The purpose of this project is to design and mass-produce kits for a floating tiny house that can sail. It combines high-tech modeling and fabrication and low-tech assembly that can be carried out DIY-style on a riverbank or a beach. This boat is a 3-bedroom with a kitchen, a sauna and a dining room. The deck is big enough to throw dance parties. It can be used as a river boat, a canal boat or even a beach house. Oh, and it's rugged and stable enough to take out on the ocean. Kits will start at around $50k (USD). The design has been tested in simulation and prototype; full-scale production will begin next year.

Monday, January 12, 2015

Concrete slab bottom

Ballast is what allows sailboats to carry sail, by keeping their masts more or less vertical in spite of the forces generated by the sails. Keelboats carry ballast in their keels, and the usual high-end material for keel ballast is lead, which is 17 times denser than water. Lead is a grey, weak, toxic metal. Some sailboat builders cast an entire keel out of lead with some bolts sticking out the top, and bolt it onto the underside of the boat with a backing plate and some nuts. Treating lead as a structural material always seemed like an odd choice to me. On such boats, when the keel snaps off and sinks the rest of the boat floats upside-down, trapping the crew, who then spend however long it takes for rescue to arrive, soaking in salt water polluted with diesel fuel and battery acid.

Such prospects never appealed to me, so when circumstances forced me to buy a production boat (a yacht, if you will) I opted for one that had an integral fiberglass shell and a steel slug in the keel for ballast. At least the keel isn't made of extra-heavy putty and won't snap off. An additional feature of lead-keeled boats is that the keel has a tendency to gradually disappear. When immersed in salt water, lead gradually turns into lead oxide, which is a light, fluffy material that easily washes away. Although some lead keels are covered with a fiberglass skin, some are simply painted, and many a marine surveyor, tapping around the keel, has seen his hammer go right through the paint and the fluffy white powder underneath. This is all very profitable for the recreational boating industry, I am sure.

On my previous boat, HOGFISH, the ballast consisted of lead bricks which the builder cast into precise shapes that fit between the frames under the cabin sole. They were certainly very effective as ballast, and since the bilge was usually dry, they did not deteriorate at all over a 30-year period. But with QUIDNON the choice of ballast is completely open (as is the quantity of it, which will be determined experimentally using a model). But supposing I used lead ballast, and used, say, 10,000 lbs of it, at about $2/lb for reclaimed led shot (which then has to be cast, generating toxic fumes in the process) that would be $20,000. So if the construction budget is $50,000, 40% would be spent on ballast? No, thank you!

Lead makes good ballast because it is compact, but QUIDNON, which will have around 3000 cubic feet of interior space, is not exactly cramped, and so using lead ballast because it is more compact than other options would be a false efficiency. Steel scrap embedded in concrete would work just as well, and many a backyard boatbuilder and cost-conscious ocean navigator has used it with great success.

But there is another consideration: one way of saving money is to make sure that each element in the design does more than one job, and as many jobs as possible. So, for instance, on QUIDNON the boom gallows will not only hold up the booms, but will also provide ventilation (they will be hollow and have openings for air), hold sheet blocks, provide attachment points for a deck awning, support an engine hoist and maybe even house some electronics.

With ballast, the situation is similar: why just use it as ballast when it can do other things as well; such as, for instance, form the entire bottom and bow of the boat—out of fiber-reinforced concrete. Fiber-reinforced concrete is portland cement with sharp sand as aggregate (something like 1:3 ratio, details TBD) with short-strand glass and polymer fibers mixed in. The result is a very tough substance, with good abrasion resistance, very resistant to cracking. It can be sealed with a penetrating epoxy sealer to be completely waterproof. The hull will still be classifiable as a fiberglass (FRP, or fiber-reinforced plastic) hull, but its core will be concrete below the waterline and plywood above.

In case you are thinking that this will make QUIDNON a ferrocement boat, it will not. Ferrocement boats were popular at one time, because of the extremely low cost of materials. They were constructed by making a “basket” out of steel rods and mesh held together by staples, which was then “plastered” by using a very dry cement mix. The cement has to be carefully skimmed to just barely hide the basket, or the end result is too heavy. What's worse, the entire plastering operation has to be done at one go, in a single marathon plastering session, so that the cement sets as a unit. This is too much know-how for most backyard builders, and so results varied from OK to awful. Nobody builds ferrocement hulls any more.

What I intend to do is pour the entire bottom and bow of the boat as a slab, using a mold. A cement truck will deliver a load of the right mixture, with the right ratio of cement to sand, and the fibers already added. This will be poured into the awaiting mold, which will already hold a structure made of rebar. I intend to be generous with the rebar, since it won't add much to the cost. The pour will be done upside-down, since the bottom of the slab will be very easy to smooth by hand, whereas the top of it will be very detailed, and include the following features (which would otherwise have to be built up out of other, more expensive materials):

1. Mast steps
2. Bottom half of the water tanks
3. Foundations for fuel tanks
4. Bottom half of battery compartments
5. Bottom half of the chain locker
6. Foundation for sewage holding tank
7. Chases for plumbing pipes
8. Chases for electrical cables
9. Ventilation ducts to draw in cool, water-chilled air for solar-powered air-conditioning
10. Drainage paths for condensation (a.k.a. “limbers”)
11. Compartment to hold the bilge pump
12. Foundation for cabin sole (floor)
13. Foundations for bulkheads and partitions
14. Lexan window for an in-hull depth sounder
15. Apertures for through-hulls (raw water in, sewage out for sanitation lines)
16. Aperture and foundation for the engine well
17. Bottom attachment points for the rails that hold the engine bracket
18. Shower sump and a well for its pump
19. Sockets for the rudder posts
20. Foundations for the centerboard trunks ...and, last but not least...
21. All-around lap joint with pre-cast perforations to hold the bolts which will make the concrete core of the bottom and the plywood core of the topsides and deck into a single, integral piece.

All of these intricate details will be drafted out using CAD, rendered in particle board using an NC mill, and then snapped or screwed together to make the mold, which will be back-filled with sand to make it hold the weight.

Once the concrete is poured, smoothed along the top and sets, it will be “hydrated” over a period of some months by keeping it covered with burlap and sprinkled with water. During the hydration period the concrete works up to full strength and stops absorbing moisture. The concrete slab will then be completely dried out and sealed with penetrating epoxy concrete sealer, making it impermeable to water. Then a fiberglass skin will be applied to its exterior surface, in effect making the concrete act as the core of a fiberglass boat.

Finally, the entire bottom surface will be covered with copper plate. This will keep it free from fouling by marine organisms for the life of the boat, so that it will never require painting. The plate will be attached to the concrete using self-tapping stainless steel screws. I do not plan to plate the sides below the waterline, because there the plating would deteriorate much more quickly and require periodic replacement. Instead, I will keep them painted with ablative paint, and scrape and paint them periodically when the boat is drying out at low tide. In this way, QUIDNON will never need to be hauled out, and will never need to have its bottom painted, eliminating a very large category of expense.

Finally, the slab will have a cage built around it, and flipped over using a crane. After that, the topsides and the rest of the hull will be built up, by bolting plywood pieces along the lap joint that will run just above the design waterline. Once the hull is built, the lap joint will be covered over with fiberglass along with the rest of the hull, faired to make it smooth and painted.

This approach achieves cost savings in the following categories:

1. Eliminates the need to form the bottom using several layers of plywood screwed and epoxied together, saving a lot of time and a lot of materials.
2. Eliminates the need to separately build in ballast: the amount of ballast is simply “dialed in” by adjusting the thickness of the poured slab.
3. Eliminates the need to separately form out of (expensive) plywood and glass in the 21 features incorporated into the bottom, which are listed above (and there will probably be more). The disposable plywood used to build the mold is only $2 for a 4x8 sheet.
4. Eliminate the need to ever haul out the boat and repaint its bottom.

In addition, this approach eliminates the problem of pounding. On certain points of sail, and sporadically whenever the sea state is boisterous or there are cross-seas running, square hulls have a tendency to pound. If the bottom were made of plywood (as it was with HOGFISH), the pounding would produce a resounding base drum-like sound that would reverberate throughout the boat. This is not dangerous, but it is not all too pleasant and interferes with sound sleep. With the bottom and bow made of a concrete slab, however, the pounding will be virtually unnoticeable. It will be like a rock hitting the water, and the only sound will be a loud splash.

Finally, the concrete slab surfaced with copper sheet would make QUIDNON go aground extremely well. Copper is quite tough, and, as Dave Zeiger's Triloboats have proven, is tough enough even when backed with just plywood, but is especially tough when backed with a slab of concrete. Thus, QUIDNON should have no difficulty with going aground and floating off again.

46 comments:

  1. Be careful about galvanic corrosion between Stainless Steel and Copper.
    Perhaps do a test with the stainless of your choice and the copper grade of your choice in a glass of seawater. For more info, see
    http://www.corrosionist.com/galvanic_corrosion_chart.htm
    Jim Rock

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  2. Hi Jim,

    Dave Z did the homework on this. (What you read is about the 5th version of this plan, as it evolved). In the galvanic series chart 410 stainless (which is what Tapcon self-tapping concrete screws are made of) and copper are right next to each other, so they should get along just great.

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    1. I should point out that 'doing the home-work' was limited to skimming some info I'm not qualified to assess... so it's still factoidal!

      While it looks good at first glance, I'd recommend confirming that impression with a real good look and some expert advice.

      Dave Z

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  3. I wonder about the rebar. In construction rebar has the nasty tendency to rust, expand, and then crack the concrete in which it is embedded, leading to shorter lives for buildings. In the marine environment especially, and puncturing the concrete with stainless steel screws here and there, is this a concern? Or is the epoxy sealant so good that the problem can be ignored?

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    1. I am not an expert on concrete, but I do know certain things about it. In construction it is common to use a very high aggregate/cement ratio, to use inferior aggregate (gravel instead of sharp sand), to largely neglect hydration and to only seal some surfaces, such as floors and driveways. I will use high-strength fiber-reinforced concrete, will hydrate it fully, will seal it with penetrating epoxy and sheathe it in fiberglass on the outside. Standing up to salt water is a matter of formulation, such as the one used in building bridge pylons and abutments. Concrete is one of those subjects that has been incredibly well researched, being something like 3000 years old, so it doesn't involve any guesswork.

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    2. If this is a salt water boat hull and you are pouring concrete on a mould, make sure that the rebar is treated with anticorrosive. Or better, use some kind of reinforcing rebar and netting that is immune to salt water corrosion. Likewise, make sure you use abundant amount of Calcium Nitrate in the concrete mix which is an inhibitor for steel rebar corrosion.

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    3. Good point about calcium nitrate. I plan to paint.the rebar witb epoxy and to seal the slab with penetrating concrete sealer.

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    4. Epoxy is rigid due to the chemical cross linkage bonds so it can crack if subject to flexing. Concrete can be porous and so it is a good idea to seal it with penetrating sealer. Rapid cure concrete is more expensive than regular but it has less tendencies to crack.

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    5. It should be Calcium Nitrite not Nitrate.

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  4. I had an "eep" moment reading this this morning, so I gave my subconscious a couple of hours to work on it. It tells me that the three way joint (concrete, wood, and frp) right at the waterline is what gives me jellylegs. I'm sure it's a solved problem somewhere, like roofing or foam insulated subflooring or fiberglass swimming pools.
    Secondly, I wonder if there needs to be some way to absorb the energy that is now translated as booming, so it's not just transmitted to some other part of the structure.
    I hope I'm not making the perfect the enemy of the good here, because I like this design a lot.

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  5. I love this design so far, as there certainly isn't anything quite like it available elsewhere. However, my "bucket list" dream (assuming that society doesn't actually in the meantime) is to sail the Great Loop. Would the mast be removable without a crane?

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    1. Yes, the masts will be stepped in a tabernacle, and it will be relatively easy to drop them and put them back up without a mast, just by following written instructions. If the boat has to be portaged, it will classify as a "single-wide," referring a highway permit as a "wide load" but otherwise unproblematic.

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    2. Make sure that there is a reliable connection between the masts and "earthing" system embedded in concrete bottom slab through copper plating and Fiberglas skins. Run your "earthing" design by a professional before pouring the concrete in the slab.

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    3. That's a good point. The concrete bottom will have to be poured with two small-diameter holes between the mast step and the bottom, so that a bonding cable can be run through, connecting each mast tabernacle to the copper plate below. Once the cable is in, the hole will be filled with epoxy.

      Since everything below the waterline will be bronze, copper or 410 stainless, I won't bother with zincs or bonding everything together (although putting a zinc fish in one of the centerboard trunks wouldn't be at all difficult).

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  6. Jon -

    I have the same trepidations, especially since they are hard to test destructively at scale without putting someone's life in danger. My thinking is as follows:

    The major concerns when joining dissimilar materials involve thermal expansion and different rates of flex. There are three ways to go, and I have been vacillating between them:

    1. Ignore the problem. Epoxy the plywood to the concrete in a wide lap joint, through-bolt it with 1/4" stainless steel bolts every 4-5" or so in a zigzag pattern, and glass the joint over. The concrete slab won't flex at all, and will convey its stiffness to the plywood. The thermal expansion difference will be absorbed by the plywood, and will be insufficient to cause it to deteriorate.

    2. Caulk the joint instead of making it rigid. Apply a layer of 3M 5200 caulk to the joint, and let the caulk absorb the stresses. Give the bolts some "wiggle room" so that they can rock a bit. The problem with this plan is that the joint will eventually get leaky, like so many of the hull/deck joints that are joined the same way. Since the leaks will be small and above the waterline, they will never become life-threatening, and will be far less annoying than hull/deck leaks because they will drain right into the bilge, but still...

    3. Make it a slip-joint by using a double-strip of slippery plastic that goes all the around the lap joint. Grease it using some very thick marine-grade lubricant that won't wash out or evaporate. The concern is that 2 feet of immersion (when heeled) plus the pressure of the waves will eventually wash out the grease, causing leaks to appear. Redoing the joint would involve taking the hull apart, which is unlikely to ever happen.

    In all, I think more calculations are needed to understand how thermal expansion difference can be absorbed non-destructively by the plywood. If there is a safety margin there, then plan 1 is the one to follow.

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  7. Hi Dmitry,

    Thanks to our conversations, I finally got off my under-hull and did a 'brain-dump' post on copper plating (plywood) hulls, here:

    http://triloboats.blogspot.com/2015/01/copper-plating-plywood-hull.html

    RE thermal joint: I've been hearing snippets of new techniques for using 'organic' fiber reinforcement for concrete, which enhance thin sheet construction. Here's a search results page (scroll down for some relevant hits):

    https://www.google.com/search?num=100&newwindow=1&q=thin+sheet+organic+reinforced+concrete&oq=thin+sheet+organic+reinforced+concrete&gs_l=serp.3...180554.181843.0.183215.2.2.0.0.0.0.0.0..0.0.msedr...0...1c.1.60.serp..2.0.0.XokK4L-sO0A

    (Eww!)

    If thin-sheet upper panels look workable, they would eliminate at least one material (plywood) from your near-WL conjunction.

    RE internal steel: In one DIY ferro-cement hull that was looking good after a decade, the builder had encapsulated his steel BEFORE applying concrete, as yet another layer of defense against the oxidation(rust)/expansion mentioned. Don't know if that or generally quality construction made the diff, but interesting.

    Other friends with (general) concrete expertise have been recommending various water-based concrete sealers and binders... may be that some of these would be a cheaper alternative to epoxy, at least for internal components?

    Dave Z

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    1. I am thinking that once I get the rebar bent and wired together, I will spray the entire thing with the right kind of epoxy paint (sickly pale green color). That seems to be what they do for the high-end concrete applications.

      The most important thing about the waterproofing I use (other than being waterproof) is that it must have good adhesion to the fiberglass sheething. I'll have to make a sample and do a "pull test" and see how many pounds it takes to pull them apart.

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    2. I just wanted to point out a technical error. Elemental copper isn't 'non-toxic', it's as toxic as any other heavy metal, due to the ogliodynamic effect. However, like silver, (and notably quite unlike lead) the human body requires copper, and has evolved chemical processes to manage the toxicity naturally; which is why it doesn't hurt us to be exposed to it. It's also why elemental copper (and brass, etc) is hostile to micro-organisms such as wood-borer larve. They can't manage the toxicity of such a concentrated copper source. It's also a wonderful material to form touch surfaces in hospitals with, since there is no pathogen known to current science that can survive direct exposure to copper for longer than 90 minutes. Silver is even better as a anti-bacterial surface, but certainly costs much more.

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    3. Another advantage of using a copper plated hull is that you can use scrap steel or iron as a sacrificial anode, since copper is higher on the galvanic index.

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    4. Thanks, Dave, that's really helpful.

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  8. Couldn't the concrete ballast perform most of these functions if poured into the formed out bottom of a marine plywood hull? If you built the hull of plywood, and then lined the form/bottom of hull with "expansion joint" padding used for concrete foundations exposed to weather anyway (for example, it's used in sidewalks to prevent early cracking due to shrinking) then the waterline won't be of dissimilar materials, and shouldn't leak as a result. Am I correct? Or am I missing something important here?

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    1. For that matter, one could use straight sand/gravel aggregate for the ballast (which is cheaper than cement, which is why it's added to portland cement in the first place) and then covered over in a layer of "leveling compound" which is just a runny type of cement that easily finds it's own level. Or one could pour a normal concrete surface, if you still had at least 4 inches to make into a concrete pad. Then the addition of fibers to the cement mixture would likely be unnecessary.

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    2. Yes, that's the usual thing: build a hull, and then, once it's afloat, pour the ballast into it, and keep pouring until the hull sits on its lines. But I want to use the concrete to REPLACE the expensive plywood bottom (adds up to a lot of sheets) to save money, and I also want to cast lots of intricate detail into the top of the concrete, for the long list of items shown above. Also, it is far more convenient to have the hull upside-down to apply the copper sheathing.

      Fiber-cement is very highly resistant to cracking, which is something I want. The amount of fiber that has to be added to get that effect is very small, by volume, and the additional cost is quite small, so there is no reason not to do it. In all, the concrete bottom doesn't add up to all that much money.

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    3. Additional question then; since concrete typically weighs about 2.5 times per volume than water, how do you intend to form into it enough airspace for the concrete hull to float with the intended waterline below it's edge? That's a lot of airspaces, do you expect that your already planned airspaces for the needs mentioned above will be enough? Or do you expect to have to add more formed airspaces in order to make that work? That seems like it would be a lot of forming work. If you are doing it all yourself, I can see the value. However, concrete forming is a skilled trade with it's own secrets, and paying an experienced form carpenter to help with that kind of work seems like it would kill any savings from using non-marine plywood as a disposable form versus just pouring concrete into the base of a marine plywood hull. Admittedly, I've made no effort to run the numbers myself, and know that marine plywood is *really* expensive.

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    4. Very simply, the bottom will form a cement barge that will float in less than a foot of water. Once the rest of the hull is bolted on, it will float on its design waterline. No airspaces, just a tub.

      If you re-read what I and others already wrote, you will understand the approach we are planning to take to forming the mold.

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    5. I'm sorry, I think I understand now. I was imagining a set of completely inclosed (or nearly inclosed) cubes/cylinders of sand impounded into a mold, which would then have to be emptied of the sand and sealed up as a float. So your basicly pouring a concrete pool upside down?

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    6. Not quite. Think of it as a slab with skirts that extend down from all of its edges, making an upside-down 4-sided box. Once you flip it over, it floats because the volume it encloses more than makes up for the higher density of the material of which it is made relative to that of water.

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  9. Hey this is fantastic, I've been thinking about concrete ballasted junks since I read about it in Gavin Menzies' blasphemous book 1421 some years ago. He wrote of "Concrete ballast from wrecked (100m x 50m Chinese junk). Waterproof concrete was cast & bonded into the junk’s hull. To stiffen the complete hull, forming a composite vessel the concrete also provided 600 tonnes of ballast". Happy to see the idea moving forward. Aside from the labour savings, what's the material cost of this method compared to a Zeiger-style plywood hull?

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    1. Wow! I knew that the Chinese formed the bottom, floated it, then built the rest of the hull with the bottom afloat using double-planking with straight planks, but I didn't know that the bottom was concrete. Looks like I am reinventing the wheel. But that's great, because by that standard all the other sailboat designers are off in the weeds.

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    2. I've only heard the concrete junk bit from Menzies and his scholarship is often considered either heretical, sloppy or both, but it's very interesting and I'd love to read more about it. I don't think I've ever heard about concrete in more modern junks, but I haven't read Worcester's "Junks and Sampans of the Yangtze" which would be the first place to look. The concrete seems like a good way to prevent hogging in large wood vessels, which is cute, given the name of your last boat.

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  10. Dmitry,
    The epoxy paint over rebar should help. I have a cast iron swing-keel on my O'Day Mariner in salt water. I got sick of grinding it every year, so I got some 2-part Micron (?) brand epoxy undercoating and brushed it on after sandblasting the rust off the swing-keel. Epoxy has to go on quickly or metal starts to rust immediately. I put the slab of cast iron flat on some 4" blocks in my garage over concrete floor. Overnight the concrete offgassed water vapor and mildly rusted the side facing the floor. Stupid me! Painted epoxy over both sides anyway. The part of the board with epoxy paint has never peeled, flaked, or rusted in the slightest. A 1/2" strip of epoxy paint about 1' long over the leading edge of the board wears off every year from dragging on the bottom. I don't bother epoxying it, just grind and bottom paint. The epoxy paint on the rest of the board has remained intact for many years now. I just lightly sand and bottom paint every season. No more grinding. Epoxy paint = good stuff!

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  11. Inspiring, and one thought: Why not do the complete hull in concrete when you are at it? With and upside down mold it would be easier to form the thin sidewalls.
    (I'm a computer programmer, not a sailor)

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    1. Concrete is too heavy. Some boats are built using a technique called ferrocement, where cement is applied as a thin plaster to a steel mesh. It is no longer fashionable, because process control issues make it very difficult to get good results reliably. It is also much easier to construct a hull out of plywood sheets and cover them in fiberglass. This technique is known to produce very good results reliably.

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  12. there is underwater concrete in Italy in various ports (piers etc.) laid down in Roman times which is still good, so you are right that there is lots of info about it. V interesting about the Chinese junks. Since many of them were almost flat-bottomed, the entire approach is very similar to what you are proposing.

    I echo the concern about joining the two different hull materials. Yes, above water-line, but a hull flexes as one whole, esp. in rough conditions, so this is a major issue for yr. design I suspect. Though: in your mold include an insert space about 9" deep into which (once the hull is flipped to bottom-side down) one 3/4" sheet of plywood can be inserted. How this works in terms of a perfectly smooth flare etc. I don't know, but structurally it seems to make sense in terms of joining the two disparate materials.

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    1. Though: in your mold
      should read:
      Thought: in your mold

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    2. I hope that there won't be much of a problem with flexing, because the concrete bottom will not flex at all. I plan to join the bottom to the sides using a zigzag pattern of through-bolts and epoxy.

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    3. I am not a designer etc. but my impression is that even if there isn't actual material flexing as you describe, there are enormous stresses in adverse conditions caused by waves, aka large masses of water impacting at high speed and often in different directions and at different point of impact (the windward side is getting pummeled by almost-broaching wave, the lee side is crashing into wave trough below whilst bow and stern are hitting at different times and angles etc. etc. ). So perhaps rather than flex, one could think of it as multi-directional, varying intensity, ever-changing stresses.

      Moreover, in terms of a single large wave strike against the hull, at the point where the two different materials meet it might be good to try to have the same weight per cubic centimetre. If X,000 lbs pressure from a head-on wave is crashing into one side, say, then probably it's best for the flat-shape receiving that pressure to have equal strength and weight, or at the least not to have a long line down the middle where there is a great difference in strength or weight because that would surely add more variegated torque, i.e. the weight of the water crashing down will push one part of the vessel more than the other despite their being joined.

      It's quite a challenge to put these simple, kinaesthnetic things into understandable English, isn't it!?

      The big damage in storms is said to come from hulls coming in contact with the surface having charged down large wave-hillsides, i.e. it's the weight of the water at speed which impacts and damages hull or superstructure. Not wind, in other words, but waves. This is one potential disadvantage of flat shapes - that they take full brunt without adequately deflecting onrush of large wave impact that smoother, rounder shapes tend to do better at.

      That said, with a Jordan series drogue and flat bottom with twin side-boards, she should run exceptionally well, super-stable, meaning that there should be no reason for her to tumble sideways down a large wave in F10 storm at sea and indeed would be far safter than any keel boat. I suspect this is why the old Chinese junks have such high sterns - having flat or relativey flat bottoms, they could run and surf very stably, so then the main risk would be being pooped (from behind on a run). That high superstructure on the stern (often the captain's cabin I believe), pretty much obviated that risk.

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  13. May I humbly suggest a alternative material for second-third world situations? Here in Mexico there are 2 ship breaking operations (Lazaro Cardenas and Veracruz) with possible cheapo steel. And way cheap skilled tradesmen labor. Then a one piece hull and built super heavy with rust ablation built in just like the big ships do ( a lot of dried out scrubbing though). Concrete and scrap steel ballast on a one piece hull capable of pounding all day on a rock reef until the tide comes back in.

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  14. I think that at this point this would be a different design. I am going with a concrete bottom and fiberglass over plywood for the rest.

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  15. I have been studying up on concrete houseboat construction. One point of interest is their use of coal tar epoxy to surface and waterproof the bottom. I am thinking of using a layer of it between the fiberglass bottom and the copper sheathing.

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  16. Dmitry-

    Has anyone else built a boat with this mix of concrete/plywood/copper? I'm wondering if a test buoy could be built with this setup, to cheaply test it. Maybe some enterprising students at Webb or any other naval architecture school might be interested.

    A.

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    1. People have built boats out of plywood with a coppered bottom (good results) and out of concrete (with good results). The concrete/plywood joint is a challenge, but I intend to tackle it by overdesigning it (wide lap joint, lots of through-bolts, the whole thing glassed over, etc.) And the more I look into the details, the less of a challenge it seems. I will consider doing some test samples, but serious problems, if there are any, are likely to manifest at full scale, under full load, and over time. The real issue is longevity, and only time will tell. Most of the techniques I intend to use have produced boats that lasted 30+ years without any deterioration of any sort—no blistering, delamination, leaks, fractures, etc.

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  17. I wonder if there is a large enough 3-D printer available to you that could print your bottom slab and maybe even the rest of the boat by layering the appropriate concrete mix. 3-D printers are printing buildings, villas with significant cost savings in labor, time, materials, and achieving consistent results in quality. Is anyone using 3-D printing for boat building on a large scale? It would be worthwhile investigating it. Here is a link to a recent article that gives some perspective with respect to cost savings printing the world's first 5 story building in China. Parts were printed and further assembled.

    http://www.cnet.com/news/worlds-first-3d-printed-apartment-building-constructed-in-china/

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    1. I am sure it's possible, but I am only using concrete for a few functions, and the cheapest/easiest, most standard way to work with concrete is to pour a slab using a cement truck and a crew. That's where the significant costs in time, labor and materials is going come from. 3D-printing an entire boat out of concrete will produce one of two results: a boat that's much too heavy, or a boat that falls apart under its own weight because it's made of eggshell. But none of that is relevant, because I am not making a concrete boat, I am building an FRP boat with a concrete core below the waterline (for ballast and rigidity) and a plywood core everywhere else. And the best way to work with plywood is to NC-mill and drill all the panels, and then screw-and-glue them together using epoxy and square-drive stainless steel screws.

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