Type D submarine

The I-361 class submarine (伊三百六十一型潜水艦), also called Type-D submarine (丁型/潜丁型潜水艦) or Sen'yu/Sen'yu-Dai type submarine (潜輸型/潜輸大型潜水艦) was a type of the 1st class submarine in the Imperial Japanese Navy serving during the Second World War. The type name, was shortened to Yusō Sensuikan Ō-gata (輸送潜水艦大型).

The I-373 class submarine (伊三百七十三型潜水艦), also called Type-D Modified submarine (丁型改潜水艦) was different from the I-361 class, however since the I-373 was a development form of the I-361 class, this article describes both of them.

Construction
After the Battle of Midway the IJN immediately planned a transport submarine. The type was based on the U 155 Deutschland. Her duties were transportation of troops (110 men, 10 tons freight and two landing craft) in the areas where the enemy had air superiority. Later the demands for her were changed in sequence. The final demands were 65 tons in the hull and 25 tons on the upper deck (freight only). In the beginning the IJN did not intend to arm these boats with torpedoes. Later, after strong demands from the front commanders, it was decided to arm them with torpedoes for self-defense. The I-372 class was designed as a tanker submarine based on the I-361 class. They were not allowed to be loaded with torpedoes.

Service
In 1944, the submarines were tasked with transport missions between from mainland Japan to remote islands. They had little success and suffered great losses. Of the 13 submarines, only four survived the war.

Kaiten missions
In 1945, several submarines were converted to be Kaiten mother ships and assigned to the suicide attack operations for the Shinchō Tokubetsu Kōgekitai (神潮特別攻撃隊). Their deck guns were removed and fittings for five Kaitens were installed on their decks.
 * JAPANESE SUBMARINE I-361
 * Assigned to the Todoroki group (轟隊) on 24 May 1945, no success.
 * JAPANESE SUBMARINE I-363
 * Assigned to the Todoroki group on 28 May 1945, sank (or damaged) one motor ship on 15 June 1945.
 * Assigned to the Tamon group (多聞隊) on 8 August 1945, no success.
 * JAPANESE SUBMARINE I-366
 * Assigned to the Tamon group on 1 August 1945, no success.
 * JAPANESE SUBMARINE I-367
 * Assigned to the Shinbu group (振武隊) on 5 May 1945, damaged USS Gilligan on 27 May 1945.
 * Assigned to the Tamon group on 19 July 1945, no success.
 * JAPANESE SUBMARINE I-368
 * Assigned to the Chihaya group (千早隊) on 20 February 1945, no success.
 * JAPANESE SUBMARINE I-370
 * Assigned to the Chihaya group on 20 February 1945, no success.

Class variants
The Type-D submarines were divided into four classes:
 * Type-D/Sen'yu (丁型/潜輸（伊三百六十一型）)
 * Type-D/Sen'yu Mod. (丁型/潜輸改（伊三百七十二型）)
 * Type D Mod. (丁型改（伊三百七十三型）)
 * Type D Mod.2/Type D 2 (S60 Type) (丁型改2/潜丁2型（第2968号艦型）). The 2968th vessel class boats were not built and remained only a design.

I-372 class
Project number S51B. She was going to become a lead ship of the Modified Type D submarines (I-373 class), at first. However, the IJN wanted a submarine as soon as possible. She was built as a tanker submarine according to revised I-361 drawings.

I-373 class
Project number S51C. Improved model of the I-361 class. Furthermore, the IJN was planned reinforced model of the I-373 class, also. However all of them were cancelled.