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.
Anti-submarine weapon
Early Japanese submarines were not very maneuverable under water, could not dive very deep, and lacked radar. Later in the war, Japanese submarines were fitted with radar scanning equipment for improved hunting while surfaced. However, these radar-equipped submarines were in some instances sunk due to the ability of U.S. radar receivers to detect their tell-tale scanning emissions. For example, sank three Japanese radar-equipped submarines in the span of four days. In 1944, U.S. anti-submarine forces began to employ the FIDO (Mk 24 mine) air-dropped homing torpedo against submerged Japanese subs with considerable success.