time context and place: 1939, europe. can a belligerent country of your choice (mine is germany) build a mobile battleship floating on a raft instead of a displacing hull?
since i don’t know how to run the numbers, i can imagine a rigid structure weighing 20,000 to 30,000 tonnes, accomodating a BB’s usual armaments (8x15", 10x5", 20x20mm) with a power plant. this structure will be mounted on a giant raft made of several small blocks of some boayant material fitted in a semi-rigid frame (ay 120,000-tonne displacement) for a total displacement of 150,000 tonnes.
the thing will probably move no faster than 5 knots, but if it’s the english channel we’re talking about, you’re probably in gun range in a very short time. it can be moved/towed to an invasion spot somewhere in southern england to provide both shore defense against naval units and fire support to the troops that had landed.
the thing will be armored and fortified like hell, with maybe a company of marines to combat boarders and frogmen. no fear of foundering either. submarines, frogmen, and torpedo planes might blow off huge chunks from the raft stucture with little effect. bombers and battleships can eventualy knock all the guns twisted and silly. but it will probably give enemy battlehips more than it’ll take, especially with coordinated support by both luftwaffe and other units of the geman navy.
knew about that one but it was supposed to be a mid-ocean airstrip with somewhat different mission requirements. flattening an iceberg looks real easy but they gave it up because it was too expensive. i’ve a feeling my raft will cost a lot more, even more than the bismark. i’m wonderign how valuable it will be if used in operation sea lion and the battle of britain, at the least, ended in a standstill.
So basically what you’re suggesting is that instead of using a lighter-than-water fluid (i.e. air) in a container, you use a lighter-than-water solid, like wood or polystyrene foam?
Even if you’re using a material with a negligible weight compared to the water it displaces, a 120,000 ton displacement still requires 120,000 cubic metres of buoyant material - equivalent to a cube fifty metres a side.
Considering the English Channel is only 40-120 metres deep, if you want a slow, unsinkable weapons platform you might be better off just making it a semi-submersible designed to rest on the bottom when its ballast tanks are full.
It would be HUGE! Why would you want a flat rigid structure when you could have something with a hull? A hull increases maneuverability and would decrease the topside surface area - which, imho, a large topside surface area is just providing a larger target for bombers. Maybe less of a target from the side for submarines, but it just seems as though it would be easier to sink.
Of course, I’m thinking structures here and perhaps you’re thinking more along the lines of a floating island? I’ve only seen them used for wetland habitats, and I wouldn’t recommend them for ocean going but it might be a little closer to your goal.
Sinking is not a big problem for a steel battle ship. For instance Bismarck, probably the most shot at war ship of all times, sank when the bottom valves were opened.
There is a structural problem beyond simple floatation. When a wave lifts your bow and aft and the trough is on the midship, there is a certain amount of weight and thus bending moment that needs to be carried from midship to the ends. This is normally carried by the hull. The most effective way to carry moment is by a tall beam. Thus the heavy double-bottom structure of most ships, it is designed to act as the lower flange of a giant beam. So I advice a lot of steel at the bottom of the structure regardless of the floating material.
In fact, if you are looking for a floating device that is light, durable,cheap, good constructability, known behaviour, may I suggest air-filled steel boxes? It has a good net buoyancy (for each tonne of steel you get to add easily four tonnes of cannons or other stuff to your raft).
If all you want is 5 knots, forget the engines, too expensive and complicated. Any structure can be made to go 5 knots with tugs. On the other hand, with a blunt bow (we are talking about a box structure?) it will stop in moderate waves, no matter how much engine power. Besides, if you want to install propellers, you have to design the aft so that the propellers get water and you lose the simplicity of shape.
In 1940, the antitank capacity of Britain was at Dunquerque, so getting a modest number of tanks across the channel would have made a difference. But the Germans had no way to do that. If this imaginary raft existed, I say screw the guns and armor (plate), just fill it to the brim with tanks (armor) and tow it over. And give Luftwaffe the task of disturbing any British ships or aircraft sufficiently for the dash to succeed. But watch the weather forecast, your box will go backwards if you try to go against the waves!
You would be pouring a hell of a lot of resources into something that was tactically static, and as such a static target for the enemy to fire at.
You’d be much better utilising scarce resources for something(s) that having done a job in one area can go somewhere else to achieve another task, rather then sitting there like a stranded whale.
Interesting idea, but I think that the effort and money would be better spent on building a wooden horse … once that is built you get inside and … wait, hold on I have already said too much.
Considering the very next person to post recommended “air-filled steel boxes,” I would think it’s a valid concern. Not to mention many rafts obtain their buoyancy from some type of air filled chamber (be it barrels, pontoons, or something else).
Add battle damage and rough weather, and I would think the results would be similar.
nice insights from everyone, especially goof and walther. for goof, it won’t be for open sea, the way i see it. germans would use it to attack england or the scandanavian seaboards. reading on the voyage of the kon-tiki, a raft whose length is shorter than the wavelength of an ocean swell can still survive without breaking apart as long as it’s porous enough to equalize pressure between the top and the bottom instantly. the kon-tiki was made of balsa logs roped together, with spaces between the logs. they put it this way: for a raft, the more leaks the better. less chance of lateral stress differences along the length and pressure differences above and below. well, this raft will be a fat elipse or almost oval, so it can glide forward easier, and to give generous swells on all sides for better protection from bombs and torpedoes (sort of usign the same principle in designing castle bulwarks and turrets.) also, large perforations along the entire structure will automatically “spout” excess pressure from undeneath.
walther, steel boxes are nice (very bouyant) but they might allow too much explosive force deep into the raft body. i was thinking of bouayant pumice or even “bubbly” glass blocks. exposed surface area will be less than in a ship since there is no need for decking. the only structures that need to rise above the wave breaks would be armaments and a conning tower. the rigid component is basically submerged, nestled within the raft body which is also mostly submerged (just like the kon-tiki.) actually, one problem is how effective the weapons would be if they were almost level with the waves.
re: dunkirk. you’re talking about june 1940, right? the british EF left all its equipment behind and if germany invaded right there and then, they would have encountered only 3 divisions around london. but britain was a populous and highly industrialized country then and by september 1940, it had fully armed and equipped 16 divisions around the southern england area, ready to repel a breakout from a german bridgehead.
You’re talking about the English Channel, right? Just checking some of the buoys, some locations along the English Channel currently have 5 foot wave heights (8 second period). It’s not open ocean, but it’s not small either.
From your previous comments on the size of your raft, you’d have to downgrade it a bit to get it to have a length shorter than the wavelength of the swell.
Is everything under deck level polystryrene? If so, all outfitting and stores need to be above deck level. Including ammunition. And power generator. And its fuel.
There is no way you can build a flexible platform of this size without some advanced material engineering. I guess rubber-steel combination with what ever floats your boat. Balsa will flex with waves but also break under the weight of the guns.
How much can guns heave/pitch relative to each other and stay accurate? Can a flexible platform measure, calculate, aim and shoot accurately? Would a group of single gun platforms make more sense? If mankind ever goes to rafts in shipping, I just wonder why we need to start with a battle ship, it’s kind of a leap from Kon-tiki…
The Japanese built a huge floating airstrip on Tokyo Bay around 2000, did some take-offs and landings and announced that it is fine. Then they dismantled it. I have no idea why.
I have never seen a photo of something called a Flak-Cruiser, but it was some sort of non-moving raft covered with anti-aircraft guns and posted near the Keil Canal. It might be close to what you are talking about. Or not.
The Russians (whe else) tried something like the back in the 1870’s, with the good ship Novgorod. Didn’t work out too well - impossible to steer, and if one of the guns fired it tended to spin like a top.
