hi daniel
What is being proposed here is not an increased output of u-boats/I-boats, rather a re-ordering of priorities, and a change in operational status. For the germans, there were massive diversions of U-Boats to the Med and the Arctic. If these boats had been diverted to the Atlantic, as donitz had wanted, ther would have been a significant increase in tonnage losses, for no additional economic cost to Germany.
In the case of the Japanese, the change would have reequired some change in strategic thinking, that is admittedly difficult to justify. However, there were constant recommendations from the Japanese submarine commaqnders to switch to a mercantile warfare strategy, similar to the german employment. These wre consistently overruled by the Japanese High command, so the resistance to the change in strategy was coming from the top, and not the operational commanders.
If you add the additional 65 I-Boats, and the additional 80 U-Boats to the battle in 1942, the equation starts to look quite different.
To maintain the british economy at subsistence level, the british calculated that they needed a minimum of 7 million tons of shipping. however, the british also had other worldwide committments, to her overseas empire , the neutrals and the dominions. For Australia alone, the committment of shipping was about 2 million tons. Without this committment of shipping, the economic benefits of Australia, who provided the lions share of wheat to feed india, and wool for various military purposes, would have been lost, and Australia would have faced economic ruin within a year. there were similar committments allover the world that had to be met or the grand allianc being forged by the british at the time would have surely failed, as well as very real economic ruin for many members of the United Nations.
I have a book "British shipping in WWII" SG Sturmey Athlone Press 1962. In this book, it discusses this issue in a fair amount of detail. To maintain Britains worldwide committments, Sturmey estimetes a minimum need of not less than 15 million tons. Thats exclusive of any military needs. So when you talk about 18 million tons, and suggest that this represented a comfortable margin for losses, i have to strongly disagree. Allied shipping in 1942 was in very short supply, and every loss was being keenly felt. The needs of operation Torch, for example, caused a reduction in strategic oil reserves in England, such that these reserves were down to about 3 months supply by the beginning of 1943. So the margin for losses was very tight indeed. any significant increase in losses over historical levels was going to have a drastic effect on operations everywhere. The loss of an additional 1 million tons, for example, would have precluded any landings into North Africa, and would have also severely affected the buildup for Alamein 9which demanded the diversion of some 800 merchantmen totalling well in excess of 3.2 million tons of shipping).
It is also important to keep in perspective the actual production of shipping in this period. In 1942, the year we are looking at, the US produced 5.5 million tons, the british 1.3 million, and "others" 0.7 million tons, for a total of 7.5 million tons. The Americans controlled a further 12 million tons of shipping at the beginning of the war, but this was pretty stretched as well, due to the losses that had been sustained by the british in the period 1939-42. in short there was very little fat left on the bone when the US entered the war.
From december 1941 through to June 1942, 4.7 million tons. I have estimated, that with U-Boats diverted from the tactical theatres, and the IJN committed to the tonnage battle, these losses would have been basically tripled, to approximately 11 million tons. At 11-12 million tons, there would have been a nett deficit, compared to history of at least 7 million tons. This is not enough to knock out the allies, but it is enough to bring a complete halt to all offensive operations for at least a year, and would severely test the political resolve of the grand alliance. just as an example, the ability to import grain to India would have been severely strained. there were sever riots in Bengal in 1943 for this very reason (which explains why the Japanese chose to invade in 1944) with even less food availble, it is quite possible that india would have descended into chaos.
It is not possible to win the war by the u-Boats alon, however, the British could only defeat the Axis by the formation and maintenence of a worldwide grand alliance. The security of this grand alliance could be threatened by shipping losses, in the manner I have described. In addition, the loss of even quite small amount of merchant tonnages, brings offensive capability for the allies to a complete stop, probably for at least a year (but depending on the scale of losses). In that period, the Japanese and the germans gain vital breathing time....time to bring Russia to her knees, time to isolate and force the surrender of Australia, time to fortify the Japanese outer perimeter, , time to build the Atlantic wall, time to introduce jet technologies.
You are right to assert that the Allies commanded a massive economic advantage over the Axis. however , where they were not at an advantage, was in the area of manpower. if the offensives of 1942 had not progressed, and the Axis had gained some time with which to prepre more thoroughly, the manpower losses for the allies would have inevitable increased. This may well have been sufficient to force th allies to the negotiating table, which in the latter half of the war was the objective the axis needed to aim for.