The Kilo class, Soviet designation Project 877 Paltus (Russian: Па́лтус, meaning "halibut"), NATO reporting name Kilo, is a class of diesel-electric attack submarines designed and built in the Soviet Union for the Soviet Navy. The class was built until the mid-1990s, when production was switched to the more advanced Project 636 Varshavyanka variant, also known as Improved Kilo-class in the West.


These attack submarines are mainly intended for anti-shipping and anti-submarine operations in relatively shallow waters. Original Project 877 boats are equipped with Rubikon MGK-400 sonar system (with NATO reporting name Shark Gill), which includes a mine detection and avoidance sonar MG-519 Arfa (with NATO reporting name Mouse Roar).

Newer Project 636 boats are equipped with improved MGK-400EM, with MG-519 Arfa also upgraded to MG-519EM. MGK 400E can detect submarines with 0.05 Pa/Hz noisiness in 16 km and surface vessels with 10 Pa/Hz noisiness in 100 km.^[3] The improved sonar systems have reduced the number of operators needed by sharing the same console via automation.

Anechoic tiles are fitted on casings and fins to absorb the sound waves of active sonar, which results in a reduction and distortion of the return signal.^[4] These tiles also help attenuate sounds that are emitted from the submarine, thus reducing the range at which the submarine may be detected by passive sonar.^[5]