Building a hull using Ferro-Cement

Discussion in 'Boat Design' started by Sparky001, Nov 5, 2023.

  1. Sparky001
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    Sparky001 New Member

    First, I know I am opening a can of worms here and going down this rabbit hole, but I want to ask this question on Ferro-Cement boat building. Has anyone used it lately, and if so have you gotten away from using steel rebar and thought about using Rock Rebar / Basalt rebar instead, or even fiberglass rebar? Before you explode with a bunch of hate and words of Oh what a piece of $#$#% it is or anything else there, hear me out and think with some logic first and foremost. I am not writing this out of fun to stir up an angry mob of haters and nay sayers, I am just throwing an idea out there and asking the question of “Has anyone done this and if so how did you get away from the main issue of steel rebar?” If you look at cement in general, it does a wonderful job of protection from the elements, used in many areas of construction, to include bridges and damns, dikes and more. Used in corrosive environments like saltwater... So why is it ok for a bridge which holds up tons of weight, or a damn which holds back a lot of water, but not a hull?

    I have had a Samson ferro-cement sailboat once. The biggest issue was the rust bleeding. There was even a 6 inch crack under the waterline that was inspected by a marine inspector and no issue of integrity. Yes it was a heavy beast and yes it did not roll as much as the fiberglass boats in the mooring, especially when a gator freighter slipped by. But again the question is “Does anyone use Ferro-Cement?” And If so have you considered alternatives when using without steel rebar? Thank you and I hope the rabbit hole was not too deep...
     
  2. wet feet
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    wet feet Senior Member

    I can't imagine anybody putting in the work to build with a material that carries a stigma and will be hard to sell. The cost of hull materials isn't that much of a factor in the total cost to get afloat. If somebody would like to pioneer a revised version of ferro, there is no reason why they shouldn't but they may have to live with it for a very long time.
     
  3. Sparky001
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    Sparky001 New Member

    The tensile strength of fibre rebar is 20% higher in comparison with a steel bar. The material bonding force of fibre reinforcement is considerably stronger compared to steel; this explains better operational endurance of FRP rebar.

    Basalt Fiber contributes to reduction in concrete coverage by 25 – 30%, and its weights 4.5 times less than steel fiber This means you'll need less manpower to lift and place the material during construction.

    Basalt rebar is 3 times stronger than steel rebar (the strength of basalt reinforcement is about 800–1100 MPA, while steel reinforcement has a strength of 225-365 MPA).

    You are right about the hull being only one end of the cost. The fixtures, motor, sails for a sailboat, mast, all the electronics and such do out weight the cost of most hulls, but I was looking at the big picture for a more affordable and something which constructed properly could last a life time...
     
  4. Rumars
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    Rumars Senior Member

    I have tought about it a lot, and the conclusion is that you need to redesign the whole structure and adapt the building methods as well.

    I guess you never used basalt or carbon fibre rebar, the stuff is flexible and can only take a certain radius. Think of it as a fishing rod, it bends a great amount in a smooth curve but will become straight again once the load is removed. If the bend is to sharp it will snap. To get tighter curves the fibers have to be bent before the resin coating, this is done at the factory. This means that depending on shape you either have to keep the rods bent in form until the concrete hardens, or you have to use custom prebends. Small boats that use only small diameter mesh are easier since the rod diameter is related to maximum bend radius, but they require a mold.
    It can be done, but you need actual textile concrete engineering to make it happen, most of what you have read about DIY ferro boatbuilding doesn't apply.
     
  5. Sparky001
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    Sparky001 New Member

    Thank you... I never thought about the bending of the Basalt Rebar. I assumed it would cover most hull forms, but you are right when it comes to sharp angles. I would think that a motorboat would be best suited, using more straight lines, or a trawler style boat might also be suited... Again thank you for the input...
     
  6. bajansailor
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    bajansailor Marine Surveyor

    The gist of it is that while concrete is very strong in compression, in tension it's strength is much less.

    A dam to hold water is usually convex shaped, so that the concrete will be in compression - if it was straight, or concave shaped, the concrete will be in tension.
    Look at a typical arch of a building or a bridge - this will generally be in compression.
    Arch bridge - Wikipedia https://en.wikipedia.org/wiki/Arch_bridge

    On a typical road bridge the columns are in compression (they are VERY strong), and the span is often supported by cables.
    Very often concrete beams will be 'pre-stressed' - here the beam has steel rods that are tensioned before the concrete is poured around them, and then the tension is released after the concrete has set, resulting in a beam where the concrete is in compression.
    Prestressed Concrete 101 https://williams-works.com/prestressed-concrete-101/
    It would be very difficult (read impossible) to tension up all the steel rods in the hull of a ferro-cement boat to create a 'pre-stressed' hull that is in compression.

    Boat hulls need to have a certain amount of strength in tension, and this can be much more easily achieved by building a GRP, wood, steel or aluminium hull rather than a ferro-cement hull.
    And as mentioned above, the cost of the hull is generally just a small proportion of the total cost of the vessel, so it is much easier (and more economical in the long run) nowadays to build in GRP, wood laminated with epoxy, steel or aluminium.

    @Sparky001 did you also see this thread which was started a couple of days ago?
    ferro cement methods at samson marine https://www.boatdesign.net/threads/ferro-cement-methods-at-samson-marine.68537/
     
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  7. rwatson
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    rwatson Senior Member

    I cant think of a single benefit of Glass in the concrete. For all their faults, corrosion has never been a major problem in ferro hulls, there is no weight advantage, there is bound to be a big cost hit, and the lack of bendability has been mentioned already.
     
  8. gonzo
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    gonzo Senior Member

    Bridges don't float, so the thickness of the concrete is much larger. Cracks in a bridge, as long as they are thin are not an issue, unlike a hull where they would make it sink. Pre-stressed and post-tensioned concrete is commonly used and can be a lighter structure. However, the shapes are limited and not always conductive to a good hull design.
    I inspect bridges, and the steel corrodes. Epoxy coated rebar is becoming more common, but he bars have to be shaped first before coating and handled relatively carefully to prevent damage to the coating.
    I don't think that a ferrocement hull can be built cheaper than a fiberglass/polyester hull for recreational use. Barges or floating docks with hulls that are several inches thick are completely different. The use of FRP reinforcement is also becoming more popular and accepted. This is driven by the fact that most damage to bridges, dams and other structures is cause by corrosion of the steel reinforcement. If you are building a concrete hull with fiberglass reinforcement, might as well build it completely of FRP.
     
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  9. skaraborgcraft
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    skaraborgcraft Senior Member

    Steel and concrete are commonly used because the expansion rates are so similar. How does balsalt rebar compare with steel? I have no issue with ferro having owned/cruised and lived aboard one, but it would not be the way i would build a new hull today.
     
  10. gonzo
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    gonzo Senior Member

    I have never seen basalt rebar. As far as I know there is no building code that includes it.
     
  11. messabout
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    messabout Senior Member

    Bajansailor, just for the record, prestressed concrete beams. flat slabs. single tees, double tees and other prestressed concrete structural elements use wire rope or cables not bars. Only precast concrete that is not structural use rebar. Precast is much different from prestressed.

    In a typical AASHO beam like the ones you see holding up a bridge over a highway , the beams use as many as 40 cables all of which are "depressed". That is to say that the highly tensioned cables are pushed downward in the center of the span all the while keeping tensions of up to 40,000 psi each. Those are dangerous muthas before the concrete sets. The depressor mechanism that pushes down the middle of all those cables, resemble a colossal bow and arrow.... When the concrete is cured and the forces that have been resisted by the forms (steel molds), tension is released, the concrete girder will bow up in the middle. When the girder is put in place and concrete slabs are laid on top, the bow of the girder is pushed down by the weight of the slabs so that the surface of the bridge is reasonably flat. The designed convexity of the bridge surface is calculated so as to estimate the weights of the vehicles that may cross the bridge. The concrete does not contribute nearly as much to load bearing as do all those cables buried within the mass. All that calculation takes some preplanning. That is what concrete engineers do.

    The cables are pretty much isolated from degradation but they do not last forever. Observe some of the failures in the last few year of Eisenhauer era bridges.

    As for boats....I suspect that ferrocement went out somewhere around the same time that buggy whips were declared inessential.
     
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  12. rwatson
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    rwatson Senior Member

    bajansailor and gonzo like this.
  13. gonzo
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    gonzo Senior Member

    Thanks for the information
     
  14. skaraborgcraft
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    skaraborgcraft Senior Member

    Same thermal expansion......interesting. Cant imagine anything made of rock being formed into a tight reverse radius that some hull shapes have.
     

  15. skaraborgcraft
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    skaraborgcraft Senior Member

    further lookie see:

    Another important consideration is that basalt rebar can be bent, but has a strong memory like a spring. If you bend a straight rebar, it requires a lot of force, and when you release it, it returns to its original straight form. This allows basalt rebar up to a certain size to be shipped in coils of 100-500 meters.

    So, not ideal for making web frames.
     
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