Type 3 Submergence Transport Vehicle (Maruyu)

The Type 3 Submergence Transport Vehicle — known colloquially as Maru Yu (まるゆ, literally "Circle Yu," the Army's designation system) — was a small transport submarine developed by the Imperial Japanese Army, not the Navy. The basic Yu 1 type displaced about 274 tons surfaced and 346 tons submerged, had a crew of 23, and could carry 24 tons of cargo or 40 troops. It carried no torpedoes; its only armament was a 37 mm deck gun.

The boats were designed purely for covert supply transport, to ferry supplies and small numbers of troops to island garrisons that had been cut off by Allied naval and air supremacy. Because the Army had no submarine-building experience, they were built by firms outside the naval shipbuilding industry, including Hitachi, Japan Steel Works, Andō Iron Works and Chōsen Machinery.

The first boat, Yu 1, entered service at the end of December 1943. Some 420 were planned across several series (Yu 1, Yu 1001, Yu 2001 and Yu 3001), but only 38 had been completed by the end of the war. Three boats were sent to the Philippines in 1944 and were lost between November 1944 and January 1945; the remainder operated in Japanese home waters and survived until Japan's surrender in August 1945.

The Maru Yu achieved enormous popularity in the browser game Kantai Collection (KanColle), where she is the only Imperial Japanese Army vessel represented, beloved for her distinctive character design and the historical novelty of an army-operated submarine. In KanColle lore she is famous for her lines referencing the army–navy rivalry.