(Imperial War Museum photo, scanned from "Submarines of the Imperial Japanese Navy" by Polmar and Carpenter)

I-73 at rest off Ariake Bay, April 24, 1939

Faster than their predecessors, these boats had the highest surface speed of any submarines in the world in the mid-1930s, although their speed was surpassed slightly by later Japanese boats.  These boats were quite successful.  I-168 finished off the damaged USS Yorktown on 7 June 1942, also sinking a destroyer with the same spread of torpedoes.  I-175 sank the escort aircraft carrier USS Liscome Bay in 1943, with a loss of 644 lives.  In return, all eight of this class were lost during the war.   I-168 was sunk by USS Scamp (SS 277) off New Ireland on 23 July 1943.  I-169 was lost during an air attack on Truk on 4 April 1944.  I-70 became Japan's first significant warship casualty when planes from USS Enterprise sank her northeast of Oahu on 10 December 1941.  I-171 was sunk by American destroyers in the Solomons on 1 February 1944.  I-172 was sunk off Guadalcanal by an old converted destroyer USS Southard on 11 November 1942.  I-73 was an early casualty, being sunk near Midway Island by USS Gudgeon on 27 January 1942.  I-174 was lost to accident 3 April 1944.  I-175 was lost to American destroyers on 5 February 1944.

I-174 and I-175 represent the KD6B type, and were about one foot (30cm) longer and 25 tons heavier than shown in the table.  Their range is stated as 10,000 nm at 16 knots.  They, along with I-171, I-172, and I-73, received the 12 cm/50 cal. instead of the 10 cm weapon shown in the table.

These boats were originally numbered I-68, I-69, etc., but were renumbered in 1942, except for I-70 and I-73, which were already casualties.