The Sōryū-class submarines (16SS) are diesel-electric submarines that entered service with the Japan Maritime Self-Defense Force in 2009. The design is an evolution of the Oyashio class submarine, from which it can most easily be distinguished by its X-shaped tail planes. The Sōryūs have the largest displacement of any submarine used by post war Japan. The class are fitted with air-independent propulsion based on Kockums Stirling engines license-built by Kawasaki Heavy Industries, allowing them to stay submerged for longer periods of time.