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.
Submarines of the Imperial Japanese Navy
The submarines were double-hulled medium sized submarines. They were derived from the (KT or K5 type, "Ro-29"-class). Several variants existed. From 1934 to 1944, the (K6 type, "Ro-33"-class) and the (K7 type, "Ro-35"-class) were built. The K6 type was equipped with a 3.25 inch (80 mm) gun and Type 95 Long Lance torpedoes. Twenty units were built: , , "Ro-35", "Ro-36, Ro-37", "Ro-38", "Ro-39", "Ro-40", "Ro-41", "Ro-42", "Ro-43", "Ro-44", , "Ro-46", "Ro-47", "Ro-48", "Ro-49", "Ro-50", "Ro-55", "Ro-56".