Materials in submarine hull construction.
Guillermo, Naval Engineer, Question on ( http://boatdesign.net ) - Interesting project, Wil. But it's going to be no easy task. Have you compared costs against an all steel one? Cheers.

Hello Guillermo, I agree that in yacht building steel is the better option. - Not so in a submarine. Walls of a 9m sub as my prototype are 18 cm thick and have spheric curves - no way to do this in steel. While you go for al light hull in a yacht you go for a pressure resistant hull in a sub. This is where advantage of making thick walls with little forming effort becomes important.

Second, steel needs maintainance, concrete not. This is why oil rig legs and submarine tunnels are made in concrete not in steel. You could not take a platform like troll out of the water every season to get new paint on it and even less a submarine tunnel. - With a light steel yacht you can. Even a small sub like my project is a lot more heavier than a similiar sized surface boat - therefore slipping cost is high - so avoiding go to slip during decades is a mayor cost benefit.

My submarine yacht had lower maintainance cost than similar sized sailing yachts under same mooring conditions. see (http://concretesubmarine.com) - This was because it never needed to be taken out of water during a decade - and due to the lack of deck and lines and sails that can be damaged by weather and UV rays.

What concerns cost, just sandblasting and painting a steel hull will be more expensive than building a concrete hull.
The steel hull for Euronaut had the following cost: 16 m x 2,5m x 22mm in 2002 for 50.000 euro about 64500 USD - the boat has a displacement of 70 tons which means hull cost of about 714 euro per ton of displacement - for a 2,2cm thick hull. In concrete you get 331 Euro/ton of displacement in a 20cm thick hull (almost 10 times thicker than a steel hull) free of maintainance cost formed to elegant blimp shape.
Experimental Concrete

Longliner45 , Senior Member, Ohio ( http://boatdesign.net )
Is this hull reenforced with steel rebarb? or anything else?

There is no need for experimental concrete in a submarine hull as the forces that appear in a submaine hull of thick walls are the same as in the walls of a submarine tunnel, the legs of a oilrig or the base of a concrete dam. So reinforcing the concrete with something else than steel would not make any sense. The reason why you use steel reinforcement in concrete is to give the concrete strenght against tension forces. Steel and concrete have a very similar extension characteristic when temperature changes - this makes them ideal partners in a composite material. Do not use other reinforcement materials in your concrete it is asking for problems... Concrete engineering is a very simple sience with a couple of quite basic rules and very forseeable results if you stay within the rules. This is why millions of constructions all over the world trust in reenforced concrete in aplications where live depends on it. The new thing in a concrete submarine is applying the proven engineering of tunnels, oil rigs and dams, to submarines - basicly adding a motor, rudder, and tanks, to the structure to convert it into a submarine. Do not leave the proven field of normal concrete - this is a dangerous way that can bring up catastrophic unexpected failure.

Experimental Concrete Fibers

marshmat Location: Ontario ( http://boatdesign.net )
Spoken like a true civil engineer.... they sure love their concrete.... you wouldn't happen to be one, by some chance? Some of the researchers here are investigating the use of carbon reinforcements in/around concrete columns... frankly I don't get the point; concrete is popular mainly because it's dirt cheap, and carbon tends to be insanely expensive... but I guess they can justify it somehow. Also some work there on fibreglass-mesh reinforcements that might be promising.It looks like you've stayed with tradition and overbuilt everything- a sound design philosophy for this project, IMHO. Very cool, indeed.

Hello Matt,
I am not a civil engineer, but i have discussed this a lot with civil engineers, and the concrete handbook for civil engineers is my bible in this as i really want to build with a material that is investigated and forseeable when my life depends on it. Fiber of any kind in concrete is a big problem: The key factor for strong concrete is compacting the concrete during pouring. So spaces smaller than 3 cm are TABU in concrete construction this is why you see workers with vibrators compacting concrete between the steelbars on every construction site - how would you fill and compact concrete between the micrometer spaces of a glass or carbon fibers - i have never seen such an application in any construction site - probably there are good reasons NOT to do it. Anyhow there are fiber concrete aplications for roofs like ETERNIT. But those are aplication where the concrete is not supposed to take forces of any kind.

joe_copeLocation: Shanghai, China What kind of concrete did you use? If I'm not mistaken ordinary cement is slightly porous and would allow water to seep in (and air to seep out) so you'd want to use a waterproof cement, right? Also, regular cement shrinks when it cures, right? so did you have any issues with shrinkage or cracking in the hull during the curing process?

What about using some kind of fiber-reinforced plastic to increase flexability, reduce the chance of cracks, and to reduce weight? I'm remembering some kid's toys back in my hometown that were made out of recycled milk jugs. You could use some kind of plastic and glass or carbon fiber and build it up in layers (paint it on layer by layer)...
My recommendation: do it as concrete engineers handbook suggest - stay away from experiments ...
seeping, cracking, filtering
The idea that concrete is a kind of sponge where water and air passes trough is not realistic - concrete made correctly does not allow pass of water at all. You see many structures without any coating that resist water under pressure. without any additional measures. (tunnels, dams, bridge fundamentations) - anyhow if you are concerned about this have a coating on your submarine - i recommend a tarmac based one as i had on my testproject [www.tolimared.com/submarine] i also was rather neurotic about what would happen when the coating would be compromised - until i noticed that the coating had been compromised in an area of 10x10 cm and i had not noticed it - no wet spot inside no filtering under pressure - just nothing. Today after having seen this in practice i would say coating is fine for estetical reasons - not necessary for tecnical ones. - If you have a imagine of a concrete submarine hull as a wet, filtering place just look at the poto in the photogallery - the hull was not only without any filtering it also was without condensation - the bilge stayed nice and dry under hot sun, and under ice cover - thing i had not expected in first place

The point is - any intent of making concrete "better" is dangerous as you go away from what is proven state of art of concrete construction.There is also no need to make it "better" as normal concrete has all properties required for a submarine hull. I do not recommend any additives - on contrary this is a way to make things more complicated to handle and increase the probability of failure. Concrete bible says that you should only use that kind of things if you have a concrete lab available - if you have no lab - stay at normal cement - in normal conditions - it is the only way to get a good result with predictable strenght. Adding flexibility does no good as concrete has a reasonable flexibility anyhow and changing form under pressure (flexible) is a problem - not a advantage.In a submarine you want a solid pressure resistant wall no layers at all - layers mean changes in force flow that are impossible to calculate and predict. Cracks are part of concrete construcction as rust is part of steel construction. Other than in glass a crack in concrete does not weaken the ability of the material to take forces - basicly for compression forces it does not matter and tension forces are not taken by the concrete - they are taken by the steelbars.

And what about shrinkage? I know that there is "non-shrink" grout for things like swimming pools. Would shrinkage while curing be a problem for larger subs or thicker hulls?
The kind of cracks you get in concrete by shrinking under normal conditions do not go trough the structure and are very small. The problem engineers worry most about is that water can reach the steelbars and produce rusting and therefore weakening on long term - therefore there is a normative that says that you should maintain a concrete cover of at least 2cm over the bars to enable them to take the full force and to avoid filtering water down to the bars. So stay within that normative and back it up with an aditional bitumen cover of 1mm - you will be at the save side and have no problems with your concrete submarine hull - no filtering.
In worst case you have a filtering it tends to stop under pressure as spaces reduce under compression. Trieste had a 11.000 m dive with a filtering on surface. Again i know the idea of cracks is a little scaring especialy if you have submarine films in mind where glass and acylic cracks just before failure. You will get piece of mind if you talk with a construction engineer about this - and his state of mind when cracks appear in the colums of a building that take thousands of tons of force ... They do not worry about cracks that appear during curing of concrete the only kind of cracks they worry about is the kind that appears years after the construction and bring up colors of rusting bars and other sign of problems and filtering in concrete due to bad execution of basic concrete rules. Shrinkage is no big issue in normal construcction. If you have walls of dozends of meters as in dam construction you get a problem because concrete produces heat while curing and must not reach more than 30degrees Celsius during cure process. Again, stick to the proven you will get a good result - no reason to see ghosts of cracks, shrinking, filtering everywhere - If you ever have seen a concrete hull in parctice you will trust it as you trust bridges, tunnels, and colums in everyday life. You ever thought about the fact that the building above you is hold in its place only by a concrete column full of cracks due to curing process - a scaring idea - similar scaring as the idea of a watercolumn of hundreds of meters above your submarine. This is the same thing as first glassfiber hulls came up - people said: oh you would cross the ocean in a fragile boat of glass? or even worse in one made out of coal? - if you ever have seen and touched a real hull and get a idea of its real strengh - all those "ghosts" simply disappear.
Acrylics, tourist sub, classification, cost, design

kach22i Architect Location: Michigan - why does your boat look so different from usual tourist subs with those big viewports )...

Acrylic viewports of dome shape take a maximum of 10000 dive cycles and then have to be replaced at prohibitive cost. A tourist sub similar as an airplane has to be certifiacted for passenger transport. So this is a segment completly apart from yachting. In a private yacht you can do and use what you consider - in public transport not. You will never get a certification for a concrete hull as there are no normative rules for certification. So the sector of tourist sub will be the LAST segment where you will see a concrete submarine. - This is why my design was NOT meant for it.
The design was made to perform as low cost low maintainance yacht being parked on a mooring in a field of sailing and motor yachts, and perform for weekend trips in a yachtintg environment. I found out that my submarine yacht had even lower maintainance cost than the sourrounding sailing and motor yachts so given the restrictions to tourist submarines i believe submarine yachting and private expedition submarines is the segment where concrete submarines will first be implemented .

To get a good idea about submarine viewports and what is state of the art read (Jerry D. Stachiw) and (http://www.psubs.org/)

What speaks against the concept of enourmous curved acrylc surfaces like in a tourist sub is cost. Viewports only can take 10.000 dive cycles then must be replaced. It is ok if you have a tourist sub you earn money with each dive. If you dive 10 times a day you have to rebuild whole ship after 3 years. This may work for a acrylic tourist bus application but- a 3 years livetime is prohibitive for a yacht. This must be why ussubmarines 65m megayacht submarine has not yet been realized - tremendously expensive and rebuild all acrylic surfaces - after 3 years...this is too much.

In my concept you have 4 viewports of 50 cm diameter. Those are disc shapes which take more loadcycles than domes. You also can change them at reasonable cost - even without taking the submarine out of water.This gives you enough light inside. You also have a fine view of underwaterworld if you bring your face close all your angle of view is covered by the viewport. So limiting the number of viewports and stay with flat disc shape instead of dome is a key factor for a reasonable life span and a reasonable cost in a submarine yacht.

Forming Shape
Thijs Struijs - The Netherlands That's a great sub you built... How did you make the mold?...How connect the viewports to the hull...?
What concerns concrete forming you should have a look at the forming process with "slip forms" as done in toronto TV tower - this consists in a small ring forming area that moves up the building a step every 5 days - just do it horizontally. You need to compact the concrete in the mold - if you build whole mold at once you can not access all areas. If you check how oil platform legs, bridge foundations, cooling towers, etc... are formed you get a good idea how to do it - they do not use a complete mold they form in small parts...of course all mold parts get removed and must be prevented from moving during pouring in the concrete.
Viewport seatings in a concrete submarine
The general purpose of viewports seatings is that you need the area of the viewport being stable and flat. In a submarine with a thin steel skin over a rib skeleton the skin moves under water pressure at dive. The viewport seat stops that movement in the area where acrylics and hull meet. In a concrete submarine you will not have that kind of movements.
A massive steel peace in your hull will interrupt uniformity of force flow. So i opted for forming the viewport seating from concrete - no steel ring. Worked fine at my pressure test model, worked fine at my submarine.