Updated Mar 09, 2021 | 16:25 IST A fuel-cell based AIP, like the ones, reportedly, to be fitted on the Indian Navy's Kalvari-class submarines, create energy by combining hydrogen and oxygen, with only water created as a by-product. The INS Khanderi at Naval Dockyard in Mumbai. | Photo Credit: PTI Key Highlights Submarines have, in recent decades, become the principal weapon in naval warfare. The challenge then for submarine designers lies primarily in increasing their underwater endurance and improving their stealth A traditional diesel-electric submarine is required to frequently surface in order to recharge its batteries giving it an underwater endurance of just a handful of days – usually 4 to 5 days and, at maximum, about a week