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An I prefix, equivalent to the Western A, denoted a larger submarine designed for fleet or cruising work. These two functions were tending to merge, for the 'fleet' concept was a hangover from earlier British ideas of using large boats with a good surface performance to act closely as an element of the surface fleet, a concept that was not successful at that time. The 'I-15' class was, therefore, derived from twin sources. First of these was the 'Type KD' fleet submarine of the mid-1930s, capable of a 23-kt surface speed and a range suitable for a return trip across the Pacific. The other was the 'Junsen', or cruiser submarine, of a slightly later date, which incorporated one or two floatplanes in a pressuretight hangar forming part of the superstructure.

 

It would seem that the idea of these aircraft was to increase the boat's scouting capability rather than for offensive purposes. Really the first variant 'Type B1' of three, the '1-15' group was 20 strong, with the hangar a low, streamlined structure protruding (usually forward) from the tower. The freeboard was high to improve aircraft handling in a seaway, and was made higher by a sloping catapult track; a folding crane was also incorporated for recovery purposes. A 140-mm (5.5-in) gun was set on a substantial bandstand. In practice, the aircraft and its equipment proved more trouble than they were worth, and several boats had such provision removed in favour of a second gun to suit them better for the attack role. As such, the boats were among the more successful of Japanese classes, being credited with the sinking of eight warships (including the carrier USS Wasp ) and 59 merchantmen of about 400,000 gross registered tons.

 

Despite these successes, the losses of the 'I-15' boats were catastrophically high, as a result mainly of their poor submerged performance and of the fact that only three full salvoes of torpedoes were carried; only one boat of 20 survived to surrender. A couple of the class, along with others from the very similar 'Type B2' and 'Type B3' variants, were modified to carry Kaiten (suicide midget submarines), The 'Type B2' was the 'I-40' class (six completed) and the 'Type B3' was the 'I-54' class (three completed).

 

Specification 'I-15'class

 

Type: ocean-going submarine

 

Displacement: 2,590 tons surfaced and 3,655 tons submerged

 

Dimensions: length 108.60 m (356 ft 4 in); beam 9,30 m (30 ft 6 in); draught 5.10 m (16 ft 9 in)

 

Propulsion: surfaced diesels delivering 12,400 bhp (9245 kW) and submerged electric motors delivering 2,000 hp (1490 kW) to two shafts

 

Speed: surfaced 23.5 kts and submerged 8 kts

 

Range: surfaced 26000 km (16,155 miles) at 16 kts and submerged 185 km (115 miles) at 3 kts

 

Armament: one 140-mm (5.5-in) gun, two 25-mm AA guns,

 

one Yokosuka E14Y1 aircraft, and six 533-mm (21-in) torpedo tubes (all forward) with 17 torpedoes

 

Complement: 100