Over the past year, China has closed multi-billion dollar deals to provide very quiet diesel-electric submarines to the Royal Thai Navy; the Pakistan Navy and the Bangladesh Navy.

Most of the submarines sold to these countries are the export version of China's Type 039A diesel-electric submarine (SSK), whose designation is S-20. The NATO reporting name for the Type 039A, which is also referred to as the Type 041, is the Yuan-class. 

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Chinese state-controlled media said Chinese submarines "are relatively cheap but offer advanced technology and designs." It also noted the Chinese subs can be modified to meet customers' requirements, "making them more competitive in the global market."

China seems particularly pleased with the sale of three Type 039A boats to the Royal Thai Navy, a deal worth over $1.04 billion that was closed last January. These submarines have been designated S26T by the Royal Thai Navy.

The Thai government said the purchase had been approved on April 18, and a budget of $393 million was set aside for the first S26T submarine, which will be delivered between 2017 to 2021. The second and third submarines will be purchased from 2022 to 2028.

The Pakistan Navy will acquire eight modified diesel-electric attack submarines from China by 2028, and the deal was estimated to be worth about $5 billion.

In April 2015, Pakistan approved purchase of eight S-20s for $5 billion. Four of the submarines will be built in Pakistan with Chinese technical assistance and four in China. China will deliver four of the boats by 2023 with the other four by 2028.

The S-20 displaces about 2,300 tons, but air independent propulsion (AIP) isn't standard with the submarines bought by Pakistan. AIP doesn't require a separate oxygen supply, an advantage that allows SSKs with AIP to stay underwater for up to four weeks without using snorkels to recharge their batteries.

Reports in Pakistani media, however, claim the Pakistan Navy's S-20s will have AIP since the navy's Agosta 90B submarines have the technology.

The Bangladesh Navy in November 2016 took delivery of two refurbished Type 035 (Ming)-class diesel-electric submarines. These obsolete boats were built in China from 1962 to 1984.