Operation Sealion - creative approach

[kalpazanin]

I would like to discuss any creative ideas with which operation Selion could have been made to be successful.
In order not to indulge in 'Alice in wonderland' type of fantasies let's discuss ideas starting implementation in july 1940 and obey strictly : laws of physics, production figures and general common sense...

[kalpazanin]

My understanding of the historical plan of Sealion is that it was flawed in every possible way, and these flaws came from 3 basic problems :
1) Lack of element of surprise.
2) Inability to quickly deliver the first wave of the invasion with tanks and heavy equipment.
3) Inability to support the invasion on the face of the royal navy.
On my humble opinion all of those could be easily remedied by:
1) No mine laying of the channel - the mine laying operation was to start a week before the invasion and would be sure indication that something is comming.
2) Returning of big part of the accumulated 1900 barges back to the Rheine couple of days before the invasion - a sign, which could be interpreted as 'no invasion' by the British
3) Sacrificing 100 merchans out of the 170 available by driving them on british beaches during high spring tide - basically those ships would consist the invasion (apart ftom the protecting KM ) Each ship would carry 4x 300 ton empty barges and 5 stormboats , 2000 troops+ 10 tanks +10 halftraks.
4) After the initial beachhold is taken the resupply will be done by the Ju52 planes.
Those need not land on english soil - they'll fly cargo in water resistant cylinders and drop it at low speed in the coast water during high tide. - During low tide the supplies will be easily retrievable by foot from the beaches.

With all those small improvements (which could be easily set up during the 2 month preparation period) the invasion would be fast, tactically surprising and would sweep british off their feet without much chance of resistance. The fearsome RN would be 'outflanked' just like the Maginot line.
The question is- why germans didn't think of that?
Any comments?

[Major Major]

My understanding of the historical plan of Sealion is that it was flawed in every possible way, and these flaws came from 3 basic problems :
1) Lack of element of surprise.
2) Inability to quickly deliver the first wave of the invasion with tanks and heavy equipment.
3) Inability to support the invasion on the face of the royal navy.
On my humble opinion all of those could be easily remedied by:
1) No mine laying of the channel - the mine laying operation was to start a week before the invasion and would be sure indication that something is comming.
2) Returning of big part of the accumulated 1900 barges back to the Rheine couple of days before the invasion - a sign, which could be interpreted as 'no invasion' by the British
3) Sacrificing 100 merchans out of the 170 available by driving them on british beaches during high spring tide - basically those ships would consist the invasion (apart ftom the protecting KM ) Each ship would carry 4x 300 ton empty barges and 5 stormboats , 2000 troops+ 10 tanks +10 halftraks.

Let me see if I read this right. Each ship contains 2000 troops, ten tanks, and ten halftracks.

Where do you get ships this large? How large are they?

Those ten halftracks are going to be stretched pretty thin supplying the troops once they get more than a kilometer or so away from shore.

Where is the artillery? Anything bigger than portable mortars?

Meanwhile, once the RN has disposed of the eight zerstorers and twenty or so torpedoboote available to cover the beachhead, you'll have HMS Revenge dropping 2000-lb bricks into said beachhead every night, not to mention cruisers and destroyers.

4) After the initial beachhold is taken the resupply will be done by the Ju52 planes.
Those need not land on english soil - they'll fly cargo in water resistant cylinders and drop it at low speed in the coast water during high tide. - During low tide the supplies will be easily retrievable by foot from the beaches.

The Tante Ju rather lacks the rear door that this implies. Everything will have to fit out the side door. Add to that the weight of the cylinders and that doesn't look good for supplies.

The World War II Databook gives a figure of 475 transports at the beginning of the French campaign. Allowing for losses in France, new construction, training units, deployments in Norway & Poland, you might have 300 available. The capacity of a Ju-52 is a ton and a half but allow for the weight of cylinders, say one ton of supplies per sortie.

That's going to take a lot of flying. Meanwhile the LW has to defeat the RAF, and protecting the morning's cargo train isn't going to be very helpful in that regard.

With all those small improvements (which could be easily set up during the 2 month preparation period) the invasion would be fast, tactically surprising and would sweep british off their feet without much chance of resistance. The fearsome RN would be 'outflanked' just like the Maginot line.
The question is- why germans didn't think of that?
Any comments?

[kalpazanin]

Let me see if I read this right. Each ship contains 2000 troops, ten tanks, and ten halftracks.

Where do you get ships this large? How large are they?

Those ten halftracks are going to be stretched pretty thin supplying the troops once they get more than a kilometer or so away from shore.

Where is the artillery? Anything bigger than portable mortars?

Of course. each ship could carry field artyllery ,.AT and howitzers with the respective supplies.
KM had summoned (according to von Tipelskirh) 168 merchant ships with total of 700 000 ton tonnage.
That makes the average german merchant~4100 ton. Considering that a Liberty class ship (14000ton)was able to carry 30 tanks + 60 trucks + various artillery and supplies to match that, I made a proportional adjustment for the average value.
The 2000 troops may find it somewhat restricting in such ship,but that would be way much better than to be in a feeble barge and besides the crossing will take ~ 6,7 hours.

Meanwhile, once the RN has disposed of the eight zerstorers and twenty or so torpedoboote available to cover the beachhead, you'll have HMS Revenge dropping 2000-lb bricks into said beachhead every night, not to mention cruisers and destroyers.

Whith the use of the 10 halftracks (average per ship) the supplies could be quickly transported couple of kilometers inland during the first day. If the KM manages to hold off until the main bulk of the RN arrives ((which could take a couple of days) there'll be nothing much to bombard...

The Tante Ju rather lacks the rear door that this implies. Everything will have to fit out the side door. Add to that the weight of the cylinders and that doesn't look good for supplies.

The World War II Databook gives a figure of 475 transports at the beginning of the French campaign. Allowing for losses in France, new construction, training units, deployments in Norway & Poland, you might have 300 available. The capacity of a Ju-52 is a ton and a half but allow for the weight of cylinders, say one ton of supplies per sortie.

That's going to take a lot of flying. Meanwhile the LW has to defeat the RAF, and protecting the morning's cargo train isn't going to be very helpful in that regard.

a) My sources place Ju52 to ~ 400 at the beginning of the sealion.
b) A 0.40mx 3 meter steel cylinder with .5sm thick walls will weight 150kg and can carry ~2 tons of supplies. - That's a 1/13 proportion,not 1/3

You're right about the lot of sorties ,but there are 2 main factors which change the whole picture:
a) The airstrips would be placed very close to the channel's narrowest pont (25-30 km)
b) There'll be no landing on the delivery point.
Those mean that each sortie will be 25-30 min long. The transports could carry 1 ton less fuel, which will improve their carrying capacity to 2 ton.
Also since the distance of the english air bases to the delivery point will be more than that Ju 52 bases (considering there is invasion after all) the only way to intercept them is for the RAF to constantly circle above the channel in considerable numbers. - This will lead to their quick decimation by the Luftwaffe's BF109E attacking from close home bases.

[Kingfish]

Something just doesn't add up here.

First off, in your first post you say to obey strictly "general common sense". You then proceed to lay out an amphibious invasion plan that calls for basically writing off 2/3 of your heavy lift assets after the first wave, followed by somehow maintaining an offensive army several corps strong via air resupply.

Take a look at other historical examples of when a German force was resupplied by air for an extended period. You'll see that the tonnage delivered was at best barely adequate to sustain that force in static defensive positions. Either the force was small in size, and thus within the Luftwaffe's limited capability, or in the case of Stalingrad far too big for the the meager amount of supplies that could be brought in. Crete is somewhat of an exception, but then again Fallschirmjagers are light by design and can operate on supply levels far lower than a mechanized force of the same size.

What you need to do is calculate the required tonnage for an army group, then multiply by at least three to account for increased usage in offensive actions (the whole point of the invasion), loss in transit, wastage, etc. Then figure the load carrying capability of the Junkers fleet and start subtracting to account for loss in combat, breakdowns, insufficient delivery methods, etc.

I would venture a guess that 'B' does not come anywhere near to satisfying the needs for 'A'.

The only way for Sealion to work is for the Germans to capture a large port early on, something like Portsmouth, and use the full sea lift capability to constantly run supplies across the channel. The German army will need all those supplies and more, since these ports are heavily defended and require strong offensive actions to overcome, while at the same time having to hold off equally determined attacks by the British land forces.

[Major Major]

Let's see: he says 200k troops. Assumng a "small" division slice, let's say that's six divisions.

I don't have figures for all divisons for all circumstances, but the figure I heard in connection with Stalingrad was 700 tons/day, for a force on the defensive.

But let's go with that. Let's furthermore say that the Ju-52 can carry its full load of 1.5 tons per sortie and ignore the loss due to dropping the cargoes at sea. That's 467 sorties/day. And you have 3-400 planes with which to do it.

Now you will have a number of planes out for repair. There will be losses due to crashes. And if the LW is busy defending the air bridge, it's going to be overstretched.

Oh, and the RN cruisers and battleships had main armament ranges of more than two kilometers.

Then too, the 6-7 hours does not include loading. And as for disembarking from a grounded ship, the less said the better. Not to mention that those men will inevitably be seasick in anything over a Force Two sea. (Presumably they cross in daylight, thus missing the fun of being attacked at night by British destroyers.)

And then they will have to unload their heavy gear. Usually this requires cranes and other such equipment. This will be exceedingly difficult, to say the least, under the circumstances postulated. Captain Mainwaring and the Look, Duck, and Vanish platoon could expect to have an even fight -- and their reinforcements have it a little easier.

[Christian W.]

once the RN has disposed of the eight zerstorers and twenty or so torpedoboote available to cover the beachhead, you'll have HMS Revenge dropping 2000-lb bricks into said beachhead every night, not to mention cruisers and destroyers.

This is where you are ignoring the Luftwaffe.



By the way, I felt that I needed to post this.

[kalpazanin]

Something just doesn't add up here.

First off, in your first post you say to obey strictly "general common sense". You then proceed to lay out an amphibious invasion plan that calls for basically writing off 2/3 of your heavy lift assets after the first wave, followed by somehow maintaining an offensive army several corps strong via air resupply.

Take a look at other historical examples of when a German force was resupplied by air for an extended period. You'll see that the tonnage delivered was at best barely adequate to sustain that force in static defensive positions. Either the force was small in size, and thus within the Luftwaffe's limited capability, or in the case of Stalingrad far too big for the the meager amount of supplies that could be brought in. Crete is somewhat of an exception, but then again Fallschirmjagers are light by design and can operate on supply levels far lower than a mechanized force of the same size.

What you need to do is calculate the required tonnage for an army group, then multiply by at least three to account for increased usage in offensive actions (the whole point of the invasion), loss in transit, wastage, etc. Then figure the load carrying capability of the Junkers fleet and start subtracting to account for loss in combat, breakdowns, insufficient delivery methods, etc.

I would venture a guess that 'B' does not come anywhere near to satisfying the needs for 'A'.

The only way for Sealion to work is for the Germans to capture a large port early on, something like Portsmouth, and use the full sea lift capability to constantly run supplies across the channel. The German army will need all those supplies and more, since these ports are heavily defended and require strong offensive actions to overcome, while at the same time having to hold off equally determined attacks by the British land forces.

Ok, I understand completely your confusion with my idea.
So let me show you the numbers and clarify out the key points:
1) Regarding the loss of the 2/3 of the merchant fleet - that is big sacrifice, but it becomes quite acceptable considering that it would improve the speed of the invasion fleet to 10 knots instead of 3-4 and make it much more sneaky (100 faster merchants instead of 1000 slow barges) also since these ships will be lying on the shore and not sunk on the bottom maybe they could be partially salvaged after the invasion succeeds.
Also considering the issues at stake 100 merchants are acceptable loss for the potential of germany becoming a superpower.
The allies sacrificed much more ships during the war in the atlantic, only to keep england alive.

As to the Ju 52 - here are the numbers:
a) The Ju52 could potentialy lift up to 3 tons (if carrying verry little fuel to a short distance) so let's settle on 2 tons in 2 cylinders attached in it's bomb droping hooks.
b) My sources point to 400 Ju 52 available at the time of Sealion. I will not argue with you so let's assume half are constantly operational - 200.
c) In the 2 months of preparation the Germans could easily set up numerous aircraft stripes in the immediate vicinity of Calais (~20 km from the shore)
d) Flight path in both ends will be ~100 km. With Ju-52 speed 250/h km that would take~30 min
d) A single sortie of the 200planes will carry 400 tons to the shallow waters and last 30 minutes.
Ten sorties would bring 4000 tons of supplies for a total 5 hours flight time for 200 planes.
Obviously the sorties will be randomly distributed trough the day in order for the RAF to be unable to intercept.
In case the Ju 52 take a beating there are 200 more in reserve + some Heinkels,Dornie or Ju 88 could be used to boost the effort.
The average need for a german division for offencive operations vary :
~100 ton/day infantry division
~300+ ton/day panzer division
Since the invasion force will be a mix of those we can assume an average of 200 ton/day
For 20 divisions that is exactly 4000 tons/day


Also keep in mind that the 100 merchants will bring significant number of supplies (~60000 ton) which could suffice for the first 2/3 weeks of operation.

The much smaller amount of the supply that Ju 52s brought to Norway, Stalingrad and Crete was becouse the option of dropping cargo without landing was unavailable there and the distance flown was muuch longer.
If you need to land under threat of enemy fighters and unload the planes that alone drops your throughput ten times at least.

[Christian W.]

these ports are heavily defended

Care to be more specific?

[Andreas]

I am moving this to the correct forum (this is not it for What-Ifs).

All the best

Andreas

[glenn239]

Other than river barges, where and what merchant ships did Germany have available for transport duty? Wasn't the single biggest problem with Sealion the fact that the transports couldn't get to and from the English coast fast enough?

[kalpazanin]

Let's see: he says 200k troops. Assumng a "small" division slice, let's say that's six divisions.

I don't have figures for all divisons for all circumstances, but the figure I heard in connection with Stalingrad was 700 tons/day, for a force on the defensive.

But let's go with that. Let's furthermore say that the Ju-52 can carry its full load of 1.5 tons per sortie and ignore the loss due to dropping the cargoes at sea. That's 467 sorties/day. And you have 3-400 planes with which to do it.

Now you will have a number of planes out for repair. There will be losses due to crashes. And if the LW is busy defending the air bridge, it's going to be overstretched.

Oh, and the RN cruisers and battleships had main armament ranges of more than two kilometers.

Then too, the 6-7 hours does not include loading. And as for disembarking from a grounded ship, the less said the better. Not to mention that those men will inevitably be seasick in anything over a Force Two sea. (Presumably they cross in daylight, thus missing the fun of being attacked at night by British destroyers.)

And then they will have to unload their heavy gear. Usually this requires cranes and other such equipment. This will be exceedingly difficult, to say the least, under the circumstances postulated. Captain Mainwaring and the Look, Duck, and Vanish platoon could expect to have an even fight -- and their reinforcements have it a little easier.

Major,
since I partially answered your questions about the airlift I'll only restate that the sorties are very short - 30 min, therefore there can be multiple of those per day.
For comparison you can inspect the Stuka's performance in the east front - they were doing sometimes dozens of short sorties per day ..

Regarding the airlift protection.
Generally the LW would carry on the fight against RAF inside england, therefore whatever forces RAF decides to spare against the air bridge, should be gladly matched by the LW - the conditions close to it's bases are much more favorable.
Becouse of the very short flight time however, there'll hardly be any need for escorts.

Regarding the unloading of the ships:

1) The ships will leave ports at night.
That fact, coupled with the desinformation caused by sending back half of the barges would lead to at least 2-3 hours before the British start reacting.

Lets examine the situation in the channel after the 'alarm' starts ringing:
The british had 3 light cruisers and 17 destroyers in the immediate area of the channel. + MTB boats and sloops.

Against that Germans could put 10 destroyers, 25 u-boats,18 s-boats and the whole LW. (At that point the one and only purpose of the LW would be to protect the fleet.)

Since the merchants would have to carry artillery anyway, several of the big guns + some AA could be placed on deck and ready to shoot. (Remember the raider Kormoran...)

The above should be completely adequate to protect the convoy for the 3-4 hours from the moment British understand it's "Invasion day" until it reaches the english shore.

In front of the merchants there will be mine sweepers and the 471 tugboats towing an additional 200 of the bigger barges equipped with ramps for direct landing.
Since the proportion of tugs to barges would be ~2/1 they can reach 10 knots and keep up with the merchants.

2) The unloading would be done in 2 parts :
a) Each merchant will carry 5 pioneer storm boats (there were 500 of those for sealion) + several small boats + 2 empty small barges (350 ton).
Those together with the 200 towed big barges would be used after hittihg the shore to start the immediate assault with some 250 troops per ship for the very first wave.
After the boats drop them they'll back up to the ship and bring second , third etc until alll 2000 troops are ashore.

b) When each ship hits the ground it will be stabilized vertically by several rods lowered from it's sides.
Then, when the tide becomes low enough the ship's front section would be blown by previously positioned explosive charges and then with some sand and wood a ramp will be positioned, allowing the heavy equipment to roll off from the lower deck directly on the beach.

Then the trucks /half tracks will be used to quickly move the rest of the equipment and supplies 2 km inland above the water level.
- This task may not finish before the next tide so the whole unloading may take a day or two.
In any case there'll be plenty of supplies allready in the first day.
Also in that short timeframe the RN will still be dealing with the KM or en route from it's bases so it could not interfere much.

[kalpazanin]

Other than river barges, where and what merchant ships did Germany have available for transport duty? Wasn't the single biggest problem with Sealion the fact that the transports couldn't get to and from the English coast fast enough?

The Germans had 168 merchants with a total of 700000 ton of cargo capacity.
I claim that the problem could be overcome by sacrificing 100 of them by grounding them on the english shores during spring tide.

[LWD]

.... I'll only restate that the sorties are very short - 30 min, therefore there can be multiple of those per day....

You aren't saying the transport planes can do a complete sorty in 30 minutes are you? In any case fields that close to Britain will very likely start getting hit by night bombers from the first night on. This will either force them back or cut seriously into their sortie rate.

Regarding the airlift protection.
Generally the LW would carry on the fight against RAF inside england, therefore whatever forces RAF decides to spare against the air bridge, should be gladly matched by the LW - the conditions close to it's bases are much more favorable.
Becouse of the very short flight time however, there'll hardly be any need for escorts.

but becuase you are dropping at high tide the RAF can predict when the transports will be there after the first sortie or two. The LW didn't do a very good job of escorting bombers why do you think they could do a better job with transports..

Regarding the unloading of the ships:

1) The ships will leave ports at night.
That fact, coupled with the desinformation caused by sending back half of the barges would lead to at least 2-3 hours before the British start reacting.

While sending the barges back to the rivers will send a signal that the invasion is off the concnetration of ships and troops will indicate that something is still up. There is a very good chance that the British will know within a few hours when the transports start loading and be watching very closely for them to sail. Those gathered in ports close to Brittain will likely be attacked at night before they have even sailed. Which brings up the point of what ports do you plan on using? How many would you need to load up a 100 transports in two or three days time. Then there is the lag for them meeting at sea.

Lets examine the situation in the channel after the 'alarm' starts ringing:
The british had 3 light cruisers and 17 destroyers in the immediate area of the channel. + MTB boats and sloops.

Against that Germans could put 10 destroyers, 25 u-boats,18 s-boats and the whole LW. (At that point the one and only purpose of the LW would be to protect the fleet.)

If it's at night what good will the LW do? Also the Germans will have to provide CAP over the invasion fleet on a contunuous basis while the British can choose the time to mass against it. In general subs didn't do all that well vs warships at combat speeds. They will also have to be pre positioned or they will have a hard time keeping up with the fleet even on the surface.

...The above should be completely adequate to protect the convoy for the 3-4 hours from the moment British understand it's "Invasion day" until it reaches the english shore.

How do you calculate that? Again what ports are they sailing from where do the convoys meet up? How do you keep the loading of that many troops secret in an occupied country (or are they sailing from Germany?)

In front of the merchants there will be mine sweepers and the 471 tugboats towing an additional 200 of the bigger barges equipped with ramps for direct landing.
Since the proportion of tugs to barges would be ~2/1 they can reach 10 knots and keep up with the merchants.

No. Among other things there is the problem of hull speed. But in any case doubling the motive power does not come anywhere near doubling the speed of a vesel. In the case of barges doubling their speed would probably mean that the British wouldn't even have to worry about sinking them.

2) The unloading would be done in 2 parts :
....
b) When each ship hits the ground it will be stabilized vertically by several rods lowered from it's sides.
Then, when the tide becomes low enough the ship's front section would be blown by previously positioned explosive charges and then with some sand and wood a ramp will be positioned, allowing the heavy equipment to roll off from the lower deck directly on the beach.

How long a ramp are you proposing? I suspect if you look at the beach topaographys in many cases the transports will run aground 10s of meters from the beach. If there is a bar as many beaches have it can be even further. Then there is the question of how likely it is the ship will flood when the tide rises again or a rogue wave comes by.

Then the trucks /half tracks will be used to quickly move the rest of the equipment and supplies 2 km inland above the water level.
- This task may not finish before the next tide so the whole unloading may take a day or two.
In any case there'll be plenty of supplies allready in the first day.
Also in that short timeframe the RN will still be dealing with the KM or en route from it's bases so it could not interfere much.

Previous discusions have shown that the Home fleet can be at the invasion beaches in less than 24 hours. Then your invasion troops and the transports are eating shells up to 15". The fleet AA can also inter fere with the transport aircraft and recovery of supplies. If the invasion starts during the day you can probably right off anything still on the ships after dark that evening.

[kalpazanin]

You aren't saying the transport planes can do a complete sorty in 30 minutes are you? In any case fields that close to Britain will very likely start getting hit by night bombers from the first night on. This will either force them back or cut seriously into their sortie rate.

Why not? Consider the distance, the average speed of the airplanes and that's what you get.
The Ju 87 were doing even shorter sorties in the russian front while providing close support to the ground troops.
As to the RAF bomber attacks - that is possible, but it will force them to "Battle of Britain" in reverse (shot british pilots cannot be recovered)- that alone could bring the RAF to it's knees.
Also don't forget that the flight path of most of the Me109E will pass that area anyway, so it will be the last place on earth for the RAF bombers - they'd be decimated.
Also the idea is to prepare enough air strips in advance. The transports will be placed in concrete hangars - so little damage to the airstrip will be acceptable price for numerous RAF bombers shot.

but becuase you are dropping at high tide the RAF can predict when the transports will be there after the first sortie or two.

Not exactly. I didn't mean during the maximum point of the high tide - I meant when tide is somewhere sufficiently above the low point - that leaves ~80% of the tidal time cycle available for the drops.

The LW didn't do a very good job of escorting bombers why do you think they could do a better job with transports..

Becouse the situation is completely different. In the cases you refer to, the bombers were flying hundreds of kilometers one way only, the RAF had plenty of time to detect them, plan where to intercept and direct adequate number of fighters there.

In my case there will be hardly any help from the radar (the invasion overruns the channel based antenaes) and the distance to RAF airfields is too long for effective reaction.
Look at this this way - for all practical purposes the transports could be flying missions inside Germany - the RAF's reaction wouldn't be of much less effect.

While sending the barges back to the rivers will send a signal that the invasion is off the concnetration of ships and troops will indicate that something is still up. There is a very good chance that the British will know within a few hours when the transports start loading and be watching very closely for them to sail. Those gathered in ports close to Brittain will likely be attacked at night before they have even sailed. Which brings up the point of what ports do you plan on using? How many would you need to load up a 100 transports in two or three days time. Then there is the lag for them meeting at sea.

I agree with you that deceiving the enemy of your intentions is a tricky business. However the british knew very well that the barges were to be used for the invasion and the merchants for a supply line afterwards.
Therefore they wouldn't conceive any german invasion without the barges.
The transports could be loaded a month before and kept in the docks - the constant threat could be perceived by the british as 'bluffing'.
Even if british try to sink the loaded transports, when we consider their success rate of 10% with the rows of barges lying packed in the ports,that indicates it's an acceptable risk.
The troops could be brought on board the ships very fast just before sailing- how long does it take for a disciplined german soldier to climb on board?

As to the lag of meeting at sea they should be synchronized to sail to their destinations on small groups (5-10ships) timed the same way as the Norwegian invasion was timed - no 'big fleet' assembly in the middle of the channel.
Considering the number of the ships and german precision that shouldn't be so difficult.

If it's at night what good will the LW do?

Since it's a spring tide there'll be a full moon. LW will be less effective than at daytime ,but still a force to be reckoned with.

Also the Germans will have to provide CAP over the invasion fleet on a contunuous basis while the British can choose the time to mass against it. In general subs didn't do all that well vs warships at combat speeds. They will also have to be pre positioned or they will have a hard time keeping up with the fleet even on the surface.

The moment the merchants exit ports the LW will send all its bombers to lay mines in all south england ports - that alone could keep the RN busy for a day...
The subs will lay waiting at various points along the path of the invasion force.
Don't forget that u-boat torpedoes did sink dozens of destroyers during the war. (some aircraft carriers and battleships as well)

How do you calculate that? Again what ports are they sailing from where do the convoys meet up? How do you keep the loading of that many troops secret in an occupied country (or are they sailing from Germany?)

As I said above - they do not meet,but sail directly to their destinations in small invasion groups. That was allready done in Norway.
The ships would sail from Le Havre, Boulogne, Calais and Dunkerque. The maximum
length would be from Le Havre~120km.
With speed of 10-11 knots it would be crossable in ~ 6 hours.

No. Among other things there is the problem of hull speed. But in any case doubling the motive power does not come anywhere near doubling the speed of a vesel. In the case of barges doubling their speed would probably mean that the British wouldn't even have to worry about sinking them.

That's debatable.
The tugs could do 11 knots and the long barge's hull speed would be less than its overall water drag.
Also if such barge is only partially loaded and towed by 4 tugs it's front hull profile would provide some lift.
Anyway in the worst case there'll be 100 such big barges sailing only from Calais and Boulogne at 7knots. That still brings some additional assistance to the main force.

How long a ramp are you proposing? I suspect if you look at the beach topaographys in many cases the transports will run aground 10s of meters from the beach. If there is a bar as many beaches have it can be even further. Then there is the question of how likely it is the ship will flood when the tide rises again or a rogue wave comes by.

While if planning properly such occasions shouldn't happen we all know that in real life things do get messed up.
For such cases each ship will carry several sections of ponton bridge in it's deck and teams of engeneers who will help to breach the lacking distance to the shore. Also all the available small boats and the small barges would be used to make a short floating bridges from such unlucky ships to the shore.

The flooding of the ship is no problem - it allready sits firmly on the ground so it can't sink.

Previous discusions have shown that the Home fleet can be at the invasion beaches in less than 24 hours. Then your invasion troops and the transports are eating shells up to 15". The fleet AA can also inter fere with the transport aircraft and recovery of supplies. If the invasion starts during the day you can probably right off anything still on the ships after dark that evening.

Most of the home fleet was dispersed away from the channel - so in order to do that they'll have to clean the mines at the port entrances placed by the LW and completely annihilate the KM - considering the forces involved that will take at the very least 2 days.

Also since according to this proposal the invasion force has artillery and armor it can proceed with little assistance from the LW at least for the first couple of days.
This leaves the Home fleet at the complete mercy of the LW close to later's bases in France.

And even if we put aside all of the above the invasion force will be quite capable of returning fire at least to the destroyers.