Submarines of the Imperial Japanese Navy
In 1904 Kawasaki Dockyard Company purchased plans for a modified version directly from Holland, and built two boats (Hulls No. 6 and 7), with the help of two American engineers, Chase and Herbert, who had been assistants to Holland. The Kawasaki-type submarines displaced 63 and 95 tons when submerged, and measured in overall length, respectively. both vessels measured at the beam. This contrasted with the original five imported Hollands-type submarines which had arrived that same year, at over 100 tons submerged, in overall length and beam. The Kawasaki Type #6 and #7 submarines had gained extra speed and reduced fuel consumption by . However both boats could launch only one torpedo, and each was manned by 14 sailors, whereas the imported Holland-type submarines could fire two torpedoes and could be operated by 13 sailors. This new type was designated the Type 6 submarine by the Imperial Japanese Navy, and was used primarily for test purposes.
6th Fleet (Imperial Japanese Navy)
Japan had prior to the attack on Pearl Harbor a diverse submarine fleet, some of which had unique distinctions: the only submarines in existence of over 5,000 tons submerged displacement, submarines over 400 feet in length (until the advent of nuclear power), the 41 submarines in its retinue (and of the world) that could carry specially designed aircraft, and submarines with the longest ranges and highest speeds of any nation. With the development of the Type 95 submarine-launched variant of the Long Lance oxygen-propelled torpedo, Japan not only had the world's most advanced torpedo, but one with the largest warhead.