Stealth: SSN > AIP > Diesel Electric
No, you don't understand.
Japanese diesel electrics can only stay underwater for a few days before having to surface to recharge its batteries, which makes it vulnerable to detection and being sunk.
Most of the Japanese submarines lack advanced Chinese AIP technology to stay underwater for two weeks. The relatively new Japanese Soryu-class AIP submarine relies on Swedish-built Stirling engines. In contrast, China builds its own AIP submarine engines.
Japan lacks the even more advanced Chinese nuclear submarine technology to stay underwater for three months, which is the ultimate in submarine stealth.
Let me find my post to explain these concepts to you.
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Type 041 Yuan-class attack submarine (SSK) has air-independent propulsion (AIP)
Yuan-class SSKs represent the new generation of PLA Navy conventional attack submarines. With AIP technology, a Type 041 Yuan SSK can stay underwater for two to three weeks at a time.
A Song-class diesel-electric submarine can stay underwater for only a few days without recharging its batteries. In contrast, a Yuan-class diesel-electric submarine with AIP can stay underwater for about two to three weeks.
This means the Type 041 Yuan-class submarine is much more stealthy. It avoids detection by lengthening the time between surfacing (when it's most vulnerable to a P-8 Poseidon sub-hunter) to recharge its batteries.
Indigenous Weapons Development in Chinas Military Modernization
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Yuan Class submarine
2006 First Yuan-class submarine estimated to have entered service.
2007 Second and third Yuan-class submarine launched.
2008 Fourth Yuan-class submarine launched.
2009 Second and third Yuan-class submarines enter service. U.S. intelligence reports that Yuan submarines may have air-independent propulsion (AIP) capability.
2010 Fourth Yuan-class submarine enters service, the fourth of which is a potential redesign of previous versions and incorporates Kilo-class features and AIP technology.
2010 China State Shipbuilding Corporation displays AIP technology.
2011 Fifth Yuan submarine delivered, conducting sea trials in Shanghai.
2011 Projected date for serial production.
As of the 2009 DoD report, China had over 60 submarines in service. Series production of the Yuan-class submarine is expected, and some analysts predict that 'twenty of the class will be built.'
[Note: Thank you to EastWind for the picture.]
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Chinese Type 093 Shang-class Nuclear Attack Submarine (SSN) can stay underwater for months at a time.