Matchanu-class submarine

The Matchanu class comprised the only four submarines ever employed by the Royal Thai Navy. They were built in Japan by Mitsubishi, and were in commission from 1938 throughout the Franco-Thai war and World War II. The boats were decommissioned following the Manhattan Rebellion in 1951 and the subsequent dissolution of the navy's Submarine Group.

Construction and design
The Royal Siamese Navy had shown interest in submarines in various procurement proposals dating as early as 1910. However, its expansion plans were limited by financial constraints throughout the early twentieth century. In 1934, Sindhu Kamalanavin, then Chief of Staff of the Navy, led a warship-procurement project which was approved by parliament in 1935. The proposal included a 6.9 million-baht budget for three submarines. Bidding was held in October 1935, and was won by Mitsubishi Heavy Industries of Japan, which offered a price of 820,000 baht each for four boats. Siamese navy officers and sailors were sent to Japan to receive training in the operation of the submarines from Imperial Japanese Navy personnel.

Construction of the submarines took place in Kobe, and the first two boats were laid down on 6 May 1936. Construction of the other two began on 1 October. The two pairs of submarines were respectively launched on 24 December 1936 and 14 May 1937. The first pair were completed and handed over to the Royal Siamese Navy on 4 September 1937, and the date is observed as Submarine Day by the Thai Navy. The remaining two were delivered on 30 April 1938.

The four submarines were named after literary characters known for their mythical diving abilities from the epics Ramakien, Phra Aphai Mani and Khun Chang Khun Phaen. They are:
 * HTMS Matchanu (II)
 * HTMS Wirun
 * HTMS Sinsamut
 * HTMS Phlai-chumphon

The submarines were of relatively small size, being designed mainly for coastal defence, and had a displacement of 374.5 t when surfaced and 430 t when submerged. They were armed with four 450 mm torpedo tubes and one each of 76/25-millimetre (3-inch) and 8 mm guns.

Careers
All four submarines departed Kobe on 5 June 1938, headed for Thailand. They stopped for supplies at Keelung in Japanese-controlled Taiwan on 9 June and at Manila in the Philippines on the 15th. The four boats arrived at Sattahip Naval Base on the 25th, and were received in Bangkok with much fanfare on 29 June. They were commissioned on 19 July, together with the Japan-built coastal defence ship HTMS Sri Ayudhya.

The submarines underwent several exercises in 1938 and 1939. In November 1940, the undeclared Franco-Thai war had begun with Thai air raids over French Indochina. The navy was mobilised to protect Thailand's eastern territorial waters, and the submarines conducted reconnaissance in the Gulf of Thailand. They were not, however, able to prevent or participate in the French naval raid which resulted in heavy Thai naval losses at the Battle of Ko Chang on 17 January 1941. Following the battle, the submarines were sent to patrol the vicinity of Ream Naval Base in present-day Cambodia, but no further naval confrontations occurred and the Japanese soon brokered an end to the war.

The submarines were in service throughout World War II, which Thailand officially joined in January 1942, although they did not see any action. However, two of them came to serve unconventional roles during the war. On 14 April 1945, towards the end of the war, Bangkok's Samsen and Wat Liab Power Plants were bombed by allied air raids, leaving the city without electricity. In response to a request from the Bangkok Electricity Authority, the Matchanu and Wirun were assigned to dock at the Bangkok Dock Company, serving as power generators for one of Bangkok's tram lines.

Following the end of the war, supplies and parts for the submarines became unavailable due to the Allied occupation and disarmament of Japan. Although navy personnel had been trained in the manufacture of batteries since during the war, its battery factory was not capable of manufacturing batteries for the submarines. The Thai submarines' service came to an end following a coup attempt known as the Manhattan Rebellion. The failed coup, led by a group of naval officers on 29 June 1951, resulted in the Navy being stripped of its power and influence. The Submarine Group was dissolved on 16 July, and all four boats were decommissioned on 30 November 1951.

The four submarines were for some time moored in the Chao Phraya River near Siriraj Hospital Pier. They were later sold to the Siam Cement Company for scrap. Part of the superstructure of the Matchanu is preserved at the Naval Museum in Samut Prakan Province.