In 1917 the Royal Navy had almost completed its latest submarine, L Class. It is surprising that the British allowed Japan to build this submarine while they were still in the thick of the war. The Japanese company Mitsubishi obtained permission to import the diesel engines, electric motors and batteries built by Vickers and build L Class boats.

L-1 TYPE (Ro-51 CLASS)

Displacement 886/1076 tons
Length 70.59 meters (231’ 7”)
Beam 7.16 meters (23’ 6”)
Draft 3.90 meters (12’ 9”)
Machinery 2 shaft Vickers diesel w/2 electric motors
Power (diesel) 2,400 hp
Power (electric) 1,600 hp
Speed (diesel) 17 knots
Speed (electric) 10.2 knots
Bunkers 75 tons
Radius of action (diesel) 5,500 miles @ 10 knots
Radius of action (electric) 80 miles @ 4 knots
Tubes (fwd) four (18 inch)
Tubes (side) two (18 inch) (carried ten torpedoes)
Deck gun none
AA Guns single 8cm and single .25 cal. MG
Depth of dive 60 meters
Crew 45 men

This boat had its main ballast tank straddling the central portion of the pressure hull, and this design was called semi-double-hull construction. This kind of main ballast tank was sometimes called a saddle tank. What was particularly noteworthy about this boat was its excellent diesel engines. Although somewhat lower in output than Sulzer diesels, the Vickers engines were far more reliable and sturdy, and were easier to maintain. The L Class boats were very successful. The first two boats were completed in 1920, designated as Boats #25 and #26 (later Ro-51 and Ro-52). This boat had, in addition to the four normal torpedo tubes in the bow, two side-shooting tubes - one on each side. There being no space behind the tube, it was made to open lengthwise to accommodate loading a torpedo.

Mitsubishi built further four L-Class boats with slight modifications. Their engines, motors and batteries were now locally manufactured. They were equipped with the K-Tube (an American made hydrophone) for the first time. The first three units were completed in 1921, and were designated as Boats #27, #28 and #29 (later Ro-53, Ro-54 and Ro-55); the fourth unit was completed in 1922, and was designated as Boat #30 (later Ro-56).

L-2 TYPE (Ro-53 CLASS)

Displacement 893/1076 tons
Length 70.59 meters (231’ 7”)
Beam 7.16 meters (23’ 6”)
Draft 3.94 meters (12’ 11”)
Machinery 2 shaft Vickers diesels and 2 Mitsubishi electric motors
Power (diesel) 2,400 hp
Power (electric) 1,600 hp
Speed (diesel) 17.3 knots
Speed (electric) 10.4 knots
Bunkers 75 tons
Radius (diesel) 5,500 miles @ 10 knots
Radius (electric) 80 miles @ 4 knots
Tubes (fwd) four (18 inch) (carried 10 torpedoes)
Tubes (side) two (18 inch) (Ro-53 had her side tubes removed shortly after completion)
Deck gun none
AA guns single 8cm and single .25 cal. MG
Depth of dive 60 meters
Crew 45 men