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.

Monday, May 13, 2019

The Centerboard


Although the Quidnon blog has been quiescent for the past three months, there has been some good progress on completing the design, and I can now report these results and see what comments, ideas and suggestions emerge. It takes time to come up with simple and cheap solutions to complex and potentially expensive problems.

One of the problems that is now solved is how to provide lateral resistance with minimal complexity and expense. The initial concept included chine runners, which are narrow ledges that extended horizontally from the hard chines at which the bottom joins the sides, and two centerboards that hung down from wrist pins and extended from slots in the bottom in such a way that they would kick up into their slots when encountering an underwater obstacle.

The chine runners were discarded when it turned out that Quidnon’s hull, being quite wide in order to provide relatively spacious living quarters (it is, after all, a HOUSEboat), doesn’t heel enough to allow the chine runners to bite into the water. All the chine runners did was add some drag (and, of course, complexity and expense).

The kick-up centerboards worked well enough, but there is a basic problem with them: since they hang down from a hinge, they are deflected when moving through water, and this makes for erratic steering behavior. The deflection can be minimized by adding ballast, but this makes the boards too heavy. It is also possible to add tensioner lines fed to cleats that pop open when the boards hits an obstacle, but this adds complexity.

The problems with the original centerboard design didn’t end there. How does one remove them for maintenance and cleaning, and put them back in? If this required the boat to be hauled out, then that would invalidate the very important requirement that Quidnon must never need hauling out (haulouts are expensive!). The copper cladding can be cleaned with the boat hard aground at low tide and there is nothing else down there that should ever need attention. And so a plan was created for installing and removing the centerboards with the boat in the water with the help of a diver. But divers are also expensive!

And then another good question arose: why are there two boards? Well, the initial thinking went as follows. Putting a single centerboard along the centerline wastes precious living space in the middle of the cabin by filling it with a centerboard trunk. Moving it off to the side makes the design asymmetric, and that’s functionally unimportant but aesthetically unpleasing. Therefore, let’s have two of them. The flaw in this logic is that the aesthetic consideration matters not at all because the centerboard isn’t visible. Your heart is on your left and your liver on your right, but nobody will ever call you ugly because of that.

If two centerboards are too many, how about zero centerboards? Well, it turns out that having a centerboard is rather important, but only when the boat moves. It is especially important when motoring in and out of marinas, because without the centerboard the boat will drift sideways instead of turning within a tight radius. It is also important when motoring, especially upwind. It is sometimes possible to sail downwind without the centerboard, but that’s about it. But if the boat doesn't move (as houseboats often don't) then a centerboard isn't needed at all, and having one that's quick and easy to install and remove would be a bonus.

And so just one centerboard is both necessary and sufficient. It will be located off-center (to starboard) with the centerboard trunk located unobtrusively in the back of a settee in the salon, sandwiched between it and the water tank. (But we’ll still be calling it a centerboard because offcenterboard is not a word.) The centerboard trunk forms an L-shaped slot that extends from the deck all the way to the bottom (and does extra duty as a deck drain). To one side of the slot is a channel that stops short of the bottom and tapers in a specific way before it stops.


The centerboard is just a piece of 3/4-inch plywood covered with fiberglass for durability. A circle is drilled out of it near the bottom and filled with lead in order to make the centerboard heavier than water, but not much heavier. It doesn’t have to sink particularly aggressively; it just can’t float up. To one side of the centerboard, near the top, is screwed a cam that rides inside the channel on the side of the centerboard trunk. At the very top of the centerboard is a hole used to attach a lanyard by which the centerboard is retrieved. If the lanyard breaks, a boathook can be used to grab the board by the hole. The centerboard is sacrificial and designed to snap without causing damage to the hull. Making a new one is neither expensive nor difficult.


The centerboard will spend most of its life sitting flat on deck. When the boat is getting ready to move, it is installed by unceremoniously dumping it into its slot.


If there isn’t enough water under the boat it won’t go all the way down, but that’s usually not a problem. (You may need to give its lanyard a tug when backing out of a shallow berth, to keep it from catching on things.)


When the centerboard hits something underwater with the boat moving forward (as boats normally move) it deflects aft, but in order to do so it needs to ride up a bit, so that the cam moves up inside the tapered slot. It can stay in this semi-retracted state, bouncing along the bottom, while the boat sails or motors across shallows. This will slow the boat down a bit, but will also provide good steering because the boat will pivot around the board as it digs a shallow trench in the sand or the mud of the bottom.


It is not necessary to remove the centerboard when anchoring above the low water line with the intention of drying out at low tide because it will be forced up entirely into its trunk.

This completes the conceptual design of the centerboard; next on the list are: the rudder; engine mount; mast steps and bow structure. These have all been reworked, and I will be detailing the new designs over the coming weeks.

18 comments:

  1. How do you prevent bio-fouling in the centerboard slot and for that matter the engine well? Will they also be lined with copper?

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    1. They will be lined with copper below the water line (the lowest 18 inches or so). The centerboard slot does get crunchy with mussels, which die a sudden and gruesome death when the boat hits the shallows and the centerboard kicks up. It can also be scraped clean with a long-handled scraper (12 feet or so) that can be wielded from the deck or by diving with a much shorter scraper.

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  2. why not use a leeboard similar to Dutch workboats ?

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    1. Because leeboards are ugly, noisy, accumulate floating debris, and while one of them is "working" the other one just bangs about uselessly.

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  3. Chris Morejohns unballasted, tough daggerboards are intriguing. Seems to work well for his bluewater Hogfish sharpie. If he runs into something he just allows it to pop up due to its buoyancy and continues on. You'd think this whole process would injure the hull but apparently he designs and builds so tough it doesn't matter. In port one could throw a small table top on the protruding daggerboard up on the deck. Super simple and maybe a bit easier to keep the case clean. No warning jangle of shallow water like a centerboard though. Oh well.....

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    1. I've discussed Chris's design with him. It has many of the advantages of what I came up with, but it also has a major shortcoming: once the board pops up, it becomes useless. It's of no use at all in tacking out over shallows. In the case of his Hogfish Maximus it is less of a problem because the narrow beam makes chine runners effective, so it's still possible to tack through 60º. In the case of Quidnon, which only heels 10º, chine runners are useless, and so the centerboard has to work under all conditions.

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    2. Series of bungees atop just retard the boards rise but would go up with bottom contact? Like the centerboard beginning to kick-up except straight up or a bit angled. Still a ultra simplistic rig and easy to clean case.

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    3. Bungees? Neoprene lasts maybe 6 months in a salt water environment. I've tried to use various schemes that used bungees, and found them to be unworkable. This is why I came up with this current scheme, which seems to be by far the most promising.

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  4. Unsightly is true, but I believe your aim is primarily practical.

    One doesn't really need 2 leeboards, one is enough, custom if one only has one leeboard is to locate on starboard side.

    Floating debris ? - a centerboard trunk will hoover up a shocking amount of stuff. Check any engine well or a CCA vessel. It might be preferable to have everything out in the open where debris is seen and easily washed off, as is the case with a leeboard.

    I honestly belive given the mission of your vessel, a leeboard should not be dimissed out of hand.


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    1. Leeboards weren't dismissed out of hand. They were dismissed after careful consideration and weighing of pros and cons of various alternatives.

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  5. I like this new simple design. Speaking of Hogfish, Chris Morejohn's boat, he has passed through the Panama Canal and currently cruising the Pacific with his wife. They have covered many ocean miles. You may be able to view his instagram photos without an account on a non mobile device. Inspiring to see the boat lying flat for bottom cleaning on a remote beach. Also cool to see his large nesting skiff/dinghy on deck.

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    1. Thanks for the update. I haven't kept in touch with Chris in Rachel. Glad to hear they are out cruising.

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  6. I did some research years ago for a sail rig design I built for Westmarine ad learned that you can move the centerboard forward of the center of resistance and get a marginal benefits. Just a thought. Most ocean racers have their daggerboards slotted in front of the mast.

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    1. The relevant math is all in the book Practical Junk Rig, which I've been using. I've also read all of Hassler & McLeod's treatise on the Junk rig, but it only adds detail. The math is the same.

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  7. Hi Dmitry, lots of great ideas here! But I'm not sure I like that cam for the centerboard. If you're sailing in shallow water, and the board touches bottom, the initial force on the board will be more aft than upwards. I don't think the board will lift enough to rotate; it will just get stuck in the down position.

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    1. I did a vector diagram to work out the geometry. The function of the cam is to make the force needed to kick up the board roughly 4 times the force needed to lift it. When pushed back, the board will slide up and tilt into the retracted position.

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  8. If the boat gets into shallow water, only the lower end of the board will touch bottom. This will result in a twisting force, not aft-ward pressure, being applied to the cam. Also, I wonder if you allowed for friction in your vector diagram? I admire your creative thinking, but I'd suggest trying a scale model of this before building it into the boat.

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  9. You are partly right. Trying a scale model is a good idea. Go ahead, give it a try, and let us know how it turns out.

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