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.

Thursday, April 7, 2016

Mast Tabernacle Rethought

Sometimes delays are helpful because they allow more time for examination, and for rethinking parts of the design. And so it was with the tabernacle design. My initial plan was to use a joint that transferred all of the tension and compression loads into sheer loads on two large bolts, which I called “Jesus bots,” after the “Jesus nuts” that hold helicopter rotors in place. But then Alan, who is designing a boat similar to QUIDNON—a houseboat that sails, but for inland waterways—pointed out that my design would require extremely high precision in the way the components are fitted, or they would start to move and flex under load, and fail. Alan has lots of experience in aerospace engineering, and a keen appreciation for structural elements. What he proposed was two flanges bolted together, connected by a hinge on one side. This approach is very standard: look at your average streetlight, and that's how it's mounted. It is so standard that it doesn't require any interesting structural analysis: one can simply look it up and plug in the numbers. Since the bolts that hold the two flanges together are subjected only to tension loads, they don't need to be fitted precisely, and the math for sizing them is simple.

Alan's idea solved one problem but created another: how to fit the mast in place prior to raising it. He is used to working in a hangar, on land, when doing such things, but raising a mast on a boat in the water is a different matter. Precisely maneuvering a heavy mast on a swaying deck in order to insert the hinge pin is no easy matter. After much back-and-forth, we arrived at a solution. The hinge pin is welded onto the bottom flange—the one that's part of the tabernacle. The top flange has two slotted tabs that fit onto the hinge pin and two latches that capture it. And so all you have to do to raise the mast is get it into position and flop it onto the tabernacle. Click! After that, the procedure is simple: insert gin pole, attach hoist, and click-click-click until the mast flops forward. Then assemble and torque the flange bolts.

Another bit of very useful information Alan provided had to do with the design of the mast itself. I was thinking of using flagpoles, which are tapered, and of using them as unstayed masts. Alan tried to calculate the righting moment that would be generated by his hull in the event of a knockdown, and realized that such a wide hull would snap any unstayed mast. We also realized that there are a few other, minor problems with the unstayed design. The first is that there is nothing to prevent the mast from swinging side to side as it is being raised. The second is that the taper of a flagpole results in a sloppy fit up top, because the parrels on the junk sail have to be loose enough to be able to lower the sail all the way down. A straight, non-tapered mast would work better. Lastly, flagpoles are not the cheapest solution, which is to use 20-foot sections of 6-inch Schedule 40 aluminum pipe joined into the correct length for the mast using plugs made of smaller-diameter sections of pipe. The same pipe can be used for both the mast and the tabernacle.

And so the revised design has a mast tabernacle that uses flanges and a hinge, a mast made of straight 6" aluminum pipe, and two shrouds to at serve three functions: 1. prevent the mast from breaking in the event of a knockdown, 2. keep the mast from swinging side to side as it is being raised, and 3. take up some of the sideways load while sailing (which can be considerable, especially when sailing to windward, when the forward force is generated by two much stronger opposing forces of the sails and the centerboards acting at a small angle to each other). The design will remain unstayed as far as fore-and-aft forces are concerned, and the risk of dismasting from pitchpoling cannot be dismissed. Here, the addition of a couple of running stays, to be deployed fore-and-aft in particularly bad conditions, seems like a good idea.

There aren't too many options for shrouds on a junk rig. Since the sail slides up and down the entire length of the mast, there can be no spreaders. Nor are spreaders needed for a hull this wide. And so the two shrouds will run directly left and right between the mastheads and the ends of the deck beams that support the tabernacle at the deck, which are part of a trapezoidal frame made of hardwood timbers on which the tabernacle is stepped.

Problem solved... at least until somebody comes up with an even better idea, or finds a problem with this one.


  1. My constant diameter steel pipe mast for my junk rig worked great with small spreaders welded on about 2 feet from the masthead and maybe 15" long on each one. I ran two shrouds out to the spreader tips on each side (4 total) and used the padeyes on deck already bolted through on the 32 Pearson Vanguard. This spread-widened the shrouds just enough that when the yard was fully raised it could swing freely all the way until it almost hit the shrouds on each side with no binding whatsoever. Pretty much like Colvin did it and lightly tensioned as per his experience as well.... basically to perform as you mentioned: keeping the mast from whipping while raised and while lowering it to the deck. My spreaders just had large notches on the ends for the shrouds to fit into but if it was going to be raised and lowered a lot a simple latch, out on the spreader ends, could be employed to make sure the shrouds didn't jump out of the end-o-spreader notches while raising and lowering the mast to the deck. One cool thing about simple pipe is welding all your fittings right onto it and quite strong at that. I enjoyed having shrouds to hang onto while going forward and such. And yet another attachment point for a clever awning arrangement.

  2. Is there a link to Alan's inland waterway design? I'd like to see that one too.

  3. @Alan,
    Thanks for the link.
    Straight sheets of plywood slightly curved in one direction only is what I have envisioned that DIY kit is about.
    Multi curved surface of Dmitry's design is a bit of puzzle to me since, what I think, it may involve custom built, vacuum pressed huge bowl-like kit elements to skin the underlying structure. Just curious.

    1. There are no compound curves in QUIDNON's design. It's all planar curves that can be cold-molded out of plywood sheets with some kerfs around the bow.

  4. The trouble I see with Quidnon is the lack of a keel. You need to get the CG near the bottom/center of the hull, and if you want to go shallow-draft, you'd better start looking at catamaran/trimaran designs (hint: the Pacific Islanders did fine with those for thousands of years).

    1. The lack of a keel is a major advantage. My last boat, HOGFISH, had no keel, drew 2.5 feet, was quite heavily ballasted and went to wind quite well.

      Multihulls are great provided you like floating upside-down. They are much more stable upside-down then right-side-up. To each his own, but I prefer to stay right-side-up.

    2. OK welp, it sounds like you've got it sussed out a lot better than I have, as long as you can have that center of gravity such that it bobs up right side up matter what.

      Frankly I think the Polynesians, if their canoes capsized, would simply untie the things making 2 or 3 single hulls, get 'em right side up, then re-tie the whole lot.

      Pretty amazing people really.

  5. I think that the combined cross sectional area of the bolts should equal or exceed the cross sectional area of the tube, given similar more bolts are required unless they are biggies. A radial pattern of more smaller bolts would distribute the stresses better though. You may know that and made your drawings with only four bolts for simplicity?

    1. Yes, it seems like 6 bolts is a reasonable number. Also, it will probably help to weld gussets between them.

  6. Dmitry,

    Seems pretty slick. FWIW in a former life I used this product on manways for sulhuric acid storage tanks:

    I know you're keen on a square base-plate and I don't think they made models in the six inch flange range but maybe you could get a unit sized for your application and avoid a custom fabbed item. I like that the universal hinges can be installed, used and removed for use on other manways (or masts in this case).

    I think having a 6" pipe with a pipe flange would be the ideal method of transferring the various forces on the masts to the tabernacles but I'm sure what you've designed is sturdy enough. The other advantage with sticking with all piping components is that the masts could be ordered from most small piping shops and arrived in the length you'd like them built. Not requiring pressure tests, xrays, etc. would save you come cash as well. They'd be able to weld on a gin-pole mount (maybe a reducing tee?) and all the stay eyelets you'd need. You could even get a threaded or pinned-style plug welded in the top to mount your wind generator on.

    Off the top of my head:
    - Gussets and more bolts are a good idea, 8 would be great since that's what a 6" flange would have.
    - You've got the bolts spaced well, always want to leave a diameter, or more, from the edge.
    -Studs rather than bolts are nice you can insert either direction and generally higher strength ratings apply for similar diameters due to lack of stress concentration at bolt head shoulder.
    -Also various locking washers options are out there I'm sure a chat with a fastener supply rep would give you some good ideas.
    -Maybe a thread repair kit onboard for the inevitable cross threading by an over-eager helper?

    All the best!

  7. Very cool boat project!

    If you're getting into selecting bolt patterns for stength, useful reference: , and Machinery's Handbook is the all-time classic, especially since its great in printed form for all you end of civilization folks.

    Best general advice I was given as an engineer: overdesign.

    Now completely ignoring that advice, here's a way to stay entertained for a while:

    Since you're going to have a weldment, consider corrosion, plating, and heat (annealing/HAZ) issues, alloy selection. Also design w/ drainage vs galvanic corrosion (yes freshwater, I know, but with dirty water, animal fluids, or cleaning fluids trapped in corners, steel-vs-aluminum is recipe for accelerated corrosion. So liners,sleeves,protective grease/goo, material and plating choices for the bolts. I had a small 1960s aluminum boat for a while (best buy ever), built with rivets, and I'd say some of the steel bolts I used to repair it did rust extra fast. Not sure about stainless ones as yet. At the same time, my boat shop told me to not worry when using steel hardware on aluminum boat masts.

    One interesting feature of your design is you can access the inside of the mast tube, so you could run things thru there if you wanted to get crazy.

  8. I'm considering a tabernacle for a junk-converted Mercury 18. Your design is the best I've seen. Has it stood the tests of time and usage? Thank you. Steve Liebig, Seattle, WA

  9. This tabernacle design has not been built yet, so feel free to test it out.

  10. Thanks for the reply--I'm reluctant to be the tester! No offense, of course. Warm regards. Steve


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