Allies invade North Sea Coast of Germany on D-Day

Yet in even more difficult ASW environment of English Channel the U-boat threat was made irrelevant.

The misunderstanding you and CalBear have fallen into is that the landing will take place in the Baltic.
There is a German North coast on the North Sea, as you know. If we need to think about this (and rule it out), let's talk about that, not an ASBish Baltic invasion that the original poster had not posited, either.

Which means that the U-Boote bases - in the Baltic - are _close_ to the invasion area; but also that their ambush lines will be in the nice deep North Sea, not in shallow, constrained dangerous water like the Channel or the Baltic. This gives the U-Boote the best of it all.
 
By 1944 the U-boat threat was negligible.

Well yes, in mid-1944 the submarine threat was negligible because the Germans had basically withdrawn them due to unsustainable losses in the Atlantic. Likewise there was no sub threat against Overlord, the German subs having not even tried to commit suicide in the Channel.
Yet in the North Sea it is another story. The Germans had, not far from there, in the Baltic, the latest models of U-Boote, which would make a comeback, especially in coastal waters, in late 1944 and early 1945. Not all of these were operational, but my own guess is that while with a cross-Channel operation an attempt was not even made, in this case it would be.

In any case, as you know, an escort carrier, assisted by 3 or more destroyers, formed by this time a fearsome ASW task force. Subs had to run away from such a group. Yet at the end of May 1944, a "negligible" U-Boot sank the USS Block Island and one of its destroyers. The sub was also lost, to the other destroyers. Yet should such a price have to be paid to sink an LST off Bremerhaven, which would be bad news for the beachhead, I suspect the Germans would find an U-Boot commander willing to pay.
 
Over the beaches in the Pacific or over Normandy, the air cover had a flight of between 20 and 40 miles. The fleet carriers would go just over the horizon and operate from there, just to provide a LITTLE confusion to the Japanese aircraft, but the Escort carrier were frequently within sight of the beaches and, of course, the gun line was at five miles or less.

Yet this was still done in the Mediterranean theater.

It is worth remembering at the end of the war, after the Germans had been pushed back onto home soil they still had plenty of u-boats (I believe the Allies split up well over 200 operational boats at the end of the war as spoils). Those boats would have a travel to patrol zone time of a couple days, tops. They could enter the most target rich enviroment a submariner ever saw, fire off their torpedoes or lay their mines and be back for a refill before the milk went bad. Be a commuter raider.

German U-boats in the Channel had much better preconditions (better equipment, including Snorts, fortified bases, more open access routes) but still failed.
 
Well yes, in mid-1944 the submarine threat was negligible because the Germans had basically withdrawn them due to unsustainable losses in the Atlantic. Likewise there was no sub threat against Overlord, the German subs having not even tried to commit suicide in the Channel.

German submarines did attempt to effect Overlord landings but the effort failed miserably. Out of 73 submarines 19 were sunk for the gain of 21 Allied ships sunk. My copy of Blair's U-boat war is tucked somewhere but I think a great deal more were severely damaged.

Yet in the North Sea it is another story. The Germans had, not far from there, in the Baltic, the latest models of U-Boote, which would make a comeback, especially in coastal waters, in late 1944 and early 1945.

And could be well dealt with the Allied ASW capability available.

In any case, as you know, an escort carrier, assisted by 3 or more destroyers, formed by this time a fearsome ASW task force. Subs had to run away from such a group. Yet at the end of May 1944, a "negligible" U-Boot sank the USS Block Island and one of its destroyers. The sub was also lost, to the other destroyers. Yet should such a price have to be paid to sink an LST off Bremerhaven, which would be bad news for the beachhead, I suspect the Germans would find an U-Boot commander willing to pay.

Sure, there would be casualties as in any war. However, conditions for U-boats in North Sea or in the Baltic would be very difficult as the nature of these seas are perfect for defensive mining, in case of possible Baltic continuation operations even for 100% sure net and boom defenses as was done OTL by Finnish and German navies in Gulf of Finland.

Take a look at the free map preview from:

http://www.1yachtua.com/charts/Chart.aspx?mapid=216

and you'll see the difficulties by yourself. Now, periscope depth for subs was around 15 meters (from surface to keel)...
 
Just to extrapolate on idea, while still considering it risky and thus unhistoric, there's the issue of aircraft strength.

According to Wiki, total Luftwaffe day fighter strength on 31 May 1944 was 1063 and bomber strength 841. The Luftwaffe total day fighter strength could be more than well be exceeded by RN/USN carrier air if deemed necessary. RN big decks available alone could operate hundreds of aircraft, add the USN's Atlantic large carrier Ranger and a number of escort carriers and necessary air strength is there. RN escort carrier strength alone also was more than required for both ASW and significant landing support. By 1944 RN had 34 Bogue-class CVE's alone, each with capability of some 20 fighters in open ocean conditions.

And this is not counting the long range planes which could be operated from UK bases and the fact that Luftwaffe could not withdraw all it's units from Eastern Front and Mediterranean. Allied medium bombers could well operate from UK bases. One should also not forget the Allied qualitative superiority at this point of war.

Thus, Allied air would dominate even in case of a North Sea landing and this would not even require substantial withdrawals from the Pacific theater. However, the aerial domination would not be as substantial as in Normandy landings and this would entail perhaps larger convoy losses if largest part of the Allied Atlantic CVE strength was used to support landings. On the other hand, Allies could well survive much more severe convoy losses.

Second question is what forces could be landed and sustained? As for the airborne troops, the range of aircraft would be enough for landings in North Sea coast of Germany or Denmark. Thus, the historical three airborne divisions could be sent.

As for the ground troops, the size of the initial assault would be roughly similar to historical strength. However, the following build-up would be slower due to larger distances. As for the initial assault, the conditions are more difficult than in the Overlord, but the defenses are much weaker. There are also several not-so-well defended harbors which can add to logistical strength of the Allied force.
 
As a final extrapolation: where to land the troops? Still don't think it as a good idea but I think this is a new thing to stab into death...

1.) Best landing area would be the area between Bremerhaven and Cuxhaven. This peninsula is fairly well divided by two significant rivers, Elbe and Weser. Allied airborne strength could be best used in helping to seize outbound islands (Heligoland etc) and to take a) Bremerhaven and b) Cuxhaven by coup.

2.) The three landing zones would be peninsula between Jade and Weser, Bremerhaven area by itself and Cuxhaven. The initial goal would be to create a secure logistics area for further assembly of troops. Historic D-day landings were spread to area of about 80km's, so this would roughly follow the setup.
 
German submarines did attempt to effect Overlord landings but the effort failed miserably. Out of 73 submarines 19 were sunk for the gain of 21 Allied ships sunk. My copy of Blair's U-boat war is tucked somewhere but I think a great deal more were severely damaged.

By sheer numbers, and a willingness to accept casualties, the Allies might win almost anywhere, in the long run. The point is that an attempt across the North Sea would give the Germans a warning, which they did not get for Normandy. The German attempt at interfering in Normandy took place against the supply runs. What I meant is that they did not attempt anything _on D-day itself_, at the Allies' most vulnerable moment. In the North Sea, they would know the Allies are coming and they would have better chances for the submarines to run interference on the very first day. With severe consequences, IMHO.

Sure, there would be casualties as in any war. However, conditions for U-boats in North Sea or in the Baltic

I can only insist that the Baltic is a red herring here. Make no mistake, I do not like the idea of a landing in Germany, but Germany had a coast outside the Baltic.
 
By sheer numbers, and a willingness to accept casualties, the Allies might win almost anywhere, in the long run. The point is that an attempt across the North Sea would give the Germans a warning, which they did not get for Normandy. The German attempt at interfering in Normandy took place against the supply runs. What I meant is that they did not attempt anything _on D-day itself_, at the Allies' most vulnerable moment. In the North Sea, they would know the Allies are coming and they would have better chances for the submarines to run interference on the very first day. With severe consequences, IMHO.

The question is where would the U-boats come from? Allies could effectively install forces which would make both Skagerrack and English Channel almost totally U-boat tight. Kiel Channel is easily minable, and with Allied tactical air power German subs operating from unfortified bases in North Sea coast would be in deep trouble, especially if and when the Allied ASW effort is aided by massive use of mines to approach routes.

Also, almost all of the operational U-boats were in France, with Baltic having older models and Norway having very few boats by this point.
Also, a total ASW offensive would be justified as occupation of Schleswig would finish German U-boat threat. The same would go for any German light forces.

Even in case that German react fast to the initial information (which they did not, in case of OTL Overlord even when the initial landing was delayed), their warning time is maybe 48 hours. This is no sufficient time to redeploy significant numbers of U-boats.

It is more likely that they believe that the armada is a) feint to landings in France or Holland b) invasion attempt of Norway (Hitler's favorite scenario).

I can only insist that the Baltic is a red herring here. Make no mistake, I do not like the idea of a landing in Germany, but Germany had a coast outside the Baltic.

I was mentioning Baltic continuation just for example. This would play upon the advantages of Allied naval and air superiority.

IMHO, if the Northern route is taken, best course of action would be an amphibious invasion of Jylland coupled with all-out airborne effort on Sjaelland. In addition to land invasion route to Germany this would easily secure superior airbase terrain and route to threaten with a landing to German Baltic coast.
 
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