ferro-cement submarine versus conventional concrete

[tugboat]

method #1
the split mold method-
submarine is made in two seperate peices by molding process, then the two halves joined together on a horizontal axis just as the way a model submarine is made. steel flanges running the circumference of the face surfaces, about 3-4 inches wide and about 1/2 -3/4 inch thick are welded onto the steel rebar protuding from each half mold subs horizontal sides to form flat mating surfaces. these are then bolted down with a gasket of epoxy layered between the two steel flanges. upper and lower. huge bolts are used to secure the two halves. and a watertight gasket is places between these steel face surfaces. so the two halves become one unit. this allows the two halves to be cheaply moved and reassembled or if the sub ever needed to be re-fitted, the bolts can be removed and the sub halved open.
the groove line that is formed at the mating surfaces perimeter outside the hull- which was not covered in cement is then filled with an epoxy/filler. sealing up the whole sub into a pressure vessel.

method 2.
a complete submarine hull is built in ferro-cement - then layered. ie. the shell becomes a mold for itself and three or more layers are built up using the same process of welded mesh and stringers.


advantages of the split mold is cost- since you are not building up layers i.e. you would have 6 inch walls after the pour,- the cost is less. however you then require two truck loads to move both halves to a marina. or other finishing off area near water. but potentially no crane or a small crane could be used. with a hydraulic trailer being the best option and cheapest.

the advantages of the layer ferro-cement method is ease of construction. you have the finished product and it is monocoque. the layers are easy to do yourself. and this is very strong 19 000 -21 000 psi impact resistance. (ref. hartley and brookes) this also allows for ease of transport(assuming both sub methods built to 50 ft loa)since the costs for transport are very large sums dealing with big cranes --rental trucks etc.

there are other advantages and disadvantages, but those are my main ones...
feel free to add your own advantages or disadvantages...

can of worms- opened!

 

[gonzo]

I think the main disadvantage would be failure after submersion.

 

[tugboat]

(laughing)--Gonzo--uh..how so? do explain...

 

[Lurvio]

Why not segments? Like a concrete culvert. All the bulkheads etc can be made in the same pour. Should be much stronger than the method 1 of the 1st post.

 

[tugboat]

Thanks Lurvio-
sounds interesting- it has been suggested before that i use a concrete pipe.
how could i pour the bulkheads in one shot?

my design is a scaled down skip jack class sub- at 51.5 ft loa, beam/dia/depth/8 ft, D.E. using pumps and dynamic diving slightly positive buoyancy, dive planes, high performance seals, and a high torque 4 hp for submerged running, ballast is pumped to raise waterline on surface. laminated lexan veiwports.-
your responses are always welcome Lurvio
maybe ill post my drawings...frame designs, systems etc...if I get enough attention on it..

 

[gonzo]

The main problem is the cement tends to crack when bent. There are huge loads on a submersible. Other threads deal with a cement submarine. Have you done any calculations?

 

[Lurvio]

Tugboat
I'm (again) talking of conventional concrete (make a mold, drop in rebar and pour). The mold can quite easily be made to include a bulkhead. If the mold is made vertical (round when viewed from the top) you can leave part of the bulkheads area open at the top to ensure no voids and can finish the surface by hand. The bulkhead can be at any height, but its probably easier to make closer to the top of the mold. The segments would make somewhere between 5 and 8 feet of the hulls lenght, the whole hull would have 6 to 10 segments. Do post some sketches at least. Im no engineer but I'm always interested in new things.

Gonzo
Can you explain in which direction the concrete is bent enough to crack it? There is of course a limit where anything fails, but to my understanding most of the loads are compressional, and concrete is very good at that, if it's the right shape.

Lurvio

 

[gonzo]

There is a huge difference in vertical and horizontal compression forces. Metal hulls can deform without breaking. A concrete submarine will crack.

 

[tugboat]

GONZO-I have done some calcs. I have done some modulus of elasticity calcs too...

I also have studied, built models of this sub. and am about to drop test a scaled hull to 150 ft. probably in June or July.

In fact i could build a 1/4 thick steel hull to a depth of about 150 feet with a good safety factor...but it costs twice as much!- and i would trust it less.

as far as stresses, well i believe that has been answered in the multitude of engineering structures that withstand stresses much greater..i.e. the movement of towers, buildings, bridges, structures etc done in concrete, and survived during a windstorm, or earthquakes etc. and they hold up quite well in most cases..

The cement would have less stresses on it than say a surface vessel in ferro-cement i.e wracking- hogging, sagging, warping, etc-- and they definitely don't crack in a seaway. one owner of a ferro cement hull says

"nothing short of a mine would destroy this hull"!

i have known of steel boats that open up from stem to stern, or break up from weakening of plate or rusted through sections or from metal fatigue, faster than a ferro-hull ever would. this is why military uses titanium now.
but plans have been drawn up for a c-sub. deep diving, for attacking at the surface...the boat would lay on the bottom in await and then fire torpedoes at the above vessel(see popular mechanics).

concrete - they are monolithic. ferro-cement hulls behave like beefed up wood hulls. but with more monolithic strength.
In fact there have been more fc boats that have crossed oceans as cruiser hulls than any other material..or so Ive read from reputable sources. Some sailors wont go to sea unless its in a ferro-cement boat. they take much more stress than a statically engaged object in water such as a submarine.

submerged-there wouldn't truly be vertical and horizontal forces on the egg shaped hull, in fact if i remember correctly my vector forces and spacing, the forces on a hull submerged would be at right angles forming at every point on the hull thus- the compression would be acting at equally at every point on the hull. The vectors of force are then deflected such the same way as radar is deflected off a round object- not on absolute horizontal or absolute vertical angles.

I have also built models and tested them. The stuff is definitely viable- and i suppose if I do decide to build this boat- then I will suffer the consequences should it fail..but someone has to try to answer this question...about concrete civil subs...anything worth doing is a risk- maybe I would fail- maybe i would succeed! but-id rather die trying than never to try and wonder what if? no i'm not suicidal--and if you think this is nuts to try--your quite right! but not sure if i will or wont build it ...just testing now..if I do though im going all the way. would start in june or july.

Lurvio- thanks-im not sure i totally understand what you mean for the mold..but if you have the time, i would love to see a sketch also of your mold design???...maybe two heads are better than one. one thing i refuse to change is the torpedo/blimp shape. so it has to conform to that shape..however i think i see where your4 going--the concrete tube is the pressure hull and the exo-hull is built around the tube???
let me know if we are on the same page?
i will get scan on here soon..just know its a rough sketch with scribbles and calculations etc written in hieroglyphics all over the design sketch..i could perhaps just redraw the sketch so it looks better-- so give me a few days and Ill work on it for you no probs!

 

[tugboat]

for anyone interested- this is an excerpt i wrote in my journal on building my own sub as posted on psubs....it was written more towards building commercially...but it applies to my one-off design...
_________________________________________________________________

i'm going to try to list pros and cons for concrete- for 1 atm designs.

concrete is extremely viable. there is really no point in debating it, as there are solutions to every problem associated with concrete c-sub construction.
and so much evidence to support those methods. remember- its already been done successfully! but there are pros and cons and each person must decide for themselves whether steel, fiberglass, concrete or otherwise- is best for them to produce their dream.
hopefully this helps...

pros: It has the advantage of being monolithic in its properties. this is huge since this is the holy grail of any boatbuilder.
piping can be placed into the concrete, or it can be mounted or even glued to the hull. Thru hull fittings can be drilled, electrical penetrations are easily done wiring through the hull by drilling a hole then filling the surrounding space around any electrical thru wires with epoxy filler. the use of epoxies is the link that makes concrete a viable and valuable material for a c-sub.
concrete can be drilled and then the reinforcing material cut using a torch, this can be done without any serious material weakening, especially if the reinforcement is done properly. the weight is an added bonus giving concrete the advantage of having more living space as the shell would be nearly perfect to act as both pressure hull and ballast,, humidity is never an issue so contraction and expansion does not exist as it does on steel. it is almost non compressible versus steel, its cheaper, it can be formed to any shape easily. It is readily available. it can be modified for strength, impact is not really an issue if done properly.
the potential for deeper dives with concrete over the average home built sub exists since a capable and functional shape in a steel hull is almost always compromised due to the costs associated with compound curvature in steel manufacturing. i.e. try getting a conical bow shape using steel. this is beyond the scope of an amateur.
although less forgiving than steel, it still is a forgiving medium and mistakes although arduous to repair, can be resolved. cracks and spalls again are easily repaired. thicknesses can be varied. and it is cheap. requires relatively unskilled labour for the most part, especially if a mold is used. vs welding where submarine welds better be good ones.


cons.
while it is true it c-subs have been successfully built, there still remains some mystification as to the building processes of a concrete hull. this is because it appears designers are unwilling to share their knowledge of the designs to the general audience due to the fear that - as in ferro-cement, many poor hulls were built and this killed the industry that was going on to be a very reputable way of boat building. thus the fear of tragedy and deaths related to poor hull design, makes designers producing c-sub hulls -reluctant to sharing their proprietary secrets and leaves many people in the dark. however there is no doubt ferro-cement can be a viable solution regardless and is proven.
There also does not seem to be any real research done on freeze/thaw cycles with concrete. hence since c-subs are meant to be kept in the water year round, there remains the question of freeze thaw cycles in areas in which those cycles occur. however steel also is subject to fatigue during those cycles as well.

Concrete is not a conventional means to build a sub and therefore it is looked upon with mistrust by a highly conventional maritime community. tradition and conventional attitudes are not easy to change in regards to radical or newer methods of construction. especially military. this does not suggest newer ideas are any worse, just they are not yet accepted.

Concrete does crack and can be hard to repair especially if the cracks are thru hull. it is suggested by myself for one, that fiberglass layers reinforce the hull after completion to stop leakages and impact damage. this raises costs. the forms for building a mold are expensive and it may well be this compensates for any savings in lighter c- sub designs. as the sub gets bigger this favours concrete however.

fittings are more difficult to design into concrete i.e. hatches etc. steel must be used for certain things in and on a c-sub, so therefore welding skills will be necessary with c-subs as well. things like hatches, bollards, crane hooks, the sail, tanks etc.

drilling concrete is a pain in the a**. time consuming and frustrating. and lack of off -the-shelf-bits for larger diameter holes are a nuisance and expensive to buy. lets face it there are many unknowns to the process of c-sub design.

summary-
given the nature of the two materials, i find personally there are no real advantages to using steel over concrete for a pressure hull, save for the simplicity of being able to weld on parts to the sub. this is a great advantage in the overall fitting out of the hull. however the advantages of concrete,especially costs, seem to outweigh the disadvantages for the nature of the purposes intended by home built sub designers and builders.

Until widely accepted and proven, concrete will continue to be debated as a material for practical sub yachting. Until its acceptance, there will be few people who take the risks involved with the material.

In my opinion. for someone on a strict budget, it is the ONLY material to use, unless you have welding skills to create near perfect welds which will not fail under pressure conditions, and the funds to roll, curve and shape expensive steel in order to get the necessary hull shape required for safe operation i.e. blimp or to some extent, tube.

I will make a point here as to the secrecy involving the processes that some designers clearly want to maintain. Since in withholding information such as the process of building; will force those who cannot afford production hulls, to brave designing their own hulls. Not everyone can afford production hulls after all. Therefore people will nevertheless, undertake projects which will undoubtedly create problems for the c-sub industry. This will create the very thing designers and sellers of c-subs don't want. Bad publicity. so by withholding info on the methods, it fulfills the cycle of lack of knowledge, leading to poorly built hulls, in turn leading to tragedy, which has the potential to ruin future c-sub marketability. my solution is- offer plans for successful designs. thereby allowing the knowledge to be distributed. this will cull out the ones who would produce their own designs, and still give the option to allow others to buy c-subs from reputable dealers. It doesn't seem logical that there should be fear that everyone will want to build there own hull, thereby reducing profit. this is an unfounded fear since many companies thrive who build production yachts. yet there are many who buy plans and build one -off boats simply because they enjoy it.

So the answer is- offer the info for a price, make some money and guarantee at least reasonable results for amateurs and reduce possible risks of stalling an industry.

 

[rwatson]

1) There also does not seem to be any real research done on freeze/thaw cycles with concrete. hence since c-subs are meant to be kept in the water year round, there remains the question of freeze thaw cycles in areas in which those cycles occur. however steel also is subject to

2)fittings are more difficult to design into concrete i.e. hatches etc. steel must be used for certain things in and on a c-sub, so therefore welding skills will be necessary with c-subs as well. things like hatches, bollards, crane hooks, the sail, tanks etc. drilling concrete is a pain in the a**. time consuming and frustrating. and lack of off -the-shelf-bits for larger diameter holes are a nuisance and expensive to buy. lets face it there are many unknowns to the process of c-sub design.

These seem to be the hardest questions to answer in my mind too. Early on I asked our favourite proponent of C-Subs how he was going to handle the mundane fittings of anchors, deck rails, hatches etc, but with little success. Having seen the impact on metal fittings against concrete in industrial land applications, I wonder how practical they would be in a sub, in the very trying marine enviroment.

The future of C-Subs is probably not so much an argument about the major structure, as being able to make it a workable vessel.

Imagine tryng to dock or handle anchors on a curved, wet slippery deck in adverse conditions, You cant really say "oh, I won't get caught in that situation", as Murphys law prevails doubly on the water.

Your comment about monolithic form pours sounds interesting - but I have read in several places that this method has been abandoned for ferro-cement boats, so I wondered what problems were encountered.

When you say "requires relatively unskilled labour for the most part" I would dispute that vigorously. My limited experience with ferro-concrete boats leads me to believe that only very, very experienced plasterers should be used to produce a sound hull, and with concrete subs, I would want to know every single workman on my hull was a dedicated, trained craftsman. No-way would a merely enthusiastic labourer get anywhere near a submersible hull I was going in.

Just to keep the pot boiling in the spirit of debate, the comment "while it is true it c-subs have been successfully built" is highly debatable. I have seen pictures of our friends sleek "edifices", but no evidence that they were ever comprehensibly used. The lack of fittings like safety rails, anchors, nav lights etc etc incline me to think they got a few limited test runs in ideal situations, but never any real work. They dont even have a "scum line", let alone any marine growth.

On a positive note, its good you are adopting a realistic, open discussion, not some sort of one sided, totally optimistic view. You might find that the lack of detail about "other peoples" construction methods is less about keeping valuable secrets and more about lack of tried and proven technology.

 

[Lurvio]

tugboat
I was thinking something like this.

Bridge segment - I-35W St Andrews Falls Bridge

Make that in a round shape and you'll get a boat. Every segment is different so a lot of formwork to do. But you get a boat that can be cast anywhere, transported to suitable place where it's assembled, fitted and then dropped to the water.

I find no benefit for a thin ferrocement structure used in boats, regular concrete makes more sence at least to me in a submarine hull. After all you want it to sink, the weight ain't the problem.

Lurvio

 

[tugboat]

rwatson;360396]These seem to be the hardest questions to answer in my mind too. Early on I asked our favorite proponent of C-Subs how he was going to handle the mundane fittings of anchors, deck rails, hatches etc, but with little success. Having seen the impact on metal fittings against concrete in industrial land applications, I wonder how practical they would be in a sub, in the very trying marine environment.


****Hi rwatson, im sorry i have to address your comments by this method..but wanted to just add input to what you have said...[/B]
so ill go through each point...

"The future of C-Subs is probably not so much an argument about the major structure, as being able to make it a workable vessel.

Imagine tryng to dock or handle anchors on a curved, wet slippery deck in adverse conditions, You cant really say "oh, I won't get caught in that situation", as Murphys law prevails doubly on the water."


***very true- I think that's an item that could be dealt with in the same way that the UC3 dealt with--they used a checker plated deck mounted to the hull topside. This acted both as extra ballast and it gave them a stable platform...one other way this might be over come is to use grab rails-
ill scan in my design sometime-..i added grab rails and a small deck area for docking...but not f'ward. just behind the sail.
Could be an issue but i don't see it as a huge one...but certainly one that needs to be considered.

"Your comment about monolithic form pours sounds interesting - but I have read in several places that this method has been abandoned for ferro-cement boats, so I wondered what problems were encountered."

**** rwatson yes fc hulls don't use a pour method- they are hand plastered. and i probably wasn't clear enough when i said- unskilled labour- it requires skilled labour on plastering day, the armature could be built by a chimp! its labour intensive not skill intensive or cost intensive- and this is why they were hard to build commercially since it is a very time consuming process to lay up a hull. of course you still gotta know what your doing to build the armature..but this is where a single handed person can be very effective alone. up until plastering day it could be done by one person. no real heavy lifting etc...- most folks hire ten people or so to get this right. people who know how to plaster- although the trend now is to plaster to the mesh and then epoxy coat to seal the whole hull.

"When you say "requires relatively unskilled labour for the most part" I would dispute that vigorously. My limited experience with ferro-concrete boats leads me to believe that only very, very experienced plasterers should be used to produce a sound hull, and with concrete subs, I would want to know every single workman on my hull was a dedicated, trained craftsman. No-way would a merely enthusiastic labourer get anywhere near a submersible hull I was going in."


****agreed!! you don't want to have people late or not showing up -or not really good plasterers. if you have put a lot of time even years into the work prior to the plastering day- you want to know who your dealing with!

it is possible to "do it yourself" but just to help- you would need need to be good at "punching through" the plaster. and knowing how to get a smooth finish...and accurate hull thickness- so this area does require skilled labour- but only one day of it

"Just to keep the pot boiling in the spirit of debate, the comment "while it is true it c-subs have been successfully built" is highly debatable. I have seen pictures of our friends sleek "edifices", but no evidence that they were ever comprehensibly used. The lack of fittings like safety rails, anchors, nav lights etc etc incline me to think they got a few limited test runs in ideal situations, but never any real work. They don't even have a "scum line", let alone any marine growth"

****Again i'm in agreement with you rwatson- I don't know what is happening in that particular case- and
i can find no cases of them actually being put into service. there were plans to do just that but i don't know what the outcome was or why--i don't want to assume anything about the reasons as to why -i could speculate forever and never know... realistically those infrastructural items being added theoretically are easily solved with epoxy bonds. and typical through hull fittings etc as i mentioned in my journal

"On a positive note, its good you are adopting a realistic, open discussion, not some sort of one sided, totally optimistic view. You might find that the lack of detail about "other peoples" construction methods is less about keeping valuable secrets and more about lack of tried and proven technology".

Thanks rwatson, I appreciate that you can see that i'm trying to show both sides and be fair...There needs to be more unbiased ideas. after all if we close our minds to either side then we overlook the great input people can offer- constructive feedback even negative feedback needs to be welcomed because regardless of a persons motives- they are in effect helping by revelaing potential problems..the smart person will rejct the B.S. and keep the rest.

being unbiased and seeing what needs to be looked at and perhaps improved or explored more deeply before risking a life, is to me the best policy. anything of course, is always a risk, but I personally don't take uncalculated ones-meaning ill look at every angle first...then go ahead and take the risk unless its so blatantly unrealistic...

briefly on Mr Ellmer - I appreciate Wils optimism. I think I understand why he does not want to reveal a lot of his things to the general public...

I do believe he has come a long way in his vision for an affordable sub..and i commend him on not giving up even in the face of a lot of criticism. Most people would have.

Anything new is bound to attract that...some of it warranted- some not.
He has personally helped me with some facets of my design. So I remain loyal and will offer my support to him until I am dis-proven that there is a red herring at work. But I really doubt this. As you mentioned any postponements would most definitely be of a "red tape" obstacle and not the actual ability of the submarine..at least i think so...

 

[tugboat]

Excellent Lurvio!-- yes this is sort of what I had in mind. the bottom would be pre-ballasted. to let you know I have looked at this method-
which is why i invented the flange type hull/split mold idea. done in two pours then bolted together and epoxied. and moved twice- using cheap excavator to move the hulls into bolting position....

this is since the craning/moving costs would be far far less.! the structure would be just as strong as a monolithic one from what i can calculate- i have seen this done in other engineering projects with great success.

the problem with the static mold, double walled(not slip formed) method is transportation costs and mold costs- for me at least -practically- I live an hour from the water. The weight of my bare hull would be
about 60 000 -70 000 lbs or about 30-35 tons! and that's not fitted out or fixed ballasted. a regular hydraulic trailer can deal with about 20 t max. and the manager told me that's pushing it.

so i inquired about a crane and truck...i almost fainted at "rental shock" it would be about as much to bring in a 80 ton crane at 250.00(about 150.00 euros) an hour and at 12 hours minimum just for the crane...then a truck at the same costs as to build the boat!!

and thats IF i could find a truck company big enough or willing to haul the 51.5 ft x 8 ft diameter structure to the water. on top of that there are overage permits etc. -just to have it un-loaded in Parry sound, then possibly have to pay marina fees for another "travel-lift" after the hull gets fitted out
to boot!..this adds up very quickly--for that price i could do it in fc. add layers and not have to pay much for a move--about 500.00 cdn.

but your idea is quite correct-- my question is-- how would you do the mold?..(i have looked at all this)- to be able to compact it properly so the aggregates do not settle into the rounded bottom , vibrate properly etc??...seems like that's a heck of a lot of troubles and expense for a one shot pour?????

please don't get me wrong..I think the idea is viable- for someone with more money than me-
personally- i just haven't found a realistic solution to 1. moving that huge and heavy hull cost effectively- and 2. the proper mold system. ideally your quite right--the concrete thick walled sub is cheaper..but stronger??..perhaps...i doubt not more than three or four layers of fc added to the hull... fc in layers would most likely be even stronger. resist cracking etc through less diminished dispersal of concrete. if one 3/4 inch fc hull resists 19 000 psi, then add that times three for three layers. but then again your really building three or four hulls into one. thus cost!...six of one half a dozen of the other???

 

[tugboat]

Sorry Lurvio- to clarify the mold would need to be rounded...this is difficult to achieve this form, from what i understand...you cannot over vibrate or you lose the aggregates settling to the bottom since you cannot see the pour- its difficult to know what going on in the mold.

and you cannot see the mixture and if there are voids etc...its tricky!! and i wouldn't want to tackle it in one pour..what are your thoughts?...btw ill have a scan for you mid next week...

however Ill share how i am going to do a two shot split mold-

first you set up your mold but the mold is integrated into the final hull form which eliminates an expensive mold.and gives you ballast and strength.

this is done by building your first of two sets of half-ring frames-since you will repeat this process twice to get two half hulls to join.

the truss frames. - they are built from 5/8th mild steel rod built in truss style 6" wide. with v's created for strength of the truss. when done you will have a half truss. 6" wide. and the diameter of the hull at the station frame.
(sketch to come)

or use 3/4 concrete re-inforcing bar or rebar bent to shape in a welded up home made bench jig. then weld up the trusses- the idea here is material doesn't matter. you could use fiberglass even just as long as its strong and you can make strong trusses rigid. i prefer steel in typical concrete structures. regardless the frame material needs to be strong and rigid for later turning over the half hull mold for the pour.
the trusses are made from my plans offsets -which ill provide to anyone free of charge.
using the offsets(very easy to draw and fair up ring frames for lofting using a homemade compass) from the plans offsets.

now prior to the frame set up a strong back must be created-- this is two parallel 60ft(spliced) lengths of hopefully cheap 4x4 spruce beams leveled perfectly.(or any other dimensional lumber or steel that is inflexible strong and cheap to use)
these are spaced to accept the mating surfaces of the frames. then the athwartships 4x4's coupled with the longitudinal 4x4's at the max beam but attached only at each framing position since that's where the weight will be concentrated. the idea is the frames will span these 4x4s. and be plumbed and leveled and then secured to the 4x4 spacers. the frames will be upside down- i.e. the round surfaces facing upwards. once the frames are erected- then its time to add the longitudinal stringers for strength. use rebar- 1/2- 3/4 inch. welded up to the full length of the hull. then weld to the INNER side of the ring frames as longitudinals stiffeners. spaced about 12" apart- on BOTH sides of the of each ring of the truss frames so now you have the shape of half a hull. but you still need to reinforce the inner shell. so you will now make trusses of the stiffeners you just added. you will add v's so the center of the wall will have rebar in it. envision truss rings running both horizontally and trusses running athwartships. all welded together!(this is difficult to visualize) but clear on paper which i will add next week.
once you have the frames all welded up- it should look like half a submarine hull. the next stage is making the permanent double mold. to do this you add a layer perhaps two of relatively cheap plasterers lath 1/4 inch. to the whole shell thus enclosing the whole shape of the half-hull. when done you will have sheathed both sides in lath(the mold walls), tightened up the mesh with plastic ties, and formed half the conical bow etc. sometimes in doing the bow you will need to cut "darts" from the lath to make the cone shape)
it will now look like half the hull.
the next step is to plaster the outsides of the layers of mesh...this is our mold.

plaster about 1/4 inch thick on each side of the truss. enclosing all but the top of the frames so you can then pour the concrete into the now plastered and hardened walls.

you smooth out the finish of the outer and inner shell/molds. and you are ready to pour.

now you must right the hull- this is tricky because if you did not build a rigid enough structure with the rebar trusses then the whole hull will pop out of alignment. so make sure when you build the frames so they are bloody rigid! there cannot be too much reinforcement. an alternative is to use cradles and build your hull so the openings will be facing up...you need to do this in stages since you must jack up the hull as you move along doing the outer bottom plastering.this way might be easier than a down turned hull since you don't risk mis-aligning the hull but it might be easier. and cold joints are ok because you will be using the inner sandwich to pour the concrete into anyway making that inner pour the pressure hull. also you may want to add a layer of 6x6 mesh at some point just for extra strength but its tricky and you have to do it in sections. although the rings should be adequate ...this is because they run through the mold and act not as passive reinforcement but as dynamic reinforcement....(see pre-stressed concrete)
remember the outside layers are just the mold. which later- you will epoxy over and make fair the final hull. the advantages of this method is you can access the concrete since half diameter of the hull is only four feet off the ground and you can pour from the trucks no problem.use vibrators and see what your doing etc. to get a consistent pour.
remember to add blanks for the hull penetrations. and set in your shaft log.
once you have plastered the inner and outer mold walls- keep them moistened for about 7-10 days so it does not crack. you will use a fine sand/ morter mix.

repeat this process for the mirror image of the second hull..be very careful about dimensions you want perfect symmetry here.