How was the Imperial Japanese Navy's I-201 submarine able to go 19?
I-201-class submarine - Wikipedia To meet the requirement for high underwater speed the designers had to: Adopt a single-hull structure Locate the main ballast tank higher than previous submarines to give a higher center of gravity and improve dynamic stability Give the pressure hull and casing a highly streamlined form Make the conning tower as small as possible Replace fixed deck guns with retractable mounts housed in shuttered recesses when submerged. Use steel plates for the upper deck rather than wood Install underwater charging system (snorkel) Fit large horizontal control surfaces at the stern instead of the more usual bow-mounted dive planes; this improved directional stability and may have decreased turbulence-induced drag.[4] Reduce the crew and crew accommodation to provide battery space; the Sen-Taka was designed for a crew of 31, compared to that the similar-sized Sen-Chu (54), and the Kai Dai 1 (60) Types (in practice the SenTaka needed a crew of 50 when it became operational, leading to an unforeseen habitation problem). Restrict the armament, also to save space; the Sen-Taka had the same torpedo outfit as the smaller Sen-Chu 2nd class submarine, and just half that of the comparable Kaidai 1st class submarine. Also the Sen-Taka had no deck gun, and the AA armament carried had to be held in retractable mounts, requiring hull space, in order to meet the streamlining requirement. They greatly reduced their combat ability.