The raw numbers of tonnage sunk seemingly favor the Germans. U-boat.net's figures cite around 14.1 million GRT sunk by German U-boats between 1939-45. Meanwhile, the American submarines sank around 5.6 million GRT of Japanese merchant ships and 500000 GRT of warships. But the raw numbers comparison is quite deceptive on a number of levels. On the whole, the American submarine effort was far more effective than the Kriegsmarine's and was a major element in the Allied victory in the Pacific.

Of course, one key piece of evidence for this is that the Allies won and the German war effort in the Atlantic failed. The overall picture of sinkings mirrored this overall trajectory of the war. The Kriegsmarine's successes picked up in 1940 after the Fall of France and crested in late-1941/1942, but bottomed out after a brief uptick in March/April 1943. The American effort in contrast had minor successes in 1942, but picked up in 1943, crested in 1944/45 and plateaued in the war's final months.

But the devil is in the details with regards to commerce warfare. One of the big factors in commerce warfare was if sinkings managed to keep pace or overtake new construction. German successes only managed to have the loss rates work to their benefit during the early stages of the "Happy Times" of the early war. However, the Kriegsmarine's early successes sowed the seeds of its later downfall. The U-boat threat energized both American shipyards to ramp up production as well as the chiefs of the RN and USN to take ASW efforts more seriously. Conversely, the Kriegsmarine's production of new U-boats and training could not keep up with its operational losses. The growth of convoying as well as better ASW methods meant that the U-boat arm had to push itself much harder in the Atlantic. The efforts of 1942/43 stretched the U-boat arm to its breaking point, and, predictably, it broke.

Added to these problems, the Kriegsmarine did not remain technologically competitive. The mainstay of the U-Boat arm, the Type VII U-boat, was a solid submarine, but it was a relatively small hull with little flex to jam it with new equipment. Moreover, the Kriegsmarine's early development of radar only narrowly focused on using radar as fire-direction for surface warships. This lack of early-investment in radar meant that the U-boat arm was behind the curve with the growing electronic battlefield in the battles of 1942/43. Much of the Kriegsmarine's electronics effort was focused on passive radar measures like the Metox radar detector to provide a submarine warning of an approaching Allied radar set. Not only were these detection efforts easily spoofed, they did not directly help the U-boats' primary mission, that of the hunter.

In terms of both hunter and prey, the American effort in the Pacific was close to a mirror image of the Atlantic. American fleet boats were large but possessed poor torpedoes and an unexceptional sensor suite. American torpedoes were on paper quite advanced, but a poor to non-existent prewar testing regimen, bureaucratic infighting, and overly ambitious torpedo design meant that a large number of early war torpedoes were duds. The torpedo problems took an incredibly long time to rectify, but the American boats did manage to incorporate search radar and other equipment that made them more effective. The USN managed to make a number of attacks in the early war period, but the submarine effort could not gain traction over other arms until the torpedo problems were ironed out.

In contrast to American shipyards, Japanese shipyards were much more lax in meeting the emerging threat. Japanese shipbuilding only barely managed to cover merchant losses from all sources in 1942 with new construction, and this, along with the failures of US torpedoes, led the Japanese to underestimate the effort needed to meet its logistical demands. Convoy discipline and escort forces remained only rudimentary and the IJN took it for granted that its inner Pacific core was secure. The IJN poured little resources into its ASW forces and tactics so that this arm did not remain competitive by global standards. IJN hydrophones and depth charges were of middling quality at the start of the war and largely remained at this level as the submarine threat grew. It was only by late 1943 and early 1944 that the Japanese began to use prefabrication for merchant construction and tighten up its convoy system. In fairness to Japan, the country's industrial base could likely not have sustained a much more serious effort to beef up its merchant fleet while fighting a ocean-wide war, but nonetheless, Japan was in a poor position to match an increased American submarine effort.

Adding to both Axis powers' woes was the success in Allied codebreaking and decryption of naval cyphers. The breaking of German ENIGMA traffic allowed for Allied ASW to become more proactive and while codebreaking in and of itself did not produce results, it did denude the U-boats of a good deal of surprise. The Anglo-Americans could reroute convoys away from U-boat patrol zones, making the U-boats work much harder to achieve results. The knowledge of these patrol areas allowed the growing Allied airpower to be vectored into areas where U-boats were operating, thus making air attacks more effective. The breaking of IJN and Japanese merchant codes also allowed American wolfpacks to vector to rich targets in the Pacific region. One apocryphal story holds that USN survey teams postwar found that American estimates of Japanese convoy tracking and losses were actually more systematic than their Japanese commands organizing these convoys!

The net result of the improvements in US submarines was that they nearly annihilated a good chunk of Japan's merchant fleet. Even though the Japanese built massive numbers of tankers in 1944, the overall tonnage of its tanker fleet fell from ca. 700000 GRT to ca. 300000 GRT. One of the problems the US submarine fleet had going into the spring and summer of 1945 was that it simply could not find enough worthwhile targets. In a brief span of a few years (1943-early 1945), the American submarine arm had effectively sunk a tonnage equivalent to Japan's peacetime merchant marine. The American submarines in the late-war period saw a proliferation of surface guns because they were a much more cost-effective alternative to torpedoes to interrupt the sampan traffic in the Sea of Japan.

While American losses did creep upwards as its submarine offensive intensified they never reached the scale of the Kriegsmarine's. The U-boat arm lost approximately 400 submarines to all causes and 30000 men. In contrast, the Americans lost 52 submarines to all causes. Even though this made the submarine arm statistically one of the most dangerous parts of American service, German submarine service was far riskier. Moreover, American losses were much more sustainable than German ones. The ratio of losses versus sinkings never really acted in the Germans' favor during the Battle of the Atlantic outside of a brief window early in the war and even when the Kriegsmarine was breaking records in terms of tonnage sunk, it was also losing veteran captains like Gunther Prien and trained crews. These losses had a detrimental effect both on the Kriegsmarine's morale as well as its own larger effectiveness.

Part of the discrepancy between the two submarine was had to do with the quality of the subs' foes. The Japanese had banked on both a short war and the impregnability of its outer defensive zones at the periphery of its newly-seized Pacific territories. Given these assumptions, it made sense not to be too deeply concerned with ASW and convoying since American submarines would be operating at the limits of their range and submarine warfare takes time to effect. The Anglo-Americans though took the submarine threat much more seriously and German early successes, while not enough to be decisive on their own, were able to prod countermeasures that proved to be effective in later stages of the Battle of the Atlantic. Neither submarine arm was omnipotent. The reluctance of a number of BuOrd and other USN chiefs to believe the reports of defective torpedoes prevented the Americans from achieving results much earlier. Doenitz and the Kriegsmarine also drastically underestimated the production capacities of Anglo-American shipyards and German grand strategy often proceeded from these faulty assumptions. But in Japanese the Americans had an enemy that both structurally and institutionally was ill-prepared to meet the demands of a full-scale industrial war. This meant that the learning curve for submarine warfare in the Pacific was much more gradual and could be mastered. The curve in the Atlantic was much steeper, and in the end, insurmountable.

Sources

Davis, Lance E., and Stanley L. Engerman. Naval Blockades in Peace and War: An Economic History Since 1750. Cambridge: Cambridge University Press, 2006.

Elleman, Bruce A., and S. C. M. Paine. Commerce Raiding: Historical Case Studies, 1755-2009. Newport: U S Naval War College, 2013.

Evans, David C. Kaigun: Strategy, Tactics, and Technology in the Imperial Japanese Navy 1887-1941. Annapolis: USNI Press, 2012.