CLT houseboat

Discussion in 'Boat Design' started by Eelco, Apr 11, 2020.

  1. clmanges
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    clmanges Senior Member

    Amazing! I had no idea. Thank you.
     
  2. Eelco
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    Eelco Junior Member

    My goal is indeed to avoid immersion and permanent wet rooms. If that proves unfeasible, indeed the plan is off. Note that the CLT would be open to the interior living spaces, and would 'breathe' along with the internal air; not epoxied in on both sides like a piece of ply typically would, so I dont think the comparison to those scenarios is apt.

    Even with CLT and its insulation id still want foam for extra insulation though. So here is the latest iteration of my most practical embodiment thus far: CLT inside. Then a layer of particle board so moisture can equilibrate from panel to panel unimpeded by structural adhesives. Then a rubberized urethane coating / some puncture resistant water barrier. Then thin non-structural chopped-glass-epoxy over 12cm or so of XPS, lapping the glass between foam blocks, so each foam blocks effectively forms its own bulkhead. Then build up extra glass/kevlar on the exterior as seen fit to provide impact/abrasion resistance where it may be needed. So from the outside, itd be foam-core construction, using wood as a template, for all intents and purposes.

    This way, inevitable cracks from everyday wear on the outer layer will not do anything to the wood. Such small cracks could be repaired by just cutting out a section of wet foam and replacing it. You need a macroscopically big and impossible to miss hole to actually risk getting water on your wood. Even then, a puncture in the final barrier leaking onto the wood at a few L per day, would still not be an immediate problem. The particle board would spread it out, and the AC would carry it away in the course of a few weeks.

    This does raise the question of what the advantage of the CLT is; indeed you might as well clad a steel hull of comparable tonnage this way, and your steel would need no more maintenance than the CLT? That may be true; but then there are acoustics, humidity regulation, and aesthetics. Also I doubt itd be cheaper in steel. The material cost of steel plate per ton is about the same as the price of CLT per ton, as installed. Could build it lighter in steel, even for the same level of overbuiltness? probably. But anyway; I dont mind paying 75k for the structure of a house I am happy with, and I dont care to save some money on this part of the budget to turn it into a tin can.

    Not sure I need a wood boatbuilder. I can get CLT delivered and assembled at any building site by crews experienced with their own product. Why not have that done inside a yard sitting ready to foam+glass it? Assembling the CLT should take only a few days; so in terms of yard costs it seems like a big saving to me. There may be some practical obstacles here; can I get the cranes the CLT guys are used to working with inside the yard, etc. But I imagine these are surmountable problems.

    With a workflow optimized for this concept you could have boats in the water ready for interior finishing in a week or two from the time the first materials are delivered. There is about 400m2 of exterior to cover, but putting on square foam blocks with some nonstructural glass is about the dumbest job imaginable, and with enough hands can be done very quickly.

    That said, ideally id find a builder that isnt dumb, and I would feel comfortable working with on taking some time to wire-cut some fairings into that foam. Again this thing is meant to sit still 99% of the time; but aesthetically id like it to have somewhat sleek lines that communicate seaworthiness.
     
  3. Eelco
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    Eelco Junior Member

    Yeah I like concrete in general, and fiber reinforced in particular. Im not optimizing for lightweight construction anyway so it is an option. But while it has much better tensile strength than plain concrete, even with fibers, the tensile/bending strength per unit weight is still kinda meh. To build a 20m vessel capable of riding out a 10m wave, you either need a lot of concrete, or prestressed tendons. And if it does crack... well it doesnt have the same lighter-than-water property of CLT, so the safety margins better be good...
     
  4. Eelco
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    Eelco Junior Member

    Some fun facts; a 25cm thick, 5x5x20m, 100t CLT houseboat, could be propped up on both peripendiculars. The maximum bending moment in the middle would be held with about a 100x safety margin. Worst case bending moment you encounter out in the open ocean is substantially less than that (10x less according to some estimate). Suspended like that, the whole thing would sag about 0.25mm in the middle.

    Just putting that out there, for those who associate 'wood' with 'weak'. I used to be one of them. But all our material science innovations notwithstanding, the reality is that when it comes to the amount of strength and stiffness you get for your money, softwoods still stand on very lonely heights indeed. Perhaps higher than ever, infact. Just got to keep it dry...
     
  5. Eelco
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    Eelco Junior Member

    Compare that to basalt concrete; which has 1/15th the tensile strength of CLT and 5x the density; and your safety margins are quickly vaporizing; not to mention its a lot harder to guarantee consistent concrete properties in an object the size of a boat, rather than a test sample. Mild steel fares better; but also scores a lot lower on specific strength than pine.
     
  6. clmanges
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    clmanges Senior Member

    You're talking about using particle board ... have you ever seen what happens to that stuff when it gets wet? It swells up and gets spongy, and even after being dried it has mechanical strength about equal to a graham cracker (and it's none too strong when new). I have first-hand experience of that. I don't have first-hand experience with wet XPS, but suspect it will suffer the same fate.

    I don't understand your thinking on this project; it seems inconsistent. On the one hand you want it to be bombproof, and on the other you expect to have to tear out and replace substantial amounts of it like that would be easy.

    You scoff at insurance; are you only thinking of replacement costs? Have you thought about liability? You're unconcerned about it getting slowly waterlogged, but when it finally sinks somewhere you'll be on the hook for the removal of it, and woe be unto you if that happens in a shipping channel. If it sinks under tow, will it pull the tugboat down with it? (I suspect they could cut the cable first, but s**t happens, and you'll pay for damages.) Black swans happen, and you're better planning for them than getting surprised.

    Just get it built out of steel, or buy a reliable barge and build the house on it.
     
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  7. Eelco
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    Eelco Junior Member

    The particleboard has no structural role whatsoever. If it does get soaked, something already has gone terribly wrong to the extent that would scrap most boats. But what it will hopefully do is buy the time to strip off all the foam and particle board altogether, and save the expensive part of the house. Sure, that would be an expensive disaster. But more like a 50k-expensive disaster; not a completely-write-off-your-500k-home disaster. The former are disasters I can afford out of pocket, no insurance needed. 500k disasters are no fun; but id still take it over getting hit by a bus, and life goes on without a guarantee that wont happen as well.

    Note that it cant sink; even with an elephant sized hole in the hull, it will float; its lighter than water. Getting the wood waterlogged also wont sink it. Waterlogging the wood is a process that would take many weeks to complete, and should that be starting to happen, it will be hard to miss if you are monitoring humidity at all; or paying attention to how it lies in the water. If that would happen it would make the thing scrap; but sufficiently floaty scrap still to tow it to a proper scrapyard.

    XPS foam is as inert to water as any plastic. When foam-core boats get water inside it tends to be quite a bad thing for them, ive seen it cause the foam to delaminate from its skin which is quite the mess. But thats also quite inconsequential here, since the foam skin is there just for isolation and as an impact buffer, and has no structural role, or role in keeping the thing afloat. Since each block of foam is in its own glass compartment, taking a saw to a cracked section and patching it over again should be an easy job for anyone thats ever worked on a fiberglas boat.
     
  8. Rumars
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    Rumars Senior Member

    The "breathing" is a myth. Plywood does not "breathe" and neither does CLT or for that matter solid wood. The glueline in plywood or CLT or particle board is impermeable to a high degree (some glues are nearer the absolute value of 0 then others). When a veneer or thicker wood layer absorbs moisture from the atmosphere the respective layer expands and the whole panel bends like a diaphragm stressing the gluelines. Coatings like oil, varnish, paint and epoxy simply slow down this exchange in order to minimize tensions in the panels. Some are better then others at blocking water vapor but even epoxy is not 100% waterproof. It's better to have the same coating on all surfaces so the water absorbtion stays the same on all sides and the panel bends less.
    Simply said, CLT is not goretex, water vapour does not pass from one side to the other, the first layer exposed to the water soaks it up, expands and bends the panel.

    Your sandwich composition does not make sense.
    1. CLT. This is the load bearing panel, at least in theory.
    2. Particle board. It is either glued or mechanicly fastened to the CLT. Does nothing but wait to get wet, delaminate and rot.
    3. Urethane water barrier. Puncture protection and waterpfoofing for the particle board.
    4. XPS. Glued or screwed to the urethane. Insulates, is waterproof by itself so it waterproofs the urethane.
    5. Thin glass/epoxy. Waterproofs the XPS.
    6. Glass/kevlar. Impact and abrasion resistance. Waterproofs layer 4 and 5. The glass must be able to resist to all structural forces alone.

    Let me explain the last point. Kevlar is weak in compression, it's there just for abrasion. All compression forces will be supported by the glass. In a sandwich, the core transmits this compression force onto the inner layer (tension). XPS is useless as a core because it has such weak shear strenght. The foam will delaminate and crumble. The inner layer (the CLT) will see no stress at all unless the object causing the compression contacts it directly. Otherwise the only thing transmitting stress will be the "fiberglass lapping" you made around the foam blocks. For that, this bridges have to be strong enough, otherwise they will break by buckling. So for the whole impact resistance thing to work the outside glass skin has to be calculated as load bearing by itself. In this case everything inside of it is structurally useless, you could just as well replace the CLT with XPS.


    A normal steel hull for a vessel that size would probably be 10mm plus stringers and frames. 19mm plate (same weight as CLT) with a multichine design is probably stringerless with a few bulkheads. The corrosion allowance and strenght is over the top. You would have to calculate the stiffness for exact scantlings. Insulation is sprayed urethane foam or glued on Armaflex panels on the inside. The interior can then be solid wood ceilings and furniture. If the insulation is properly done, there is no way CLT is better in any of the mentioned points. You can even up the foam thickness to the point of making the whole thing insubmersible.
    Yes, wood is wonderfull if you design for weight. In small sizes even carbon can not compete because of the practicalities of everyday use.
     
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  9. clmanges
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    clmanges Senior Member

    My mistake about the XPS; I had it confused with flakeboard. I somehow wiki'd the wrong acronym.

    I don't share your confidence in the unsinkability of this boat. Wooden ships have sunk in great numbers throughout history.

    If your hull does need repair, there's a chance that reputable shipyards would refuse to touch it due to your choice of materials; they have their own liability to consider.

    If you must have a foot-thick wooden hull, at least consider making it out of marine plywood. Any shipyard can deal with that, and it won't delaminate as the CLT likely would.
     
  10. Eelco
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    Eelco Junior Member

    Given that scenarios like these are hard to mentally model or engineer from first principles; I am genuinely curious; what do you think it would take, for a few layers of continuous kevlar to be pressed through 12cm of hard foam, and then deliver a sufficiently pointy jab, to put a tear in a urethane-rubber membrane that has something like 300% elongation at break, and is backed by solid wood? Or rather gauge a good chunk out of it; just a tear likely wont leak fast enough to get significant water in before you plug that hole with some putty.

    Frankly, I cant think of any realistic scenario that would cause that kind of damage. The realistic risks are bumping into a pier at a few knots, or running aground on rock with your sloped hull. Lets say we are being really stupid and we run full steam ahead into a concrete river bridge. It wouldnt look pretty, foam would be crushed; wood would creak and maybe crack a little. But id say the urethane membrane still wont have any reason to let us down; especially below the water line.

    I think youd really need to run at a good speed into an underwater steel beam poking right at you from a block of concrete, to do the kind of damage, to get a lot of water fast onto the CLT. I think those are risks im willing to take.

    But instead of the urethane, I could also order a truckload of 2mm polycarbonate sheet. Epoxy on a first layer, then solvent-weld on a second one so there are no seams. Now poking a hole in 4mm of continuous polycarbonate backed by solid wood... you could do it with a high velocity rifle round obviously; but reasonably blunt object moving at a speed measured in knots, would have to tear through the whole CLT panel before actually making a tear in the PC, id think. Could also put some steel plate nice and dry under that PC sheet on the bow. Pieces of steel poking at us from underwater better have good buckling strength now, if they want to get anything done.

    Dunno; I suspect this is all kinda ridiculously overkill for a houseboat that, if not sitting still 100% of the time, will be towed during pleasant weather by professional tugboats with sonar. And I suspect the purported inability to keep a wooden boat dry is a bit of an article of dogma. I know a layer of paint isnt going to be very reliable. But infact I know a guy with a houseboat and a wooden table that is totally dry; so dry wood on a boat can be done, its just a matter of figuring out what he knows that we dont.
     
  11. Eelco
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    Eelco Junior Member

    What is your source for that? Ive read a study on moisture migration in CLT during unprotected install in rain; they measured moisture over time and along the thickness of the panels, and moisture clearly migrated by double digit percentages over to course of weeks. Without scrapping the panels, other than some superficial cracks and cosmetic damage it wasnt the end of things. I imagine it depends on the adhesive used; I could imagine that forming a hard barrier; but that is not a necessity apparently.

    The XPS has sufficient compressive strength to carry the static weight of the boat by about a factor 60. (10kpa at 1m depth, vs 600kpa compressive for dense XPS). 100kpa seems to be the long term creep limit, which we will also stay well under. Indeed it only weakly couples the layers of glass on all sides, but thats fine. The outer skin is really just there to keep the foam from being eaten away by UV, critters, or piers.

    The stresses as seen by the CLT will be mostly from normal forces on the foam. If a 1m wave hits the bow, that would make the local pressure go up by another 10kpa, which the foam will happily pass on to the wood, where it builds up to more serious bending stressed along the length of the boat. Ofcourse the skin will flex at the similar rate, but its contribution to the stiffness is minor. Even continuous kevlar only has 10x the stiffness as softwood; and then im looking at figures for UD fiber, not laminate, so 1mm relative to 300mm is still nothing.

    Shear forces on the skin due to impacts will also translate into normal forces sooner or later, given that the outer skin encapsulates the whole boat, from keel to roof. Youd really have to rip out a chunk of kevlar to start grinding away at the foam directly.

    I dont want to rely on the water proofness of the XPS. In one piece its a good water barrier, but it cracks easily even with a light tap through the outer skin probably. Its there for thermal isolation first; and second it should help a tad by providing a crumple zone in case of blunt impact, and keeping sharp bits away from the actual moisture membrane.

    I am not saying thats a terrible option. This thread is about exploring if CLT is a feasible one. FWIW I have not yet seen a quote for a new built steel hull of such a size that wasnt at least 2x the cost of CLT; but I am happy to be proven wrong. Any of the small bumps that would crack my kevlar skin and I could honestly forget about if I didnt have OCD, would strip enough paint to have to get that steel hull into a drydock though; am I wrong?
     
  12. Rumars
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    Rumars Senior Member

    If moisture migrated trough the panels the glue they use is permeable to a high degree. Probably selected to be very flexible to accommodate wood movement. It would be interesting to know if the panel was constrained at the edges or free to move, it makes a big difference on how the layers crack. The problem is of course there is exactly one wood glue rated for immersion (resorcinol), and another one wich is not rated but widely used (epoxy). Both are highly imprevious to water.

    As I said before the problem is not the CLT per se, it's your expectations not matching the material. Boats have been constructed out of plywood and solid wood without any epoxy for years and they have no problem with just cheap varnish and paint. Your expectations regarding lifespan, rot and impact make it so difficult. A normal wood or plywood boat has clear requirements: keep the paint (or varnish) in good shape to slow down moisture transfer and minimize movement and repair the thing when needed. If rot develops you cut out the bad part and glue in new wood. Simple as that and inspection and maintenance is part of the routine. Sure, it helps if the wood is rot resistant by itself, but pine was a normal planking wood. Spruce and fir are worst but you could impregnate them. If you accept this facts you can use even CLT to make a boat. If you want better abrasion just glue on a harder wood on the outside as a wear layer (actually you have to glue two layers in order to keep the panel balanced). The underwater part has to be protected from teredos (if present), so you either keep the antifouling fresh or you put something over the wood (metal, plastic, etc.). If you tell me CLT is mandatory I would say cut out the panels to size, then vacuum bag two layers of black locust or other stable hardwood on the outside and build the boat. If teredos are a concern you get some fberglass on the outside and three coats of epoxy on the inside to match. Then paint and forget. If you hit a pier smear some thickend epoxy in the scratch and paint. If you rip out wood, or crack the boat, cut and glue in new wood. If you want insulation put it on the inside.

    XPS has good compressive strength (especially the heavier sorts used under foundations) but very poor shear or impact strenght. This means the fiberglass or kevlar will simply delaminate from it when it comes under stress (tension or compression). You hit something the glass is going to crack and delaminate on a wide area and crush the foam. There is no reason to have it on the outside, just live with the wood. Laminating polycarbonate or UHMWPE onto the entire surface is possible, but again why? You could just as well make the whole boat out of those materials, they are strong enough. What makes sense is making a rubrail (or two if you like) out of UHMWPE and bolting it to the hull at the most exposed location.
     
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  13. Dejay
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    Dejay Senior Newbie

    About 3 meters by 15 metres. From my research that is what you can fit on a normal 45' semi trailer with a bit of overhang and extra wide special permit that is cheap to get and doesn't cause big problems (no convoy vehicle required etc).

    If you let off your idea of using 30cm of solid wood you might get more boat for less money :) I'd reconsider what you really need and what you want and get inspired by existing proven designs.

    There are a lot of construction methods, including planking that is very similar to your CLT idea but instead you only need like 3cm thick wood. 30cm is just crazy. And planking for a box should be pretty easy to DIY.
    Construction Methods — Index https://smalltridesign.com/Trimaran-Articles/Trimaran-Construction-Methods-INDEX.html
    Boatbuilding Methods: How its done https://www.glen-l.com/submethd.html

    For a large and low performance "barge" houseboat like you're envisioning, I'd look at steel. If speed or energy efficiency doesn't matter that much I'd build with steel because it is easy, cheap and less work. Especially for a box shaped boat with little curves. I'd suggest you look into mig welding youtube tutorials. Then add XPS insulation and decorative wood on the inside.

    This is a sketch for my own "dream boat concept". A 50' purely solar powered trimaran cruiser that could be build relatively easily and moved via semi truck in pieces. But it would need to be very light weight. The freedom to go and stay anywhere "off grid" with plenty of electricity is my main requirement. A barge like yours would require careful planning and renting suitable docking space I think. Not sure if this is something you're thinking about.
     
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  14. Eelco
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    Eelco Junior Member

    PUR and urea glues are the ones ive seen mention most often. This paper contains a review section of dozens of papers on moisture and CLT; different sources also find rather different values, and there seems to be disagreement specifically about the influence of glue on diffusion in the literature too. But the impression I get is that under the right circumstances, water diffusion is not that different as through solid wood; which is high in the first place in softwoods.

    True; for a houseboat, I think its desirable to arrive at a concept that needs significantly longer maintenance intervals than is typical for a shipping vessel. That may add complication; or be too optimistic altogether. It seems to be attainable for cement houseboats at least though.

    CLT with hardwood exterior definitely sounds appealing aesthetically. But I dont think it will provide more protection to the CLT than kevlar and foam, or require less maintenance, or be cheaper. Or insulate as well.

    Making a vessel of this size out of PC would not be very economical. In terms of strength/stiffness per euro, its at least 10x the cost of CLT; even when comparing installed CLT vs the cheapest sheets of PC with shipping and taxes unpaid on alibaba. Combining materials to combine their good properties is not that radical or novel idea. CLT is great for economical house-sized stiffness assembled to spec. PC is great for impact resistance and water tolerance, and unlike most other rubbers or ductile plastics, it can be glued and bonded very well. They make a nice union.

    To be honest I am not very confident about the best way to execute this external foam concept; there are many details I do not have experience with. But I do know foam-core construction is an established concept, with properties that are acceptable to me. May be more expensive than I hope, to get it done right, though.
     
    Last edited: Apr 15, 2020

  15. Eelco
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    Eelco Junior Member

    Im mostly mentioning the 30cm to illustrate it would be affordable. Probably, with some more detailed calculation, turns out you can get away with a lot less, agreed.

    I dont think there is much easy about DIYing anything house-sized. Thats subjective of course and is different for everybody. I build my own furniture and maintain my own bikes; thats about the extent of my DIY ambitions.

    Nope, not interested in welding my own house. Do you watch the SVseeker youtube channel? Its really inspirational stuff to me, but i think it teaches me all i need to know about the realities of welding my own boat, and I dont think I have even a tenth of the grit that man has.

    The netherlands has a large houseboating community, and many locations zoned for houseboats, including all utilities. That may be less true in other parts of the world; but ill worry about that and perhaps upgrade off-grid systems accordingly as that becomes useful.
     
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