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 four-bedroom with a kitchen, a bathroom/sauna, a dining room and a living 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. It's rugged and stable enough to take out on the ocean.

Friday, February 8, 2019

Frame Joinery Redux

Although most of the problems with hull structure have already been solved, there remained one problem that stood in the way of completing the design: how to join together the frame. It consists of 4x4 softwood (fir) timbers (3.5x3.5 finished size) combined into a box structure that reinforces the bottom the deck, the bow and the transom and provides support for mast steps. After working out a design that included a dozen different steel brackets that had to be custom-fabricated at considerable expense, I realized that I don’t like it at all: too complicated and too expensive. And so, as usual, I sat back and waited for some new ideas to filter in from the ether.

Eventually this happened. An unrelated project required me to build a rectangular frame by joining together some square cross-section sticks using L-brackets. To avoid the wood at the ends of the sticks splitting as I drove in the screws, I wrapped the ends using several turns of fiberglass packing tape. It worked just fine. The tape took up all the force that would have gone into splitting the sticks along the grain, and the resulting joins were impressively strong.

Transferred to Quidnon’s frame design, this technique will make it possible to assemble the frame using just 3-inch-wide perforated steel strips cut to two or three different lengths and bent to various angles. Some of the brackets will need to be bent to specific angles other than 90ยบ, but this is not a complicated procedure.

The procedure for fabricating the frame now consists of the following steps:

1. Using a chop saw cut 8-foot 4x4 timbers to required lengths.
2. For each timber, shave down the first 6 inches off each end on all four sides using a planer.
3. For each timber, router off the corners on the first 6 inches on all four sides using a router.
4. Roll on a layer of epoxy to the prepared ends, wait until it “tacks up.”
5. Wrap each end in three layers of 6-inch-wide fibergass tape.
6. Saturate the tape with epoxy; let it cure.


Frame assembly then consists of matching up the timbers and the brackets and connecting them together by driving in a lot of self-tapping screws using a cordless drill. There is no need to worry about the wood splitting, and the resulting joints are strengthened by the fact that they are pre-stressed: the wood is compressed between the screws and the fiberglass. In a humid marine environment the timbers will gradually absorb moisture and swell, increasing the pressure on the fiberglass and the screws, holding them in place securely. (The choice of softwood for the timbers is critical: when hardwood swells, it generates enough force to burst fiberglass.) The strength of the joint is determined by the force needed to crush the wood fibers, which is somewhere around 6 times greater than the force it takes to split them along the grain.


There are still several more complicated pieces that will need to be fabricated: engine bracket, mast tabernacles, masthead fittings, tiller and keelboard hardware and bow rollers. These are all key elements of the design and there is no way to simplify them. But the frame joinery is now very well in hand and can be done cheaply using components that can be locally sourced in many places around the world.

21 comments:

  1. To avoid the wood splitting, simply pre-drill the holes to the minor diameter of the screws. It works a treat, and eliminates the fiberglass wrap. Just my 2 cents. Cheers!
    Chris

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    1. Pre-drilling isn't necessary if the ends of the timbers are taped with fiberglass and weakens the joint by reducing the compression of the wood. The point of the exercise is to compress the wood between the fasteners and the fiberglass.

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  2. I would prefer a scarf joint to a butt joint. I'm also hesitant to use non-wood components in wood framed structures. Metals and composites are stiffer than wood and this can cause wear at the interfaces. An end wrapped in composite is essentially a ferrule that hides the eventual deterioration of the wood because of trapped moisture. A scarf joint with nonferrous fasteners would be an acceptable compromise.

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    1. Given that the entire design is a screw-and-glue structure encased in fiberglass your preference for an all-wood structure with fancy carpentry is not really applicable to it.

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  3. seems like a complicated method. Why not just use triangular pieces of plywood to make the joins as is usually done in wooden boat building?

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    1. This is not a wooden boat, this is a fiberglass boat with a wooden core. Wooden boats are a terrible idea. Many marinas simply don't allow them, mostly because they tend to sink a lot, but also because wooden is routinely equated with obsolete.

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    2. Im building a Dory for when I move up to Grand Marias on Lake Superior next Summer.
      I hope to set it up for Line fish trolling for Lake Trout and Salmon. TY for your project updates. Maybe Ill try a Quidnon once I get settled in my new town. Winters are long there, the community has some top shelf carpentry talent. What could go wrong?

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    3. There is a school in Grand Marais that teaches practically all the skills you'd need to build a Quidnon right there.

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  4. If I become a customer and am doing the construction, I would specify non-ferrous materials throughout. Common construction wood screws rust through in listvenitsa wood on a fence in Far East of Russia humid simmers in four years. Perforated metal plates can be used for extra strength at joints and I really agree with the recommendation for marine plywood gussets. After all they are consistent with wood 4x4's and don't interfere with idea of fibreglass hull in any way. Are titanium screws available and affordable? I don't mind so much to make a mistake with exterior accessible components, but dread any regrettable mistake in materials or construction of infrastructure. Your design is a great wide hull idea. Lon Ball

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    1. The screws have to be either stainless or bronze. The most effective material for the brackets is galvanized mild steel. Galvanized anchor chain and anchors last for decades immersed in salt water, and there have been no problems with stainless screws holding together fiberglass-over-plywood boats over a 40-year period. Titanium is not an option. The choice of materials is ultra-conservative and time-tested.

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  5. The following is a possible alternative for consideration. It may offer additional simplicity, cost reduction, and lends itself to being conformed more easily with requirements for hull curvature.
    Instead of creating a single butt-joint to mate each 100mm x 100mm timber, run each timber through a table saw to give 3 off 100mm x 30mm planks. The would allow each 100mm x 100mm (now 90mm) timber to be re-created as a 3-ply construction assembled so as to eliminate all contiguous butt joints. This would be achieved by offsetting all end-of-plank joints from one-another; with the intent of removing the need for butt-joint re-enforcement. If you were to lay up the planks, with the plank-ends offset at 1m intervals, and bonded them with water-proof wood glue, whilst clamping the 3-ply construction at 50mm interval to ensure good adhesion, this may satisfy your requirements of: cost-effectiveness, ease-of-constriction, commonly available materials, fitness-for-purpose, and strength. By constructing the timbers in this manner you could then remove the need for: epoxy, fiber-glass, metal fabrication, screws, and the associated costs.
    In a similar vein, it always struck me as magnificent that the ancient Egyptians were able to transport a granite obelisks, weighing hundreds of tonnes, hundreds of km’s, down the Nile on a barge constructed from nothing more than the stalks of the ubiquitous swamp grass of that region. Given the materials and technology available 1000BC, these barges were likely constructed by binding together dried stalks, in an innumerable quantity to achieve sufficient bouyancy; and with a rigidity and structural integrity necessary to navigate a river course over the large distances involved. The strength to be found in a laminated construction of simple and lowly materials, as demonstrated by those papyrus barges of 3000 years ago, can be truly awesome.

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    1. I've looked at the alternatives of steam-bending bow timbers and slicing them up into slats that can be cold-molded to the right curve. All of these approaches involve more work and tooling, and a higher level of craftsmanship than a beachside/riverside builder can reasonably provide. The need for epoxy/fiberglass, metal fabrication, screws, etc., would remain the same regardless of whether straight or curved timbers are used in the bow.

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  6. I'm glad the steel brackets have been eliminated. This seems like a simple, strong, and reliable approach.

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  7. Good trick to know for other boatbuilding projects as well. Probably provides that extra level of confidence when the boat is hove to and taking some shots occasionally. But... still a place for all wood watercraft in beach microcruiser-campers and wood surfboards!! 8) Resolute men proudly sport a woodie.

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  8. Apparently a bad idea to mate steel brackets to bare wood without a lot of bedding (George Buehler book?). The fiberglass tape might solve that problem. And 3/4" ply gussets? Been there, done that and creates big voids perfect for spiders and trash. Re the brackets rusting out: evidence from a boatbuilder who took two steel frames (both freshly sandblasted) and coated one with epoxy and one with Tremclad brand paint. A year or so in the rain and the epoxy ones were rusting. The guy figured he could just use Tremclad paint without sandblasting. So much for epoxy coating steel.

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    1. My experience with slag-blasting and epoxy-coating vs. painting welded brackets is that epoxy doesn't really adhere. In the future I will slag-blast, then galvanize, then follow the standard procedure for painting galvanized steel. As far as bedding, a pad of 3M 5200 between the steel and the fiberglassed wood is probably the best strategy. Small pilot holes for the lag screws and a bit of 5200 on the screws would probably help too.

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  9. While I am neither a boat builder nor expect to every have a Quindon or similar boat, I find these continued discussions fascinating to try to follow. Someday I hope you can actually build this boat so we can see if your engineering design decisions actually work or not.

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  10. I agree with some of the other comments that a scarf joint is best. When you are making a long straight or semi-straight length the goal is to have it all have about the same strength. What you have done with your proposal is take a long somewhat evenly flexible section and create an absolutely inflexible section aka a stress riser that will transfer the stress of the entire length to the ends of the fiberglass wrap and metal piece. One way around this is to graduate the termination of the glass wrap over a longer area. Or just do a 10” angle cut scarf joint, glue with a good resorcinol or my current favorite is a polyurethane moisture activated glue and screw. I assume you will have at least one side of this beam glued and screwed to ply so that is your bracket. If you really want more strength there make a nice ply piece 12 to 18" long with tapered ends and g&s it in place.

    I would not bother with metal brackets except maybe where you have a 90 degree.

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    1. Scarf joints result in scrap, and scrap is expensive. A 10" angle cut will waste 2 feet of material. If for each 8-foot 4x4 2 feet are wasted, then that's 25% wasted as scrap and another 25%, plus another 2 feet of effective length reduction, blowing up the lumber bill 50%.

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  11. I am so glad this project continues to move forward, after one or more thoughtful time-outs, as which often happens with hard projects. Dmitry, applause! Y'all, commenters included, are above my pay grade, and super-helpful. I'm sensing a downhill slope to a boat in the water. That's what I'm waiting for!

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  12. I'm late to this post, but I'm wondering if eliminating the curvature of the hull wall will also allow the elimination of the need to build the entire hull upside down, then flipped by force. Maybe just the bottom section upside down, then flip?

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