Beachable fortress for cross-channel invasion

I am trying to be sensible with the title and not hurt peoples sensibilities too much, but there is no escaping what this is. A What If for another half-baked idea to make the unmentionable mammal work - in another world.

Here goes: Imagine a somewhat different 1930's were the following four conditions are met.
1) The Germans are better off total resource wise so the can afford the proposal below without to many knock-on effects (through any number of means, earlier power grab, better economic management, not losing as much of Silesia post war, whatever!)
2) They feel confident they can defeat France! Otherwise they would never prioritize the stuff below. Again, this could be for better internal strength or realization of French weaknesses, most likely a combination.
3) They really want to defeat Britain! Anything goes, maybe Hitler becomes fascinated with Wilhelm the conqueror, anything goes, but the Germans want to invade Britain early on.
4) The German realize that a conventional naval buildup towards superiority is impossible as the British will outbuild. Temporal asymmetry is the best possible and they go for trying to defeat the British army in France and build sea lift and a protection force for the channel crossing.

Todays what if is how the proposed trinket would fare as part of such a protection force. The inspiration comes from the last voyage of the mighty Yamato. Coulda design be made, purpose build to be a beached gun platform? A design that would be constructed to be beached at high tide and then function as an invunerabile gunplatforn around the narrow parts of the British channel. Maybe on the shallows a few km off the british coast. Coupled with a pre-planned gun installation on the french side, no ship in the channel would be more than 20 km from heavy German artillery.

Now the design below I assume for a while would not obviously look like a warship until the guns are installed, which could be ordered way in advance and it has very low engine power.
I imagine this design as having 3-4 inch outer armor plates for decapping. Then a 5-6 meter space followed by a 16-17 inch armor plate and that followed by a 2 inch splinter protection plate/torpedo bulwark from top to bottom of the hull.
When it is beached, a large portion of the hull and space between the armor plates would be filled with water (maybe sand, but I dont know hos fast that could be done). When looking at the specs from shipsharp, please consider that the deck armor in the simulation is spread over the entire width of the ship, but it only needs to cover half so the deck armor thickness could be 12-13 inches.

As far as I can see, the vessels would be quite immune to anything until the tallboys comes along?

If we imagine 4 of these build, two on either side of the invasion site, maybe another version with rapid firing 8 or 6 inch guns, and connected with wires to provide ease of communication and joined firecontrol and redundancy in the same. How would it work? The cost would be similar to 2-3 battleships (a lot saved on engines though)

Another note. If you wonder why such an invincible ship was never build in real life, one reason could be that it is not invincible. The armored citadel is about a quarter of the ships volume so it couldnt keep it afloat. That doesnt matter if you intentionally fill the ship with water anyways.

Germany Monitor laid down 1938

Displacement:
23.698 t light; 25.866 t standard; 26.135 t normal; 26.351 t full load

Dimensions: Length (overall / waterline) x beam x draught (normal/deep)
(421,44 ft / 410,11 ft) x 85,30 ft x (32,81 / 33,05 ft)
(128,46 m / 125,00 m) x 26,00 m x (10,00 / 10,07 m)

Armament:
4 - 15,98" / 406 mm 55,0 cal guns - 2.204,08lbs / 999,76kg shells, 300 per gun
Breech loading guns in turret on barbette mounts, 1938 Model
2 x 2-gun mounts on centreline ends, evenly spread
8 - 5,04" / 128 mm 55,0 cal guns - 69,07lbs / 31,33kg shells, 450 per gun
Dual purpose guns in turret on barbette mounts, 1938 Model
2 x Twin mounts on centreline ends, evenly spread
2 raised mounts
2 x Twin mounts on centreline ends, evenly spread
2 double raised mounts
12 - 1,46" / 37,0 mm 65,0 cal guns - 1,71lbs / 0,78kg shells, 1.200 per gun
Anti-air guns in deck mounts, 1938 Model
4 x Twin mounts on sides, evenly spread
2 x Twin mounts on centreline, forward deck aft
2 raised mounts
64 - 0,79" / 20,0 mm 65,0 cal guns - 0,27lbs / 0,12kg shells, 2.500 per gun
Breech loading guns in deck mounts, 1938 Model
12 x Twin mounts on sides, evenly spread
4 x Twin mounts on centreline, aft deck forward
4 raised mounts
Weight of broadside 9.407 lbs / 4.267 kg

Armour:
- Belts: Width (max) Length (avg) Height (avg)
Main: 19,7" / 500 mm 246,06 ft / 75,00 m 32,81 ft / 10,00 m
Ends: 1,97" / 50 mm 164,04 ft / 50,00 m 26,25 ft / 8,00 m
Upper: 3,94" / 100 mm 246,06 ft / 75,00 m 13,12 ft / 4,00 m
Main Belt covers 92 % of normal length

- Torpedo Bulkhead - Additional damage containing bulkheads:
1,77" / 45 mm 246,06 ft / 75,00 m 45,93 ft / 14,00 m
Beam between torpedo bulkheads 42,65 ft / 13,00 m

- Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
Main: 21,7" / 550 mm 13,0" / 330 mm 15,7" / 400 mm
2nd: 5,91" / 150 mm 3,94" / 100 mm 3,94" / 100 mm
3rd: 0,79" / 20 mm 0,79" / 20 mm -
4th: 0,39" / 10 mm 0,39" / 10 mm -

- Armoured deck - multiple decks:
For and Aft decks: 6,30" / 160 mm
Forecastle: 0,79" / 20 mm Quarter deck: 0,79" / 20 mm

- Conning towers: Forward 21,65" / 550 mm, Aft 21,65" / 550 mm

Machinery:
Diesel Internal combustion motors,
Direct drive, 2 shafts, 3.033 shp / 2.263 Kw = 10,14 kts
Range 2.000nm at 9,00 kts
Bunker at max displacement = 485 tons

Complement:
1.027 - 1.336

Cost:
£10,793 million / $43,171 million

Distribution of weights at normal displacement:
Armament: 2.311 tons, 8,8 %
- Guns: 2.311 tons, 8,8 %
Armour: 15.116 tons, 57,8 %
- Belts: 8.694 tons, 33,3 %
- Torpedo bulkhead: 741 tons, 2,8 %
- Armament: 1.894 tons, 7,2 %
- Armour Deck: 2.966 tons, 11,3 %
- Conning Towers: 822 tons, 3,1 %
Machinery: 83 tons, 0,3 %
Hull, fittings & equipment: 5.588 tons, 21,4 %
Fuel, ammunition & stores: 2.438 tons, 9,3 %
Miscellaneous weights: 600 tons, 2,3 %
- On freeboard deck: 200 tons
- Above deck: 400 tons

Overall survivability and seakeeping ability:
Survivability (Non-critical penetrating hits needed to sink ship):
46.312 lbs / 21.007 Kg = 22,7 x 16,0 " / 406 mm shells or 10,2 torpedoes
Stability (Unstable if below 1.00): 1,10
Metacentric height 4,7 ft / 1,4 m
Roll period: 16,5 seconds
Steadiness - As gun platform (Average = 50 %): 68 %
- Recoil effect (Restricted arc if above 1.00): 0,47
Seaboat quality (Average = 1.00): 1,35

Hull form characteristics:
Hull has a flush deck,
a normal bow and large transom stern
Block coefficient (normal/deep): 0,797 / 0,798
Length to Beam Ratio: 4,81 : 1
'Natural speed' for length: 23,88 kts
Power going to wave formation at top speed: 26 %
Trim (Max stability = 0, Max steadiness = 100): 50
Bow angle (Positive = bow angles forward): 20,00 degrees
Stern overhang: 6,56 ft / 2,00 m
Freeboard (% = length of deck as a percentage of waterline length):
Fore end, Aft end
- Forecastle: 20,00 %, 13,12 ft / 4,00 m, 13,12 ft / 4,00 m
- Forward deck: 30,00 %, 13,12 ft / 4,00 m, 13,12 ft / 4,00 m
- Aft deck: 30,00 %, 13,12 ft / 4,00 m, 13,12 ft / 4,00 m
- Quarter deck: 20,00 %, 13,12 ft / 4,00 m, 13,12 ft / 4,00 m
- Average freeboard: 13,12 ft / 4,00 m
Ship tends to be wet forward

Ship space, strength and comments:
Space - Hull below water (magazines/engines, low = better): 86,3 %
- Above water (accommodation/working, high = better): 77,6 %
Waterplane Area: 31.898 Square feet or 2.963 Square metres
Displacement factor (Displacement / loading): 100 %
Structure weight / hull surface area: 159 lbs/sq ft or 775 Kg/sq metre
Hull strength (Relative):
- Cross-sectional: 0,92
- Longitudinal: 2,11
- Overall: 1,00
Adequate machinery, storage, compartmentation space
Cramped accommodation and workspace room
Good seaboat, rides out heavy weather easily
 
Did they not plan some ships to be beached for D-Day but in the end decided against it? Might be a useful place to look at doctrine and actual usefulness.
 
With a speed of 10 knots you will have anything with a torpedo will be able to get a successful launch solution on it quite easily. You don't have to worry about tallboys in this case, unless you have a large group of destroyers and minesweepers operating in front of it on the way to the channel, you will be able to hit it below the waterline easily.
 
The problem is, pre-conflict on paper the French looked quite good - best tank, lots of aircraft, outnumber the Nazi's ... no one (not even Hitler) expected them to fold as fast as they did ...
So you need a POD that makes them look 'bad' on paper ... allowing Hitler to be confident of beating them ... BUT if that's the case, then the British are likley to send even more troops to Europe in order to stop them being beaten .. and H. would know that, so would be looking to dedicate even more resources to tanks / aircraft and less to ships .....
Here's an idea == Communist Party elected in France in depression era (say 1933), attemts to sieze 'the means of production' utter chaos in heavy industry, France collapses into civil unrest, national strikes, lock outs, destruction of the manufacturing base. By 1938 the communists are out but this leaves the country without tanks / aircraft and no means of making / buying any ....
In the 1930's, the British will be more worried of Communism being exported across the Channel than the possability of Hitler making a grab for France ... SO LONG AS Hitler sticks to his 'we go east' speeches and actions (so hands off Czechokslovakia == go for Poland instead **) the British are unliley to start re-arming early enough ...
[** IMHO Hitler has more justification for the return of Danzig / Polish corridor and without the example of Czechoslovakia I can quite easily see Chamberlain agreeing to the Danzig first, half the rest of Poland second in the same way as he rolled over on the Czechs == even more so when Stalin claims the other half and it's a choice of agree or declare war against Germany and the Soviets both]
 
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Did they not plan some ships to be beached for D-Day but in the end decided against it? Might be a useful place to look at doctrine and actual usefulness.
They sank ships as wavebreakers, not as gun platforms
What's it for?
To keep away British naval ships and protect the Dover-Calais transport route.
With a speed of 10 knots you will have anything with a torpedo will be able to get a successful launch solution on it quite easily. You don't have to worry about tallboys in this case, unless you have a large group of destroyers and minesweepers operating in front of it on the way to the channel, you will be able to hit it below the waterline quite easily.
That is a given, but at the narrowest part, it’s only two hours across the channel. Besides, it can take a few hits.
 
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The problem is, pre-conflict on paper the French looked quite good - best tank, lots of aircraft, outnumber the Nazi's ... no one (not even Hitler) expected them to fold as fast as they did ...
So you need a POD that makes them look 'bad' on paper ... allowing Hitler to be confident of beating them ... BUT if that's the case, then the British are likley to send even more troops to Europe in order to stop them being beaten .. and H. would know that, so would be looking to dedicate even more resources to tanks / aircraft and less to ships .....
Here's an idea == Communist Party elected in France in depression era (say 1933), attemts to sieze 'the means of production' utter chaos in heavy industry, France collapses into civil unrest, national strikes, lock outs, destruction of the manufacturing base. By 1938 the communists are out but this leaves the country without tanks / aircraft and no means of making / buying any ....
In the 1930's, the British will be more worried of Communism being exported across the Channel than the possability of Hitler making a grab for France ... SO LONG AS Hitler sticks to his 'we go east' speeches and actions (so hands off Czechokslovakia == go for Poland instead **) the British are unliley to start re-arming early enough ...
[** IMHO Hitler has more justification for the return of Danzig / Polish corridor and without the example of Czechoslovakia I can quite easily see Chamberlain agreeing to the Danzig first, half the rest of Poland second in the same way as he rolled over on the Czechs == even more so when Stalin claims the other half and it's a choice of agree or declare war against Germany and the Soviets both]
Whichever works, I want to know how the vessel would perform
 
That is a given, but at the narrowest part, it’s only two hours across the channel. Besides, it can take a few hits.
This point often comes up in discussions around Seelowe, that although the shortest distance across the Channel is only 2 hours at 10 knots, you have get your ship from wherever is is coming from to the starting point for the shortest crossing, and it will be noticed then. Well, it would be noticed when under construction. The British had pretty good aerial reconnaissance, and spies. This would be very difficult to pop as a surprise.
 
To keep away British naval ships and protect the Dover-Calais transport route.

But I'm not sure how it would help, really. No gun platform can change the basic math that the Royal Navy totally controls the ocean and the Royal Air Force is at last at parity in the skies. What are a few guns on a immobile sandbank going to do?
 
I suppose the Royal Navy's battleship crews would enjoy the gunnery practice. Is the armour on this vessel thick enough to stop the RN's AP shells?
 
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It might be "invulnerable" but how long do you reckon the crew would survive in there before the sheer number of rounds hitting it from the UK coastline just drove them insane? You'd have everything from 18lb to 15'' having a crack at it along with everything the RAF could put up.
 
Where did you get the performance figures? Tonnage?

What is the actual design of the hull/protection ? Size of transverse bulkheads?
It’s shipsharp 3beta.
So the details missing are not specified. It annoys me as well that the transverse bulkheads are unspecified. Obviously they would need to be there as well, although at the ends of the citadel, a lot of water/sand could help decelerate a shell.
 
It would be a waste of steel
That would be felt during Barbarossa
As for the fortress itself, it would be getting nightly visits from the RAF
 
It might be "invulnerable" but how long do you reckon the crew would survive in there before the sheer number of rounds hitting it from the UK coastline just drove them insane? You'd have everything from 18lb to 15'' having a crack at it along with everything the RAF could put up.
It wouldn’t go there alone. It’s a surprise factor to locally counter the RN superiority. It would accompany an invasion. If the land forces focus on these, well, much better for the vulnerable ships.
 
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