The Project 633 class (known in the West by its NATO reporting name as the Romeo class) is a class of Soviet diesel-electric submarine, built between 1957 and 1961. A Chinese variant, Type 033, was built in China from 1962 to 1984. Project 633 Project 633 was a further development of the Soviet navy's post-war 611 (Zulu-) and 613 (Whiskey-class) designs. The project was undertaken by Lazurit Central Design Bureau of Gorky. Only 20 of the Soviet Union's originally intended 56 were completed between October 1957 and the end of December 1961 because of the introduction of the nuclear submarine into the Soviet Navy. By today's standards Romeo class submarines are considered obsolete, but they still have some value as training and surveillance vessels. Chinese Type 033 Romeo class and its variants Under the 1950 Sino-Soviet Friendship and Mutual Assistance Treaty, the Soviets passed to China (and later to North Korea) the documentation necessary to produce Romeo submarines in 1963. The Chinese variant is known as the Type 033 Romeo. A total of 84 Type 033 submarines were built in China from 1962 to 1984, plus several exported to other countries. The Chinese Type 033 incorporated some improvements over the original Romeo, including noise reduction of 20 dB. Sonar on board was also continuously upgraded: the original Soviet sonar was first replaced by domestic Chinese Type 105 sonar, which consequently was replaced by H/SQ2-262A sonar built by No. 613 Factory. Today most of the Type 033 subs have been retired or preserved, with a few remaining in service for training purposes. A total of six Chinese Romeo-class submarine variants were developed: Type 6633: Original Chinese built Romeo, China was to have assembled these Romeos from knock-down kits provided by the Soviet Union, six were planned, but only two were completed. The construction program came to a complete stop when the USSR stopped delivery of parts following the Sino-Soviet split. Construction of the third unit at Wuhan was stopped, and available parts were diverted to complete the first pair, but many parts had to be developed indigenously. The primary improvement of Type 6633 over the original Soviet boats is the use of domestic Chinese batteries, which had slightly superior performance to that of the original Soviet batteries. Type 033: Complete domestic production in China was achieved in 1967, and subsequently the project was renamed the Type 033. However, experience from deployment