November 4, 20178 yr I think from memory "I" types were cruiser/long range oceanic patrol types, and "Ro" smaller, coastal and older types, but don't find any concrete reference now, but as hints: https://en.wikipedia.org/wiki/Submarines_of_the_Imperial_Japanese_Navy#Type_J1_.28I-1.2C_I-2.2C_I-3.2C_I-4.29 http://www.combinedfleet.com/ss.htm https://en.wikipedia.org/wiki/Junsen_type_submarine First "I" type class is J1, with I-1: http://www.combinedfleet.com/type_j1.htm
November 4, 20178 yr At last, as stated in Chapter 6, page 69 of "Submarines of the IJN 1904-1945", by Dorr B Carpenter and Norman Polmar, Conway Maritime Press 1986: Three categories stablished in 1924: - "I" type (Greek letter lambda, actually): first-class or large submarines. - "RO" type: second-class or medium submarines. - "HA" type: third-class or coastal submarines. - Ex-German submarines received from 1919 were designed as "O" types. - The system suffered of omissions and redundances, and a renumering from 20 May 1942 was even worse, renumering l-52 through l-85 to l-152 to l-185, and midget submarines were designated as "HA" types. - German and Italian submarines adquired in WWII were classified as "I" or "RO" types.
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