The No. 31-class patrol boats (第三十一号型哨戒艇,, Dai Sanjūichi Gō-gata Shōkaitei) were a class of patrol boats of the Imperial Japanese Navy (IJN), serving during World War II. 9 vessels were converted from Momi-class destroyers and 1 vessel was converted from a Wakatake-class destroyer in 1940. During WW2 all were lost except one.
Background
In 1939, the IJN was liberated from London Naval Treaty, and they built many Kagerō-class destroyers. On the other hand, aging of the Momi-class destroyers was serious. Their boilers were degraded due to the operating conditions present in destroyers. Some Momi-class destroyers were not able to show 30 knots speed. The Navy General Staff made the Confidential Document No. 456. It was an order to rebuild them into patrol boats - about four Minekaze-class destroyers and six Momi-class destroyers. The IJN chose nine Momi class and one Wakatake class. Three Momis and one Wakatake were increased, because two Minekazes were decreased.
Rebuilding
Ten vessels were sent to dockyards for rebuilding. "Q" turret, all of torpedo tubes and one boiler were removed in 1940.
In the second half of 1941, the IJN rebuilt them once again for war preparations.[1] They were rebuilt to the landing craft carrier. The Y turret was moved to a place of the "Q" turret. Furthermore, a slope for Daihatsu-class landing craft was installed to their stern, and added deck house for one company of Navy Landing Force. As for the No. 31 and the No. 46, those rebuilt were not done.
First quarter of 1940 at Kure Naval Arsenal. Renamed Patrol Boat No. 36 on 1 April 1940.
Survived war at Surabaya. Seized by Indonesian People's Security Army in June 1946, recaptured by Royal Netherlands Navy in July 1946. Decommissioned on 10 August 1946; scrapped in 1947.
^About the time of their entering a dock, correct historical materials were not left.
^第三十一号哨戒艇 (Dai 31 Gō Shōkaitei). The same shall apply hereinafter.
^1 April 1940, Notice No. 72, Reclassified old destroyers to auxiliary boat. (昭和十五年四月一日 海軍大臣官房達第七十二号 舊驅逐艦ヲ特務艇籍ニ編入シ左ノ通命名ス 海軍大臣 吉田善吾), Minister's Secretariat, Ministry of the Navy. The same shall apply hereinafter.