Submarines China Russia and the US

Kockums makes submarines with Stirling engines. Their site has some descriptions on how stirling engines work and details on the subs.

The submerged endurance for the Gotland Submarine is weeks according to the specs at the bottom of the page.

Apparently it is also pretty darn quiet. The US Navy wants to rent one for a year to practice against it. cite (in Swedish though)

Kockums also makes the Visby class ships that are stealthy. Kinda neat ships.