U-31 is a highly advanced non-nuclear submarine. It is the quietest conventional submarine in the world. It features diesel propulsion and an air independent system using hydrogen fuel cells. The latter enable the vessel to remain submerged for up to three weeks. Four days is the maximum for most other conventional submarines.
It is capable of operating in only 17 meters of water. The hull and internal features are constructed of non – magnetic materials, there is no exhaust heat, machinery is vibration free and the prismatic hull cross-section and shape improve its stealth characteristics.