Thonburi and her sister ship, HTMS Sri Ayudhya, were designed following the incorporation of the earlier Rattanakosindra-class gunboats into the Siamese Navy in the 1920s. The Rattanakosindra class were British-built ships which featured six-inch guns in two turrets and light armor. Under Plaek Pibulsonggram's command, the Siamese Navy began a series of modernization efforts. Priorities for the navy consisted of protecting the extensive Thai coastline, and coastal gunboats were viewed as the best resource. Several foreign firms from European countries offered a variety of designs, but in the end the Japanese company Kawasaki won the tender.
The new vessels were basically larger versions of the earlier Rattanakosindra ships. The ships were laid down at Kawasaki's facilities in 1936, and Thonburi was launched on 31 July 1937. The resulting "battleships", as they were referred to in Siam at the time, displaced 2,265 tons, featured increased armor protection (protecting machinery and gun turrets), and were powered by twin diesels produced by MAN of Germany.
Armament consisted of four 8-inch (203 mm)/50 calibre guns mounted in two twin turrets. The Japanese 8-inch guns were of the same type as in early Imperial Japanese Navy heavy cruisers and the aircraft carriers Akagi and Kaga. In fact, they were second hand guns left over from Japanese modernization efforts. It is unknown which ships the guns came from, but the carrier Kaga is a possible donor. The main armament had a maximum range of 24,000 metres (26,000 yd) at 25 degrees of elevation. A tower above the bridge featured a fire control tower with a gun director sitting on top to direct the guns. Additional armament consisted of four 3-inch and four 40-mm guns.
The new ships were enthusiastically received by the Royal Thai Navy. Purchasing further vessels of the type was considered by the government, but ultimately it was decided to purchase two Italian-built light cruisers in 1938. Both ships were seized by Italy in 1941 before construction had finished (they were never completed), leaving Thonburi and Sri Ayudhya as the most powerful combatants in Thai service.
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