new way to build concret hulls?

Discussion in 'Boat Design' started by Baltic Bandit, Jan 22, 2014.

  1. Petros
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    Petros Senior Member

    It might not be worth doing, but the concept of a using a concrete mixture below water line is interesting in that it putts the weight down low where you want it on a sailboat. so it would not necessarily have to have costly lightweight filler material. and yes, concrete has to has tensile reinforcement for it to be useful as structure other than pure compression. I have designed hundreds of concrete reinforced structures in my current position as an engineer, the idea of building a boat from it is not practical on anything other than a barge. But with some advanced construction methods and admixtures, there may be some merit in it for a cruising sailboat, certainly not a racer however.

    But consider if you will, a welded steel frame work, either galvanized or even out of stainless, a lead keel, and than a fine metal or even polyester, kevlar or even carbon mesh stretched over the frame, and than molding a concrete mix into a complex hull shape around it.

    Yes, it would need to be sealed with coatings both inside and out. That weight low in the hull improves rough weather handling, it is low maintenance, and the infilling material is relatively low cost compared to other materials. there is no reason to go super light weight (which may have durability issues as well as cost) because you want the weight down low anyway on a cruising hull.

    Than perhaps cold molded wood and fiberglass can be used on the upper hull and deck. Although a thin concrete deck surface has some attraction in terms of durability, but unless it is stiff enough not to flex, it is not really practical on a smaller boat. However, there are other means of making durable decks.

    And than use conventional aluminum/stainless rigging, or even carbon mast and booms. You keep the weight low, and the larger bulk of material is relatively low cost compared to most other hull materials, and not nearly as labor intensive to build. I would be willing to try it on a 24-28 ft pocket cruiser, and I like the concept, if executed properly, it seems to me like a worth while consideration.

    I have seen some old DIY type plans that used an all concrete keel, inexpensive but not as efficient as a lead or even bronze keel, so would it not be better to get some use out of the concrete as part of the ballast that you have to put in a monohull anyway?
  2. Skyak
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    Skyak Senior Member

    For a monohull sailboat I would not spend anything on making the concrete lighter. I would focus on making use of its strength, stiffness, form and maintenance advantages and use it lower in the design. The point I made is that there is value in not separating the material function between strength and ballast. When you realize that the value of light weight is proportional to the height relative to the waterline you will see a high value in concrete composite in the lower part of a monohull sailboat.

    Ilan V. -that is certainly a bunch of words, some common knowledge about boat design mixed in with loopy threats of danger but it all fails to support the first sentence which claims you are proving my assertions wrong.

    -concrete is very capable of reproducing any Bonjean curves

    -About tensile strength, it can be derived from concrete's superior compressive strength. Using tension rods on concrete is common practice.

    -About your comment "Dorade which shows now it's very great old age of conception. You're going closer to naval archeology than boat designing."


    Dorade shows that great sailboats were built long before the 1970s fiberglass revolution. More importantly to my assertions, great boats can be made of heavy materials in forms concrete and wood is very capable of.

    -Your comment about 'pitching uncontrollably' is completely unsupported opinion. If you like I will build the boat I describe with your money and insure it for the full amount with you being the payee, insure myself and a young female companion each to your investment -again with you being the payee, and give you the film rights to to our tragic death ( which sounds incredibly dramatic in your account). How about it! 3+ times your money! How can you refuse?

    I find it amazing that I am in the position of defending historic sailboats capabilities arguing against PAR. He has slapped me around so many times with his knowledge of their capabilities any time I have questioned them.
    PAR I understand and accept your preference for common FRP-you design boats, you don't build them or sell materials. You charge a fraction of the boat cost -cheaper material and construction is less pay, and it is far more work designing to engineering rather than common scantlings (and risky even if it is sound). The one thing I ask is that you consider offerings constructively. If someone is willing to do the homework you can grade it objectively or be quiet. High strength concrete is a long way from being important to you professionally, but it should be of interest to someone with your experience and portfolio of classically inspired designs.

    For those who see the value
    -about the ability to produce hulls in female molds -that's what I was thinking early on -with molds produced in sand (using additives common to casting cores) with some specialized cnc gear. With developments since then I am thinking that a male mold could work with 3d printing style extrusion and troweling. High strength concrete is very stiff -as in low slump rate. Vibration and vacuum can be used to get the air out and get it up to density and strength.

    -3D printing is already being used to produce casting cores at scales of 3x6M to 600dpi resolution. It shouldn't be hard to scale to 2 or 3 times the size with a corresponding reduction in resolution but I am not sure of the cost of the additive to the sand.

    -There is another aspect of using concrete with another material above that should be given consideration. The concrete will take the majority of the loads because it is stiffer. The concrete has no shortage of strength but the upper can be lighter due to this decreased loading.

    There was a comment somewhere back that my boat was a dream (my boats are real, my concept is a dream). It reminded me of an old saying 'if dreams were boats we'd all be sailing'. The reason I spend my time here is to push those dreams closer to sailing reality.
  3. Skyak
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    Skyak Senior Member

    You are following the same reasoning I did. I was reading about traditional boat building and I was amazed at how much time, skill, and high quality wood went into the main beam and deadwood -and in the position they occupy in the boat, density less than water is actually a big negative. The buoyancy of the wood takes up space and adds to the need for ballast. It is pretty damn simple to make a beam and deadwood out of concrete. Then I started looking at a better connection to the ribs and I realized that all the wood deep in the hull should be replaced with concrete. The only question left was how far up to make the transition to wood. Lower has the advantage of being into thicker concrete that can be lower strength with plenty of room for metal attachments. Reaching up to the waterline is more demanding on the concrete material properties, but it does wonders for reducing maintenance and raising reliability. Old heavy designs make it much easier to achieve full concrete below the waterline. Your comment about good for a cruiser not a racer is a bit off -it won't make the fastest boat, but it can make a fine race boat because of rating rules. Based on the opinions expressed on this thread I would venture to say that your boat will be widely underestimated and likely to win many bar bets.

    It sounds like you know more about reinforced concrete than me but it also sounds like you are over complicating the design. You are still trying to stress the wood hull which stresses the keel. The concrete is stiff and will take all the loads. Run the rig attachments directly to the concrete and run the steel reinforcements in the concrete like they are tensioning wires on concrete compression members. The rest of the loads are fairly light and well distributed because they are supporting buoyancy. I found that a bolt on crash box bow and stern overhang made the design much easier to tension the concrete. If you need shallow draft or are in a strong tidal location concrete seems to work great with twin keels. The windward keel adds extra righting as it lifts out of the water.
  4. Petros
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    Petros Senior Member

    the problem with your statement that the concrete will take the load is not exactly accurate: it will only reliably take loads in compression. A boat hull in heavy seas has all kinds of changing and dynamic loads on it, so any "compression" member will also need to take some tension loads too. the safest way to design a boat hull is to have all elements of the design to take both tension and compression, which means a full cage of tension members throughout the hull. and concrete is not waterproof, so the metal tension members would themselves need to be resitant to corrosion, so either heavy galvanized or stainless, or perhaps carbon ribbons? And it would have to be a pretty complicated arrangement to accommodate all possible load combinations.

    consider that a hull sitting statically in the water has the keel in tension and the gunwales in compression. But slicing through deep chop, as large as the boat length, when going over the crest of a wave, but the gunwales in tension. likewise, the forward part of the hull cutting though deep chop would have alternating loads on the front bulkheads, almost wanting to "oil can" the sides in and out, so you need tension members on both surfaces of the skin. An alternative is to use perhaps metal internal stringers bonded to a thin a wire mesh reinforced skin.

    And a concrete hull patterned after a historic large tall ship would actually be lighter than its all wood counterpart. I once worked with a guy that built a replica of a historic ship out of steel reinforced concrete, and all fitted out it weight the same as the historic vessel. If it was optimized, and used some modern admixtures and reinforcement, it could actually be made lighter. On a smaller vessel, it would be more difficult I suspect, but a deep water cruiser is already very heavy, so an optimized reinforced concrete design could be competitive in terms of weight and cost.

    Another issue, as often pointed out on this forum, the hull cost is only a small part of the total cost to fit out a boat ready for cruising. With such nay sayers as PAR and others, whether justified or not, who would be willing to take such a risk with an unproven concept? so unless you have so much money you are willing to take the risk yourself, not likely you will have many takers. If it we could innovate a lower cost, lower maintenance, and stronger hull, there is not exactly are large market for large luxury sailing boats. I doubt anyone would get rich from it, even if was proven to be a viable construction method.

    One place however where it might have wide spread acceptance, but still no profit, might be to build fishing boats for third-world countries where specialized materials and manufacturing methods are not always available.

    I like the concept very much, and have actually thought about it before. Right now it is just an intelectrual exercise. I might consider it on a smaller boat for my self just for the sake of an experiment, but I would not think that there would be a realistic chance of having anyone actually order such a hull from us.
  5. hoytedow
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    hoytedow Carbon Based Life Form

    The re-bar in the "New" Sunshine Skyway Bridge are coated with epoxy to protect them from the salt.

    That could be done with the wire mesh for the canoe but it would not save weight or money.
  6. frank smith
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    frank smith Senior Member

    Start with swimming pools and go from there.

  7. Skyak
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    Skyak Senior Member

    you misinterpreted my statement. I understand concrete's strength is more in compression than tension and that it is best to put it in compression. It's in my earlier posts. What I was saying is that a boat made of concrete for say the lower third with the upper two thirds being a shell of wood or FRP, the stresses will all be highest in the concrete because of it's dramatically higher stiffness. Consider your boat as a beam (like your example) calculate the deflection of the concrete at failure, now take that deflection and apply it to the wood upper portion -how much stress does it see? Not much. How much force is it contributing to help the concrete not fail. Not much. Of course this calculation changes as the parting line moves lower the upper gets relativity stiffer.

    It would be more efficient if you could load the entire shell, my point is I am prety sure you aren't. Maybe we are talking about different parting lines but you can derive a parting line where the upper contributes equal force -it's very low. So I figured you likely needed a reminder. I don't know what design you are considering, but you likely need to consider that the strength is all being supplied by the stiff concrete lower.

    About those reinforcements -we need to clearly separate between rebar reinforcement and tensioning rods.
    -rebar is placed in the concrete unstressed and must lock into the concrete like aggregate. It adds some tensile strength but it brings failure modes. Particularly if it corrodes it loses it's lock to the concrete and actually can add stress because iron oxide is larger than the steel it came from. (Petro I know you know this but it is for the benefit of the thread) I don't trust galvanized or stainless. They both still corrode but their oxides are just more stable. Stainless is expensive and many alloys still corode in the absence of oxygen. Common high quality rebar uses that green coating (filled epoxy?) and I think it is likely best. Petro, I presume you know more about this than me. Is there a higher quality product for salt water use? I expect that this coating is reasonably priced but it must be applied after the rebar is bent to shape. My last word about steel rebar is that historically it is what limits the life of concrete boats -they would be better without it.

    Tensioning rods are completely different from rebar. They need to NOT lock into the concrete. They ARE stressed after the concrete sets. They can be designed to be inspected and even replaced. Tensioning rods don't add strength, they shift the compressive strength the concrete has into tensile strength it needs. Galvanized? Stainless? maybe, but why not some tensioning fiber that does not corrode? Carbon? Any high strength fiber might be considered.

    About old ship designs, or any sailing ship designs for that mater, they have design waterlines and weights. If you cut the weight you violate the design. If someone wants a boat to Dorade's design, but lighter, the hull must be redesigned and it isn't Dorade anymore. They can match some properties, improve some, but 'better' will be an argument. The point you are making is that weight is not a problem making old designs with concrete lowers.

    About business viability, risk, and return -I can't think of a faster way to turn this forum into a worthless pile of ranting naysayers than to suggest there is money to be made making yachts this way. You want one? I can help you make one paying less, little, maybe nothing. Per the above I dare not say you could make a profit, but I do say it could present a compelling opportunity to improve your life and the case could be more compelling for others.
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