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Japan had what was easily the most diverse submarine fleet of any nation inthe Second World War.  These included manned torpedoes, midget submarines,medium-range submarines, purpose-built supply submarines (many for use by theArmy), long-range fleet submarines (many of which carried an aircraft),submarines with high submerged speed, and submarines that could carry multiplebombers.

Because of the vastness of the Pacific, Japan built many boats of extremerange and size, many of which were capable of cruises exceeding 20,000 miles andlasting more than 100 days.  In fact, Japan built what were by far thelargest submarines in the world, indeed, the only submarines over 5,000 tonssubmerged displacement, or submarines over 400 feet in length until the adventof nuclear power.  These same boats were credited with a range of 37,500miles at 14 knots, a figure never matched by any other diesel-electricsubmarine.  These large boats could each carry three floatplane bombers,the only submarines in history so capable.  Japan built 41 submarines thatcould carry one or more aircraft, while the vast submarine fleets of the UnitedStates, Britain, and Germany included not one submarine so capable.

During the Second World War, there were 56 submarines larger than 3,000 tonsin the entire world, and 52 of these were Japanese.  Japan built 65submarines with ranges exceeding 20,000 miles at ten knots, while the Allies hadno submarine capable of this feat. By 1945, Japan hadbuilt all 39 of the world's diesel-electric submarines with more than 10,000horsepower, and all 57 of the world's diesel-electric submarines capable of 23+knots surface speed.

The Japanese navy also built submarines with the fastest underwaterspeeds of any nation's combat submarines.  They employed 78 midgetsubmarines capable of 18.5 to 19 knots submerged, and built 110 others capableof 16 knots.  As the war was ending they completed four medium-sizedsubmarines capable of 19 knots submerged.  This exceeds the 17.5-knotperformance of the famed German Type XXI coming into service at the sametime.  As early as 1938, Japan completed the experimental Submarine Number71, capable of more than 21 knots submerged.

Japanese submarines employed the best torpedoes available during theSecond World War.  The Type 95 torpedo used pure oxygen to burn kerosene,instead of the compressed air and alcohol used in other nation'storpedoes.  This gave them about three times the range of their Alliedcounterparts, and also reduced their wake, making them harder to notice andavoid.  The Type 95 also had by far the largest warhead of any submarinetorpedo, initially 893 pounds (405 kg), increased to 1210 pounds (550 kg) latein the war.  All Japanese torpedoes made during the war used Japanese Type97 explosive, a mixture of 60% TNT and 40% hexanitrodiphenylamine.  Mostimportantly, the Type 95 used a simple contact exploder, and was therefore farmore reliable than its American counterpart, the Mark 14, until the latter wasimproved in late-1943.  Japan also developed and used an electric torpedo,the Type 92.  This weapon had modest performance compared to the Type 95,but emitted no exhaust and, therefore, left no wake to reveal itspresence.  Similar electric torpedoes were used by several nations.

Given their size, range, speed, and torpedoes, Japanese submarines achievedsurprisingly little.  This was because they were mainly employed againstwarships, which were fast, maneuverable, and well-defended when compared tomerchant ships.  Japanese naval doctrine was built around the concept offighting a single decisive battle, as they had done at Tsushima 40 yearsearlier.  They thought of their submarines as scouts, whose main role wasto locate, shadow, and attack Allied naval task forces.  This approach gavea significant return in 1942 when they sank two fleet carriers, one cruiser, anda few destroyers and other warships, and also damaged two battleships, one fleetcarrier (twice), and a cruiser.  However, as Allied intelligence,technologies, methods, and numbers improved, the Japanese submarines were neveragain able to achieve this frequency of success.  For this reason, manyargue that the Japanese submarine force would have been better used againstmerchant ships, patrolling Allied shipping lanes instead of lurking outsidenaval bases.  Bagnasco credits the Japanese submarine fleet with sinking184 merchant ships of 907,000 GRT.  This figure is far less than achievedby the Germans (2,840 ships of 14.3  million GRT), the Americans (1,079ships of 4.65 million tons), and the British (493 ships of 1.52 milliontons).  It seems reasonable that an all-out blitz of the American westcoast, the Panama Canal, and the approaches to Hawaii, New Zealand, Australiaand India would have caused the Allies more difficulty than did the navaldeprivations that were actually achieved.  Losing a significant number ofmerchant ships, and also needing to spread meager defenses even more thinlyalong two coasts, would surely have had some substantial consequences for theUnited States in 1942.

The Japanese did, of course, make some attacks on merchant shipping in thePacific and Indian Oceans, but these were the minority of missions. Frequently, they waited for fleets that were never seen, supported spectacularlybrave but inconsequential reconnaissance flights, or toted midget submarinesabout, all of which achieved rather less than was possible with so valuable aresource as the Japanese submarine fleet.  Worse from a naval perspective,Japanese submarines were increasingly employed in running supplies to thestarving garrisons of isolated islands.  The Japanese expended hundreds ofsorties in this way, which might have otherwise been used offensively againstthe Allied war effort.  A submarine's cargo capacity was much less thanthat of a relatively inexpensive freighter.  However, Japan wasunderstandably reluctant to let island garrisons starve.  Additionally,many practically unarmed submarines (including 26 built for Army use) were builtspecifically for the supply role, consuming production resources as well.

For their disappointing achievements, Japanese submarines paid heavily. Japan started the war with 63 ocean-going submarines (i.e., not includingmidgets), and completed 111 during the war, for a total of 174.  However,three-quarters of these (128 boats) were lost during the conflict, a proportionof loss similar that experienced by Germany's U-Boats.  Most of thesurviving boats were either dedicated to training roles or were recentlycompleted and never saw combat.  Of those which saw significant combat, thetoll was very grim indeed.  For example, of the 30 submarines thatsupported the Pearl Harbor attack, none survived the war.

Compared to German submarines, Japan's huge boats were relatively easy tosight visually and with radar, slow to dive, hard to maneuver underwater, easyto track on sonar, and easy to hit.  Japanese hulls were also not asstrong as those of German boats, and therefore could not dive as deeply norsurvive such rough treatment.  Also, they lacked radar until the first setswere installed in June 1944, and never had sets as good as the Alliespossessed.

Compounding these deficiencies, Japan was at war with the United States andthe United Kingdom, two nations embroiled in a vast conflict with hundreds ofU-Boats in the Atlantic, and hence two nations which poured lavish resourcesinto anti-submarine warfare (ASW) research and development.  As an exampleof the fruits of this research, in June 1944 the US Navy sank the I-52 by usingcode-breaking to discover her schedule, finding her at night with radar-equippedcarrier-based aircraft, tracking her underwater with sonobuoys dropped by thoseaircraft, and sinking her with acoustic homing torpedoes dropped by the sameaircraft.  The Japanese could achieve none of these technological feats atthat time.

In the face of such disadvantages, morale declined within the Japanesesubmarine force.  This is reflected in a post-war report prepared by the USand British Navies which states, "It was frankly impossible to believe thatsubmarines could spend weeks on the US west coast 'without contacts,' or spendmore than 40 days running among the Solomons during the Guadalcanal campaign'without seeing any targets.'  Even the Japanese commanding officers couldnot disguise their embarrassment when recounting these tales.  Furtherenlightenment is found in the extremely large number of times the target was'too far away to attack.'"

Sources:

Polmar, Norman and Dorr B. Carpenter.Submarines of the Imperial JapaneseNavy, Conway Maritime Press, 1986.

Boyd, Carl and Akihiko Yoshida.The Japanese Submarine Force and World WarII, Naval Institute Press, 1995.

Bagnasco, Erminio.Submarines of World War Two, Naval Institute Press,1977.