Sentaka I-202 at high surface speed
Class overview
Builders	Kure Naval Arsenal
Operators	Imperial Japanese Navy
Preceded by	I-400-class submarine
Subclasses	I-201-class, Project No. S563 I-207-class, Project No. S56B
Built	1945
In service	1945
In commission	1945
Planned	23
Completed	3
Scrapped	3
General characteristics
Type	Submarine
Displacement	1,291 long tons (1,312 t) surfaced 1,450 long tons (1,470 t) submerged
Length	79 m (259 ft) overall 59.2 m (194 ft) pressure hull
Beam	5.8 m (19 ft) pressure hull 9.2 m (30 ft) max. across stern fins
Height	7 m (23 ft) (keel to main deck)
Propulsion	Diesel-electric 2 × MAN Mk.1 diesel (マ式1号ディーゼル, Ma-Shiki 1 Gō diesel), build by Kawasaki and Mitsubishi. 2,750 hp (2,050 kW) 4 × electric motors, 5,000 hp (3,700 kW) at 600 rpm 2 shafts
Speed	15.75 knots (29.17 km/h) surfaced 19 knots (35 km/h) submerged
Range	15,000 nmi (28,000 km) at 6 knots (11 km/h) 7,800 nmi (14,400 km) at 11 knots (20 km/h) 5,800 nmi (10,700 km) at 14 knots (26 km/h) Submerged: 135 nmi (250 km) at 3 knots (5.6 km/h)
Test depth	110 m (360 ft)
Complement	31 (plan) approx. 50 (actual)
Armament	4 × 533 mm (21 in) bow torpedo tubes 10 × Type 95 torpedoes 2 × Type 96 25 mm AA guns

The I-201-class submarines (伊二百一型潜水艦, I-ni-hyaku-ichi-gata sensuikan) were submarines of the Imperial Japanese Navy during World War II. These submarines were of advanced design, built for high underwater speed, and were known as Sentaka-Dai type submarine (潜高大型潜水艦, Sen-Taka-Dai-gata sensuikan, "Submarine High speed-Large type") or Sentaka type submarine (潜高型潜水艦, Sen-Taka-gata sensuikan, "Submarine High speed type"). The type name, was shortened to Suichū Kōsoku Sensuikan Ō-gata (水中高速潜水艦大型, Underwater High speed Submarine Large type).^[1]

They were one of the fastest submarine class built during World War II, second only to Walter Type XVII closed-cycle powered submarines. Twenty-three units were ordered from the Kure Naval Arsenal under the 1943 construction program. Due to the deteriorating war situation, only eight boats were laid down, and only three, numbered I-201, I-202 and I-203, were completed before the end of the war. None of them saw operational use.

In 1938 the Imperial Japanese Navy constructed an experimental high-speed submarine for evaluation purposes, which was designated Vessel Number 71 (第71号艦) for security purposes. Based on previous experience with high-speed, short-range midget submarines, Number 71 displaced only 230 tons surfaced with a length of 140 ft (43 m). She could attain a submerged speed of over 21 kn (39 km/h; 24 mph), making her the fastest submarine of her day. The results gained from experiments with Number 71 formed the basis for the I-201 class submarines.^[2]

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.^[3]

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)
• Fit large horizontal control surfaces at the stern instead of the more usual bow-mounted dive planes; this improved directional stability and may have decreased turbulence-induced drag.^[4]
• Reduce the crew and crew accommodation to provide battery space; the Sen-Taka was designed for a crew of 31, compared to that the similar-sized Sen-Chu (54), and the Kai Dai 1 (60) Types (in practice the SenTaka needed a crew of 50 when it became operational, leading to an unforeseen habitation problem).
• Restrict the armament, also to save space; the Sen-Taka had the same torpedo outfit as the smaller Sen-Chu 2nd class submarine, and just half that of the comparable Kaidai 1st class submarine. Also the Sen-Taka had no deck gun, and the AA armament carried had to be held in retractable mounts, requiring hull space, in order to meet the streamlining requirement.

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,^[5] 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 53 cm (21 in) 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.

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. The Hawaii Undersea Research Laboratory found the wreck of the I-201 near Hawaii using submersible craft in 2009.^[6]

The I-201 design and technology influenced Japanese Maritime Self Defense Force JDS Oyashio (1959).

A refurbished I-203 is used by the characters in the film Hell and High Water (1954).

• German Type XXI submarine
• Ha-201-class submarine
• Vessel Number 71
• JDS Oyashio (SS-511)

• Lengerer, Hans (2006). "The High-Speed Submarines of the I 201 Class". Warship. 28. Chrysalis Books: 59–77.
• Stille, Mark; Tony Bryan (2007). Imperial Japanese Navy Submarines 1941-45. Osprey Publishing. ISBN 978-1-84603-090-1.
• Stille, Mark. Imperial Japanese Navy Submarines 1941-45. Osprey, 2007.
• "Rekishi Gunzō"., History of Pacific War Vol.17 I-Gō Submarines, Gakken (Japanese publishing company), January 1998, ISBN 4-05-601767-0
• The Maru Special, Japanese Naval Vessels No.132, Japanese submarines I, Ushio Shobō (Japanese publishing company), February 1988

• The Transpacific voyage of the I-400
• Submarines of the Imperial Japanese Navy