Let's grant that there is some level of U-boat investment Hitler could theoretically have made to so badly impede British (and if the Alliance lasts long enough, Allied--that is to say, US mostly) shipping that Britain would have no choice but sue for terms. I haven't looked over the above numbers to begin to guess at it. Of course the Allies would respond by diverting more effort into ASW measures, but that costs them too, and from a diminished resource base given sufficient Kriegsmarine magnitude.
How to do it?
{ninja'd by Astrodragon!}
Ok, so it looks like the British started the war with 19,500,000 tons of shipping, and the allies built 270,000-470,000 tons of shipping per month.
If the Germans sank 750,000 tons per month for 24 months, the British would be losing 280,000 to 480,000 tons of shipping per month, roughly 1.5 to 2.5% of their fleet size. In two years, the British would be down to 7,980,000 to 11,280,000 tons of shipping, roughly half their pre-war fleet.
This still seems like a lot of shipping. Could the British stay in the war with about 10,000,000 tons?
If the Germans want to sink more shipping, they can either:
- Increase the number of U-boats (more pre-war production, better survivability, or give U-boat construction priority during the war)
Hitler suffered several constraints before the war limiting pre-production. For one thing, U-boats though far cheaper than the capital ships the KM wanted, still cost, and before the conquest of first Poland then western Europe resources were limited. To build more U-boats he'd have to flatly deny the KM the surface ships they desired. He was also under considerable political constraint, as building U-boats was a red flag waved in Britain's face. He hoped to keep the British out of the war, and if they did happen to come in he wanted them doing so as late as possible, and even hoped to persuade them to drop out if they did declare war. So threatening Britain's shipping and the RN was dangerous for him to do. In those circumstances, he did better to gratify the KM admirals with their surface ship plans since the RN was confident they could contain these. Thus, unless he felt he could keep mass production of U-boats secret from the British--a very bad bet--his hands were tied.
Making U-boats "more survivable" is a design issue, dealt with below.
I believe he did give them priority during the war, at least after they failed to drop out after the fall of France. Given hindsight knowledge of the relative effectiveness of different systems, perhaps funding that went OTL to V-weapons and grandiose tank schemes and so forth might have been better channeled into U-boat production, but it was going great guns OTL anyway.
- Increase U-boat time on patrol (better fuel economy, bigger fuel tanks, or underway replenishment)
- Increase tonnage sunk per U-boat per day at sea (OTL this number was about 750 tons. The Germans could increase this number by fixing their torpedoes during the opening months if the war, or by using effective air reconnaissance to vector U-boats towards ships, instead of U-boats having to make a picket line and hope to stumble on a ship)
All of this is counsels of perfection! Given a time traveller with blueprints of the best designs of 1944, surely each boat made could have been far more effective. Short of this--even if he were to give the KM secret funds to run hidden yards in which to design the most advanced forms they could think of before the war, with systems untested in battle, they'd be guessing wrong a lot. Some technology just plain requires time. Other nifty design features would become infeasible with Allied denial of resources to the Reich.
And all of this risks exposure of German apparent intent to wage heavy war on Britain, which as with churning out mass numbers of known designs would perhaps raise British resolve to face Hitler down say in the 1938 Czechoslovakia crisis, which if the British could get France on board would spell total disaster for Hitler. Neither France nor Britain could do much to aid the Czechs directly, though their diplomatic channels might open up supply routes via Romania for instance, or Poland via Romania--but this is a long shot, as would be persuading anyone in Eastern Europe to allow passage of Soviet allied troops. But France would pose a formidable threat on the German western border, British command of the sea would seal off German seaport access, Hitler would have no access to say Portuguese tungsten except perhaps through Italy, and anyway the Wehrmacht and Luftwaffe at that point were so weak that the Czechs alone could give them a long hard fight, and in the time and attrition they bring, France could invade with crushing force. Many people claim the German Army would mutiny if the Czech crisis escalated to an anti-Entente war like this; even leaving that aside Hitler could not risk war in 1938.
I daresay quite a lot of advanced design work was conducted before September 1939, but after all the various flawed systems you cite were products of that cleverness. Mistakes will be made.
Once France had fallen, Hitler had a lot more resources to work with, including guaranteed access to Swedish iron ore and Portuguese tungsten. And the politics of lulling the RN was moot. The German war machine did have all stops pulled out. We witnessed the OTL results.
- Increase the tonnage sunk through other means (mines, aircraft, surface raiders, etc) I have no clue how to do this.
The Reich tried OTL; probably if you think of something they did not try you'll find it was either in the works but took too long to attain, or was practically unattainable for them. With a time traveler's hindsight they might have dropped dozens of lines of development and focused on just a few and been more effective with these. Effective enough to cut off Britain? Maybe, that depends on numbers. But anyway they did not enjoy this sort of hindsight and had no plausible means of getting it.
If the KM used air reconnaissance to vector U-boats, how much more effective could they have been? I think the majority of ships slipped through the U-boat picket lines undetected durine the first few years of the war. Could the KM make a complete, unobstructed picket line with Fw 200s?
But they did do this too, as soon and as well as they could. I've wondered if airships could have helped them--being inflated with hydrogen of course they'd be vulnerable in the narrow passages past British air defenses, but airplanes managed to evade intercept, and once out over the Atlantic they might be able to stay out for months or even years, provided they could figure out how to survive storms. OTL the USN blimp service conducted something called "Operation Whole Gale" in the late 1950s, to demonstrate they could indeed remain on active duty during the worst stormy season. Prewar the Zeppelin operations were confined to the calm summer months, even the legendary captain and firm head (until demoted by the Nazis) Dr. Hugo Eckener did not want to risk one of his few creations defying Atlantic storms. I think Whole Gale demonstrates they could have, but there would be few opportunities to test this and gain experience before the war even if Zeppelin production were increased a hundredfold. Possibly the non-rigid construction of the much smaller American blimps had something to do with their ability to recover from adverse weather too, but I don't see how.
Perhaps it is a good thing for the West that Hitler, Goering and most Nazis generally despised Zeppelins as symbols of the effete pre-Great War monarchial/aristocratic order, and for being slow and fragile versus virile, manly warplanes! If a half dozen or so Zeppelins could get out over the Atlantic, with hook-on airplanes to widen their scouting range while keeping themselves over the horizon of British observers, I think operating as pure scouts they could indeed vector and coordinate U-boats more effectively, while some U-boats could be detailed to keep the airships supplied with fuel and perhaps ammo. In certain circumstances perhaps airships could even serve to strike at some seaborne commerce directly, with radio controlled glide bombs, perhaps with rocket assist or Argus pulse jet engines, while staying out of range of retaliatory AA. Hook-on fighters would be a poor defense against a squadron of proper land or carrier based fighting planes (even if plane for plane they are equal the surface borne planes would come in greater numbers than an airship could sustain) but might serve quite well to shoot down light seaplanes and so forth. Perhaps not against a Sunderland or something like that!
So I've had some fun imagining an ATL where the Great War KM Zeppelin commander Peter Strasser is not killed off and winds up taking Hitler's place as dictator of an aggressive Germany that goes to war against Britain, but this time with a hundred or so war Zeppelins bearing a few fighters of the Me-109 class or maybe eventually something better, and some light scout planes to serve as eyes for the U-boat fleet. Given fuel, ammo, food supplies and hydrogen refills, I suppose such Zeppelins might remain on station for years, evading RN/RAF attempts to hunt them down, vectoring subs and detecting and attacking Allied submarines too.
But it is pretty marginal. Zeppelin company learned to churn out Zeppelins by the dozens a year during WWI and might, with backing from the government, learned to produce greater numbers of bigger and much more capable airships in the 1940s, at a cost perhaps much lower than even a U-boat. The hitch is their vulnerability once detected and located. As with submarines, there would be no way to build up capacity before the war, though plausibly the Germans could get away with a small fleet of a dozen or so to test out the most state of the art tech to include in the wartime built standard models. So it would be production line versus production line, and perhaps I underestimate the cost per item even under ideal conditions pretty badly, as well as their viability once they manage to find station over the wide Atlantic.
After all, the way to survive a storm is to run with it. This would take them off station and to predictable locations where British fleet and air elements might be lying in wait for them.