Unfortunately they seem to breed like b***y bunnies...
As it is hitting them over the head with facts is turning into AH's version of whack-a-mole..(whack-a-sealion?)
Everyone loves a successful Sealion as it is the only military way to achieve German victory in WW2 after 3rd September 1939. The alternatives such as a successful Barbarossa can be countered by an eventual Anglo-American nuclear attack if conventional warfare proves too slow and costly. The only real alternatives are political PODs involving changes in the British or American leadership, either in 1940 or due to a loss of patience in 1943-4. German victory in WW2 is attractive because it produces very significant differences in all aspects of subsequent history. For example, the list of banning offences on AH.com will be very different!
The problem is that describing a plausible successful Sealion is challenging. There does not seem to be a POD after May/June 1940 giving any chance of success. Nobody even came up with a plausible POD starting from January 1940 in a recent series of posts https://www.alternatehistory.com/discussion/showthread.php?t=162318. Unfortunately, I tried to find a POD starting from September 1939 and in consequence have wasted a significant amount of time. On the principle of “our men shall not have died in vain”, I am now wasting further time to inflict it on you (including some repeated ideas from the earlier thread).
The POD has some weaknesses or at least improbabilities. It starts from Adolf Hitler, who we all know was a stable and rational individual, and assigns to him both some rather unusual beliefs and a slightly unusual manner of thinking. The beliefs basically come down to the idea that the worms who had yielded at Munich would only dare to oppose him now (September 1939) if they had been promised assistance from America and thus that he is confronted, at least in embryo, with the alliance of France, Britain and the USA that had defeated Germany in 1918. Such an alliance has much greater economic strength than Germany and would justify the French slogan “Nous vaincrons parce que nous sommes les plus forts” (we shall win because we are stronger). It is not totally implausible that Hitler might have had odd beliefs about America. Adam Tooze argues that America was central to Hitler's world view: “In Hitler's mind the threat of world war, the Americans and the Jews were inextricably intertwined” - Tooze “The Wages of Destruction”, Page 424. However, even Tooze is only talking about 1940. There is no evidence that he did see America as standing behind Britain and France in 1939 nor that he believed that America might have rushed into the war if France were defeated. However, had he had such a belief, it is likely that he would have decided that simply defeating France would not end WW2. In fact, had he believed that America would quickly declare war, he should have realized that there would be no time to successfully conquer the USSR before the Anglo-American threat pulled his forces back to the west. If, again following Tooze, we assume that Hitler understood America's ability to produce aircraft, ships and equipment of all types, he would had little excuse for believing that Germany could win a long war. Only a few people would have followed such a train of logic all the way to accepting that the only chance of victory was to defeat and occupy first France and then also Britain during 1940 when even defeating France seemed very difficult but this POD assumes that Hitler was one of them. It seems likely that Hitler was seeking a way to defeat France rapidly and that this attracted him into gambling on a thrust through the Ardennes. However, in OTL he believed that Britain would make peace if France fell and thus saw no need to make any plans to invade Britain. My POD has Hitler believing that France will fall quickly to a German attack but also that the war would continue after the fall of France. Thus he starts preparations for an invasion of Britain from September 1939.
The next improbability is that Hitler and/or other German leaders would have set a group of fairly intelligent people to work on plans for this Sealion and would use his authority to support them in clashes with the other power centres of the Reich. I tried to model the group on the planners of Operation Weserübung, the invasion of Norway and Denmark, but obviously planning for Sealion would be a much larger and more complex project. There is no reason that a group of competent people could not have been assembled but it requires luck. A hostile analysis might liken collecting such a group to having the bottom of a flask of water freezing while the top half boiled but it is in fact slightly more probable, especially if the initial competent members recommended/ co-opted others. However, there would clearly be a danger of Luftwaffe – Kriegsmarine conflict especially if Raeder and Goering, both capable of great incompetence, became involved. The OTL story of the German use of magnetic mines could serve as a fine example of how not to wage a war involving LW – KM “co-operation”. I am not sure if Jodl, Marschall and Milch with Hitler's support could have kept Keitel, Raeder and Goering from snatching defeat from the jaws of victory but it is at least untested. In one area LW – KM distrust is helpful for my POD as I feel that KM criticism of LW cipher security would have been a plausible result of asking the two to exchange information freely. It even seems plausible that the LW might have adopted the KM method of sending indicators as it is so obviously better that their own system (the LW system seems in retrospect so utterly stupid that one might almost suspect sabotage).
One obvious question with my POD is whether Germany could have produced additional aircraft and gliders, ideally while training additional pilots, completed surface warships quicker, built landing craft and additional S-boats and R-boats, produced additional mines, converted ships to lay them, designed and produced effective aerial torpedoes and otherwise prepared for a successful Sealion. Much of this goes back to assessments of the German war economy. Immediately after the war, Galbraith argued that Germany had not initially fully mobilized its economy and explained the huge surge in production in 1943-4 as due to belated mobilization. This view has been largely discredited leaving the views of Richard Overy (for example in “War and Economy in the Third Reich”) that the German economy was mobilized but very inefficient and the view of Adam Tooze, “The wages of Destruction”, that the 1943-4 surge was a result of the huge investment in industrial plant over 1938-43. If we accept Tooze's view, the question for this POD is what sort of output was possible if it had been decided that the war would be won or lost by the end of 1940. Cancelling synthetic fuel plants that would not be completed within a year in 1939 rather than as OTL by Speer in 1942, not rebuilding shipyards for U-boat construction, not completing the Volkswagen plant, http://en.wikipedia.org/wiki/Volkswagen, which produced its first Kubelwagen in 1940 but was not complete until 1941, the huge Flugmotorenwerke Ost factory for the Jumo 222 or the Nibelungen-Werke tank factory and many smaller investments will free large quantities of steel and labour. It is much less clear how much extra output is possible by the middle of 1940 although many plants were not working for 24 hours because of lack of labour. For example, He 111 production doubled in 1942 after more labour was provided. Would extra labour have revealed other limits? Aero-engines seem a possible problem but the USSBS report notes that “The capacity of the industry was more than adequate for the aircraft program during the first years of the war with the result that production rates in individual plants were below an economic level. This excess capacity, however, turned out to be valuable insurance.” Looking at the data in the report http://www.sturmvogel.orbat.com/airrep.html for DB 601 production at the main Genshagen plant does not show a sharp increase until 1942, so I am guessing that production only moved to a 24 hour basis in 1942 and that extra labour would have raised production over 1939-40. Of course, if the main problem is inefficiency, different orders from above may make no difference.
The proposed production of mines and torpedoes might also raise some eyebrows. In September 1939, 1250 550 kg LMA and 1150 960 kg LMB magnetic mines had been ordered but only 143 had been delivered. Approximately 1000 magnetic mines were laid during the first year of the war. Clearly, producing much larger numbers would have required explosives which were in short supply due to need for a huge increase in ammunition production in 1939. However, the situation seems to have eased by Late Spring 1940. In any case, the availability of additional steel would prevent the construction of 12,000 emergency concrete cased bombs which were never actually used and allowed the explosives to be used for mines. Torpedoes were a major problem for my initial efforts. Unfortunately, I have not read “Der Lufttorpedo: Entwicklung und Technik in Deutschland 1915-1945” by Friedrich Lauck except for table 5 http://www.luftwaffe-experten.org/forums/index.php?showtopic=2270 which shows German production of around 200 torpedoes per month once production had geared up. Production of the F5 for September 1939 was 36 torpedoes from the same table. The critical question is how much redesign occurred between the near useless F5 and the usable F5b. Could the facilities for F5 production have produced F5b like torpedoes in early 1940 had tests in September to November 1939 found most of the defects? Another question is whether discussions with the Italians and Japanese who both used inertial fuses would have led to a comparison of these with the German whisker contact fuses (there is a description of Italian torpedo fuses from a USN mine disposal handbook at http://www.ww2aircraft.net/forum/weapons-systems-tech/italian-torpedos-11487.html) and much earlier deployment of reliable contact fuses (were there any examples of British ships being hit by dud Italian torpedoes?). The weakness of the Italians was probably aiming their torpedoes from either aircraft or ships and the SM79 did not carry any aiming devices such as the German ToKG-1B http://www.ww2aircraft.net/forum/weapons-systems-tech/german-torpedos-11482-2.html. Thus Italy might also have benefited from German ideas. However, the pre-war trained Italians hit more ships by eye than the Germans hit with their predictor. I am assuming that German interest slightly accelerates the establishment of an Italian torpedo bomber force.
Finally, the curse of all timelines are the butterflies. There are some obvious unaddressed problems. For example, several French ships took refuge in British ports, most at Plymouth and Portsmouth, before the Armistice was signed on 22nd June 1940 and remained there undisturbed after it came into effect on the 25th June. They were boarded and taken over by the British at first light on 3rd July. If the Luftwaffe had been attacking Plymouth and Portsmouth over the week before the 3rd July, it seems likely that something would have changed. One possibility would have been that Adm. Darlan might have ordered the ships to sail to neutral or French controlled ports, either because of German pressure or to avoid damage from bombing attacks. Would the British have allowed the ships to sail? If there had already been Anglo-French clashes, then in turn the events of 3rd July would surely have been altered. Even if the ships had remained quietly in port and not suffered damage, they might have manned their anti-aircraft guns at first light on the 3rd and this could make boarding them more exciting. I decided to ignored the problem and assume that Anglo – French relations went exactly as OTL. There is a second issue in that I have assumed the same world wide deployment of the RN as OTL including a strong fleet at Alexandria despite fewer German loses off Norway. The ships at Alexandria were certainly needed and the strength at Scarpa looks adequate, so the deployment is not improbable especially if Churchill was already involved in those details. I have also assumed that the British would interpret the failure of the Germans to deploy a magnetic mine up to June 1940 as evidence that they had not developed a working mine although they had received reports of German magnetic torpedo fuses and the Oslo Letter mentioned magnetic mines. OTL few counter measures were started before an air dropped mine was examined in November. It is worth pointing out that the British magnetic mines operated on the rate of change of the magnetic field while the German mines used its direction. Thus sweeping methods need to be different. I am assuming that neither could sweep enemy mines in July 1940. It is improbable that the British would have produced very many magnetic mines in the absence of German use because they would have feared that the Germans might capture and copy the fuse.
Finally, it could be noted that the Germans are given some luck in this timeline. However, I would argue that they had much more luck in May 1940. An Icelandic saga writer would simply remark that Germany's luck was good up to late 1941 but we should not fall into the opposite extreme of arguing that Germany had used up its luck after June 1940.