, allowing underwater diesel operation while recharging batteries.
I-201 displaced 1,291 tons surfaced and 1,451 tons submerged. It had a test depth of 360 feet (110 m). Armament consisted of four 21 in (53 cm) torpedo tubes and 10 Type 95 torpedoes. The two 25 mm anti-aircraft guns were housed in retractable mounts to maintain streamlining. The submarine was designed for mass production, with large sections prefabricated in factories and transported to the slip for final assembly.
Read more about this topic: I-201 Class Submarine
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