The Japan Maritime Self-Defense Force just launched what could be a seriously advanced non-nuclear submarine.
Diesel-electric boats had an extended lease on life when the first nuclear-powered SSNs hit the water due to the fact that the German-originated snorkel system became standard post-WWII. Coupled with enhanced hull shapes (also largely pioneered by the Kriegsmarine) snork boats are still viable, although ASW countermeasures then started concentrating on detecting snorkel pipes and targeting same.
Then came X-shaped sterns and Air Independent Propulsion (AIP) which allowed SSKs to sever their ties to the surface in exchange for adding weight and space to the boat in the form of a Stirling system that allowed the vessel to remain operational while submerged for weeks at a time, sans coming to shallow depths to snork.
Currently, at least 10 nations are building AIP submarines while 20 nations are operating them.
Now, the Japanese could have just flipped the desk on the AIP argument by coupling it with better batteries. You see the newest member of the Soryu-class diesel-electric submarine, JS Oryu (Phoenix Dragon), uses enhanced lithium-ion batteries capable of much better performance– more than double the electric storage capacity of traditional lead-acid batteries– and still has an AIP system. Now, we could be talking months without coming to the surface, not weeks.
She launched this week at Mitsubishi’s Kobe yard.