Germany was obviously a leader in submarine technologies in 1940. What if the KM, and Hitler, decided to build a submarine similar to the Japanese I 400 class as a potential means of attacking the US? The Luftwaffe was trying to construct a bomber that could reach the US. Let's say the KM decides to be a player in that game too in late 1940 and begins design and construction of say, 10 large aircraft carrying submarines designed to hold two aircraft each.
By early 1944 these boats are now finished but the Germans now have two options:

1. Continue with the original plan and launch the subs on major US cities using their two aircraft each or

2. Modify the plan some and use them to launch several V-1 each.

I assume here each sub has a catapult capable of launching a single or twin seat single engine float plane so it would also be capable of launching a V-1. V-1's would take up a fraction of the space a plane would so 4 to 6 might be carried.

So, given the completion of some or all 10 of these boats the Germans possess a sub capable of reaching the US, capable of launcing two aircraft or several V-1 while surfaced. These could obviously also be used against other targets than the US.

While I doubt it would have had an appreciable effect on the outcome of the war, such a weapon combined with attacks on the US, Britain from unexpected directions, or other targets that were not particularly considered vulnerable would have had considerable moral and propaganda value.

What are the chances the Allies would have had fore knowledge of such a weapon if it were built in relative secrecy? Would the Allies stand a good chance of stopping it once at sea?

According to Clay Blair in Volume Two of "Hitler's U-Boat War", the USN did have intelligence indicating that the Germans intended to attack the US East Coast with missile-launching subs, late in the war. A massive plan was formulated to thwart this effort and was actually put in motion due to a mis-interpretation of intercepted German signals to U-boats at sea. Thus, the Allies had an excellent chance of stopping, or at least, minimizing the effects of such an effort.

As for building such large subs, (the I-400 class had a surface displacement of 5,223 tons), the Germans did plan a series of aircraft-carrying boats in 1939, but these were canceled in the interest of building more attack boats of the Type VII and Type IX classes. It would seem illogical to waste so much time and effort to build boats which could make only relatively pin-prick attacks on the East coast of America. Even assuming that a U-boat could be built that could carry ten V-1's, that would be only 100 V-1's against the entire coastline of the US. Since these were unguided cruise missiles which could only be used against area targets to create an atmosphere of terror, their effects against even the heavily populated areas of the American coast would be minimal, as they proved to be against England when launched in much greater numbers.

And these boats would be vulnerable to the heavy air and surface patrols they would encounter off the US East Coast in 1944-45, so it's entirely likely that only a few would survive to launch their V-1's or seaplanes. It just doesn't make any military sense to embark on a project with so little likelihood of any significant return in terms of damage to the enemy. Being able to post some propaganda victories in 1944-45 doesn't mean much, and neither would the minimal hit-and-miss damage they could achieve.

But then the Germans did some pretty dumb things during the war, so the whole thing isn't beyond the realm of possibility, but I doubt even the Germans would be that stupid.

The type III boats could have been dangerous because of their long range but as WMD carriers they are no threat. Let's be seroious the Arado Ar 231 is no strategic bomber.

Arado_231.jpg
(scan is from an old W.Green book).

The interesting think is that the 231 development continued long after the Type III were cancelled, first flight was in 1941 but was finally scrapped in favor of the Fa 330 "rotary wing gliders.

IMO a more serious threat would be a Scirè like installation to carry human torpedoes ad attack the less guarded ports. The defensive effort and disruption to guard against those would be probably worth the the effort of equipping a dozen subs. Of course the allies had coastal patrols but as the Gibraltar, Alexandria and Tirpitz episodes showed, you need special procedures against midgets.

And if we assume Hitler gets the bomb a sub is the only credible delivery platform against the USA.

AFAIK a sub launched missile was attempted late war but I don't know much about it, but IMO sub launched missiles are cost effective only if you go nuclear.

I think the V-1 option would have been out. Because of the German belief that their codes were absolutely secure (and of course they weren't), and their habit of being very chatty with communications to HQ (huff-duff fixes), I doubt a single sub would have survived in 1944 to get within range to do any damage. Particularly if intercepts gave an indication of something "special" about to happen. In 1944, the allies owned the Atlantic.

As far as 20 Arado's, each with a pair of small bombs? Far less damage than Doolittle's raid, and meat on the table for any fighters in the neighborhood. And then would they even survive to get back to Germany? I'm betting there would have been an all out effort to find and sink those subs.

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The Arado 231 was clearly intended as a recon plane to extend the sub's search horizon. Donitz was always complaining that his subs couldn't search enough ocean to find the convoys they were supposed to attack. To make a German I-400 sub-class feasible, the Germans would have had to develop, or adapt, a different attack aircraft than the Arado 231.

The Japanese I-400's were much larger than the Type III's, or any other sub the Germans contemplated building. Thus a notional German sub would have been capable of carrying single engine attack aircraft only. But even the I-400's would have been incapable of launching more than pin-prick raids on the US.

There are two pretty unlikely assumptions here.

First, the Germans weren't anywhere near developing the atomic bomb for several reasons.

Second, launching a WW II era nuclear bomb from a sub is pretty much a non-starter because of the weight. The two American bombs used in WW II weighed around 10,000 pounds. The largest German missile, the V-2, had a maximum payload of around 2,000 pounds. In any case, the V-2 was unguided and capable of delivering a war head only at random. Any WW II aircraft capable of carrying an atomic bomb any distance had to be as large as something like the British Lancaster or the American B-29; impossible to carry on a sub or launch from a sub.

WW II conventional subs were not "credible" delivery platforms for nuclear weapons, unless one conjures up a one-way suicide sub and crew.

As you pointed out an A-Bomb armed German sub in unlikely but IMO it's still more likely than a German B-36.

My point was that submarines are terribly inefficient aircraft carriers, a sub relies on stealth for survival and WW2 air ops are not stealthy, planes can be traked by radar and without radio guidance losses to navigation errors are going to be something horrible.
Even normal WW2 aircraft carriers are bad platforms for strategic bombing. The Ar 231 was hopeless as a bomber but any plane small enough to fit in a sub would still be a very poor strategic bomber. Immagine a one tonn bombload for the plane and a sub large enough to carry fuel and weapons for four or five missions, that's a total of five tonns for a two month or so cruise with a good chance of the sub getting caught after the first raid. The economics are awful and strategic bombing is really economic warfare. The I-400 was a waste of resources and a German I-400 would be the same.

Only the A-Bomb can change that calculation. We don't really know what a German A-Bomb would have looked like, the fact that the US came up with a nearly ten tonn monster is no proof something smaller could not have been designed to a different reqirement, smaller A-Bombs appeared rather fast after the first ones. A sub large enough to carry a modified A4 with a shorter range but a larger payload is not impossible or more likely something like the US regulus weapon system based on an evolution of the Fieseler Fi 103. Not sure it will work, putting a liquid fueled missile in a sub is not a great idea, but theoretically possible. A 50 Km range weapon is enough for the sub to fire at ports from outside coastal patrol zones and that can probably allow the required payload.

Why even launch the missile? US coastal anti-submarine was awful, so it's not impossible for a German submarine to sail undetected right into New York harbour (under robotic control, or with a suitably fanatical crew), surface, and detonate.

The Germans could have easily gone through the coastline undetected. Heck, even now it's fairly lax security wise.

For the most part, I agree entirely with what you have posted. The I-400 class was a typical example of wasted Japanese time and resources. Nothing would have been different for a German I-400 derivative.

But I am skeptical about your comments on a notional German Atomic bomb. The Germans thought an atomic bomb would necessarily be many times larger than even those the US dropped on Japan. And I seriously doubt the size of atomic bombs declined much before the 1950's. Ballistic missiles capable of carrying nuclear warheads didn't appear until then.

A 50 Km range would doom any sub attempting to launch a missile at the US East Coast in 1944 to almost instant destruction. That was well withing range of coastal surface radar and standing air patrols off the US coast could have responded within a very short time frame. This in fact, was part of the secret USN plan mentioned by Blair in his "Hitler's U-boat War", The whole idea of a German atomic bomb, small enough to be carried on a V-2, and launched from outside the range of US coastal patrols in 1944, is just too fantastical to consider.

While these statements may have had some validity in the Spring of 1942, by 1944, when this "what-if" is posited, they are completely false.

The US East Coast in 1944 was heavily patrolled by both aircraft and surface vessels, and covered for most of it's length by a system of surface and air radar stations. There was practically nowhere in the entire North Atlantic in late 1944 where a U-boat could surface without risking air or surface attack. This, after all, was the reason Donitz decreed that all existing operational U-boats must be equipped with a snorkel; without it a U-boat in the North Atlantic was doomed.

I see this scenario as certainly doable. But, it would have only really been good for the propaganda effect. But, by 1944 propaganda in Germany that showed them bombarding America and claiming exaggerated effects would have definitely been a morale booster.

As for getting there, the Allies would likely have been aware of the subs sailing and possibly known their approximate operating area as well. While this would have helped track them down it is also unlikely that Ultra intercepts alone would have allowed a sufficent amount of information to lead directly to a timely sinking. The Allies simply don't have sufficently good sensors for that. Now, that doesn't mean that the boat might be tracked down after surfacing and starting to launch missiles or planes. Radar would have allowed a much greater search area and with airborne targets along with the sub could have provided a means to track them down.
But, by then it would have been too late. At least some missiles or aircraft would have been on the way.
I do think it would have been worthwhile for the Germans to pursue such a project on a limited scale as it would have increased the Allies industrial slice being put into ASW along with causing the US to increase air defense at home, if for no other reason than to show the population they were doing something about this danger.

But surfacing here is just an issue of enhancing the effectiveness of the bomb. There isn't any need for the U-boat to linger on the surface. It'd just be a second, and however good they are, allied coastal defences aren't going to kill that submarine in the seconds it takes between breaching the water and having Lt Hans push the big red button.

No, surfacing, and remaining on the surface long enough to assemble and launch either a plane or missile, is the issue. In the late 1920's the USN experimented with sub launched recon aircraft and found that any sub that operated aircraft was extremely vulnerable to ASW counter measures due to the length of time required to launch and recover the aircraft. With a missile, there is no recovery issue, but there is still the time to assemble to missile, position it on it's launcher, and launch; the time might seem insignificant, but by 1944, the Germans were finding that even minutes might mean losing the sub.

By 1944, the Americans were reading the German code used to signal German subs, so it would be likely that the USN would know the general area to be used for launching. Moreover, due to the limited range of the V-1 the launching area would have to be in a fairly limited distance offshore, say 100 miles. It would be possible to cover this thin offshore strip with radar-equipped blimps. A hunter-killer group, consisting of an escort carrier would be sent to that area, and planes from the carrier would fly patrols. In addition, long range air patrols would cover the area from onshore bases. In the fifteen or so minutes it would take to surface, assemble the missile, position it on the launcher, and launch, it would be likely that the sub would be spotted and destroyed. Even if it managed to dive, USN policy by that time was to hunt a sub to destruction with relays of surface vessels and aircraft. So a sub might possibly be able to launch a single V-1 before being detected and destroyed. That's not a very good return for the millions of Reichsmarks invested in each sub.

As I posted above, a sub would probably have a 50-50 chance of launching one missile before being destroyed, but Allied ASW was so good by 1944 that it almost certainly would be destroyed before it could launch a second missile. In any case the sub would have to move to another area to attempt a second launch and because of the requirement to remain submerged that would take days or even weeks using a snorkel.

If each sub was able to launch one missile and these impacted the coast in a random pattern, perhaps half (five?) might do serious damage and produce casualties. It would be entirely possible for the US government to suppress the news of these attacks (as it did the news of the Japanese balloon attacks on the West coast), thus depriving the Germans of any propaganda value.

As for increasing the Allied ASW burden, no, it would not significantly increase the required ASW effort because, by 1944, the ships and planes were already built and committed to ASW warfare and had already driven most German subs from the Atlantic. It was necessary to keep them on patrol to keep the Atlantic free of German subs but they were not in 1944 pressed by any enemy sub activity.

As for air defenses, a handful of V-1's over three or four week's time wouldn't require much more than was already available. The Germans wouldn't be able to sustain such attacks for more than three or four weeks before their missile launching subs were hunted down and destroyed. It would take much longer than that to move additional AAA or fighter units to the affected areas, thus the threat would be past long before public pressure would build to unbearable levels.

In reality, the only benefit the Germans would realize would be whatever random damage and casualties they could achieve, and this would be insignificant in the overall scheme of the war.

Sorry, there's some confusion here. I was talking about if the Germans had the nuke, in which case it would be possible to circumvent the issue of getting a workable missile launch system and sufficient miniaturisation of the warhead, by simply having a submarine move submerged sufficiently close to New York, and detonating the nuke whilst it's still on the submarine.

My mistake, that part wasn't clear to me.

In that case, the biggest problem would be getting through the USN's defenses which would by that time of the war by no means be certain. Of course, the assumption of Germany getting a nuclear bomb, even a crude one, is so far fetched as to be considered a non-starter.

Just to give some idea of the defenses that would be brought to bear, there is a page on Wikipedia that describes the historical actions of Operation Teardrop, the USN's response to the intelligence it had regarding possible German missile attacks on the East Coast in 1945.

See; Operation Teardrop - Wikipedia, the free encyclopedia