Building a kinematic solar chicken tractor (A Philosophy of Outbuildings, 2nd worked example)

In late 2019 I wrote a post called A Philosophy of Outbuildings, attempting to capture the lessons from 40 years of making and using utility structures on the land in Five Islands, and now a bit here in Gorham. In the summer of 2020, as part of our garden expansion I took down our oldest, smallest ground-mount PV array of four modules, which was mounted on a rack of untreated 2×4 that was rotting up from the ground. We needed a new rack for the PV, and we have also talked for a long time about eventually getting chickens here. I had already worked up a mostly-effective technique for using quality aluminum foil tape to weathershed the seams between modules on a ground-mount array, making a pretty nice garden shed that might become a third essay in this series. So a solar chickenhouse seemed like a decent idea.

One of the principles of the philosophy is that Small Outbuildings Should be Portable – because they can be, because it increases the value of the time put into building them, and because otherwise they often get in the way of bigger plans that come along later. In the case of grazing livestock, portability is a primary requirement, to expose the critters to fresh pasture, and the Chicken Tractor is at this point a classic DIY project. My parents have taken that concept to the extreme in Five Islands with two small barns for laying hens, built on the four-wheeled frames of retired haywagons from the Holbrook dairy operation in Woolwich. These are truly impressive structures, complete with large winch-up flyways, and those hens get moved every 2-3 days, but a conventional stick-frame rectangular building on a long, spindly four-wheel wagon will come under some pretty intense wracking strains when pulled over uneven ground, and indeed the first iteration ended up somewhat diagonal due to this fundamental challenge of kinematics.

The wracking of the four-wheel chicken tractors offended my engineer’s sense of propriety, much as an out-of-plumb outhouse grates on a carpenter’s, so I resolved that our portable solar chicken house would be kinematically correct, with a three-point stance to avoid wrenching when transported. This complicated things significantly, but the end result was satisfying enough to document in this post, with some lessons learned that could be useful in designing similar buildings.

The size of the building was roughly set by the four PV panels to be mounted, older 185W Chinese mono modules that measure a bit over 2×5′ each. To minimize footprint and provide a bit of standing headroom inside, I arranged them 2×2 in portrait format, leading to a collector a bit over 5’wx10’h, set at 45 degrees (we’re at 44 degrees north here), giving a right-triangular wedge for the main mass of the structure.

When tilted up, the run of the collector array fell just short of 8′. But for efficient use of materials, and to provide a bit more floorspace, I went for a full 8′ in the north-south dimension. To cover the gap at the north end I put in a small north-facing roof facet which I covered with twinwall polycarbonate to let in a bit of light; this also echoes the design of the (decidedly non-portable) solar garden shed that sits just to the north. In the end this resulted in a lot of picky carpentry; it might have been best to tilt the array slightly flatter than 45 degrees and deal with the reduced headroom, but I am happy with the result. With our without the north-facing rooflet, the north wall is the only one big enough to receive a door, and is also a convenient place to implement daylight/ventilation.

Inspired by a coop Gerry Carroll built in NJ, I was keen to try accessing the laying boxes from outside, and the building could use a bit more width to give it stance against blowing over, so I slung two rows of laying boxes on either side running north-south, designed to have hinged access hatches on the side walls and de minimis shed roofs to cover. I clad the laying box sheds with salvaged mobile-home skirting in a nice gray-brown faux woodgrain color, scored from Dave’s salvage collection. We’ll see how the external-access laying boxes work out if we ever get around to populating it with chickens.

The kinematic foundation is the most unique element of the structure, and fortunately I have some decent pictures. Because the shed would be located within our large fenced garden area, it would probably move relatively infrequently (on an annual or slower-than-annual rotation) to allow the chickens to fertilize a fallow area, so I decided to use skids rather than wheels. The kinematic principle could (and I would say should) be applied to a wheeled structure with minor variations. Like the portable tractor shed I built upriver, the skids are made of ordinary treated lumber, 4×6 flatwise in this case. Here are some shots of the substructure under construction, showing how the floor can pivot freely and be adjusted independent of the base:

Built upward from the skids are a front crossbeam (2×10, sculpted top and bottom to increase clearance) and a back crossbeam (C-channel construction of PT decking, similarly sculpted below for ground clearance), plus 2×6 diagonal braces to keep the substructure square. The front crossbeam has metal plates sandwiching it fore and aft, through-bolted (in this case the plates happen to be leftover tail vanes from a prototype wind turbine). The curved metal plates project well above the crossbeam, and form a trunnion that accepts one of the joists of the floor deck with a large pivot bolt. This gives the superstructure a roll degree of freedom relative to the substructure, which is key to preventing the building from wracking as the skids move semi-independently to conform to the surface contour of the soil. Additionally, the flex in the metal plates, the wooden joists, and general slop in the system allows sufficient pitch degree of freedom such that the floor can be leveled.

The ability to level the floor is not necessary to the fundamental goal of a kinematically appropriate structure, and the chickens probably won’t care, but as the son of a carpenter I feel that certain standards must be upheld. So the north end of the structure is supported off the rear crossbeam by two salvaged scissor screw jacks from light cars, which allows the floor attitude to be adjusted, taking advantage of the roll and (modest) pitch degrees of freedom of the front pivot point described above.

Between the solar panel roof and the specialized kinematic base, I tried hard to keep the structure from getting too heavy. The floor is 1/2″ PT plywood framed with a 2×6 perimeter, but joisted with 5/4 decking. The wall framing is similarly light, mostly decking ripped in half lengthwise, and the side walls of the structure are diagonally planked with Hammond Lumber’s thinner, nicer-grade shiplap that is rough one side but measures an actual 3/4″ thick (where regular shiplap is actual 7/8″ thick). I also incorporated some hardware cloth and twinwall polycarbonate for light and ventilation. Here are some photos of construction:

We got an early snow in the fall of 2020 that then melted, and I took the opportunity to skid it into the garden to its final (for now) home:

The structure moved nicely, and it was almost a letdown how trivial it was to level it in place with a couple cranks of the screw jacks. Because the superstructure is intentionally built light, it is secured downward to the skids and the jacks with light chain and preloaded by turnbuckles left over from a fencing project. I bolted on the PV modules, and it sat through the winter and spring while other projects took priority, including growing and harvesting a nice crop of rye (which deserves a post of its own):

I finally got to ‘finishing’ it, roofing the laying box sheds with the trailer skirting and building a door on the north wall this summer. All in all, a satisfying project, currently storing gardening sundries and awaiting a shipment of chicks someday.

Naturally in the course of this type of exploratory project, I came up with some things I’d do differently next time. Most basically, the construction makes the superstructure a few inches taller than I think is necessary, and fitting the kinematic degrees of freedom into a shallower package would be cool. This might be accomplished by dispensing with the metal trunnion plates and fitting the front crossbeam between two closely-spaced floor joists. This would also require a more compact arrangement for the adjustable elevation on the north end, and while the scissor screw jacks were free from Dave’s dump collection and well-suited, they have a temporary, inelegant feel about them; in building another one I might try to substitute some stumpy homemade turnbuckles with welded end plates, fabricated from a couple large-diameter left-hand threaded nuts and bolts. This would obviously also provide downforce, eliminating the need for the turnbuckle/chain.

Reflections on another year of serious gardening (2021)

A post to capture findings from another year of trying to do a good job in the vegetable garden (last year’s post).

The weather was odd; spring seemed ordinary, then June was absurdly dry, then July was ridiculously wet right into August. then things seemed to even out, but I can’t remember a late summer period that seemed this green. We irrigated steadily early in the year, but never once after it started to rain in July.

Made an effort to drive up fertility with good effect; got the usual 3 yards of surf-and-turf in the spring from O’Donal’s, and probably made and spread nearly that much of our own compost.

Weed control was OK through most of the summer thanks to the hacked BMX-bike wheelhoe and a spiffy collinear hoe I bought from Johnny’s. Got sloppier late in the year as things turned rampant and other projects intervened.

Fencing is in good shape, thanks to big investment last year. No woodchucks in the squash this time; deer only got in when we left the gate open.

By crop:

• Brassicas: did pretty well in general; planted all in one row and sprayed 2 gallons of BT (organic-listed bacterium that parasitizes moth larvae) on the row 3, maybe 4 times over the course of the summer, which made a major difference – e.g. the Brussels sprout tops were getting skeletonized and recovered immediately right after I sprayed.
  • Broccoli: direct-seeded 2-3 fedco varieties, one or two failed so got a dozen starts from O’Donals. Couldn’t tell the difference between varieties; froze several gallon bags’ worth on cookie sheets, since we always use a lot. Kept producing off side shoots well into the fall.
  • Kohlrabi – grew for the first time after being inspired by a picture of Holly’s son with a huge one. His description was pretty accurate – like a giant hunk of broccoli stem. The root is functionally similar to jicama but a bit zippy instead of sweet, and makes pleasant fresh eating as a low-cal snack, or grated into a salad; I blanched and froze a few quarts to try in soup. Surprisingly good stir-fried with sesame oil and salt, per HG advice. Next year: attempt to succession-plant; start some late for fall.
  • Purple cabbage – I direct-seeded this early, and thinned/transplanted in the rows. They did well, but ripened while there was a ton of more compelling food in the garden. Plant late for fall/storage
  • Kale: didn’t plant kale early, and didn’t miss it much. Planted some late which is doing well now; seems like the right move. get some other varieties next year besides Russian
  • Brussels sprouts: still coming along – look pretty good.
• Cucurbits – very satisfactory this year. We experimented training cucurbits and tomatoes on the garden perimeter fence; the tomatoes did great but the squash got munched by deer – so in the future we should either string electric fence outboard or confine the squash to the interior of the garden.
  • For summer squash I started some early in the greenhouse in 4″ pots – Yellow crooknecks, the same anonymous green pattypans I bought in the spring of 2020 at O’Donal’s, and a green zucchini variety I bought from fedco without reading the description very carefully. These latter turned out to be zucchini rampicante, which are in the moschata family, and they worked really well – resistant to the bugs that eat the stems of other squash, long-lived, and prolific. They would have been even more impressive if the deer hadn’t decimated them where they climbed the fence, and if I hadn’t planted them over mowed red clover that came up pretty think and competed for light. The pattypans were OK but nothing like last year (maybe try a different variety next year), and the yellow squash faded fast but I planted a succession probably around July 4 that did well (and is still fruiting in late Sept, while just starting to get mildewed).
  • Cucumbers likewise we planted along the perimeter fence (as well as in the greenhouse). The deer got them, but still got plenty for salad and sandwiches (not sure if Kelsey pickled; I think we’ve had more in past years…) – Plant fewer next year, and succession-plant them
  • I grew a few pots of honeydew, cantaloupe, and watermelon. Got a few of each that were pretty good. They got a bit lost under the tomatoes that were also planted on the fence; in general I have a sense that if we committed space and babied them, we could grow some serious melons.
  • For winter squash I planted 3-4 hills of waltham butternut, and got about 120lb of fruits. This grew in an area about 15×20′; this represents a pretty decent people-feeding rate of around 4.5 people per acre, although at 200 calories per pound it would be tough to live on butternut squash…
• Tomatoes: did OK, planted mostly along the perimeter fence, and the deer didn’t touch them. Didn’t pay enough attention to varieties, and ended up with a bunch of sungold and small red cherries, and not so many large eating/canning tomatoes. The elongated red cherries were pretty nice for fresh eating, and overall we got plenty of fruit – I canned over 6 gallons of diced tomatoes in enriched sauce made by reducing mass quantities of the little ones in a crockpot and running them through Kelsey’s squeezo. The ones in the greenhouse did OK but actually came in later than the outdoor plants; the greenhouse seems less necessary for them than for peppers and eggplants. Plant more large ones next year.
• Basil: planted around 12 row-feet in the greenhouse, and similar outdoors. Both did great, made several ~quart batches of pesto and froze in ice cube trays. The outdoor planting started to brown pretty early in September; it definitely seems to be among the most sensitive to cold of the stuff we grow.
• Eggplants: did great in the greenhouse. Didn’t bother outdoors based on last year
• Peppers: bought some starts at O’Donal’s, mostly planted in the greenhouse where they did OK. Two plants outdoors kinda fizzled; not clear why
• Carrots: did a good job of planting successions, mostly of Yaya. Frustrating since the cutworms really like them, but still got plenty. Carrots seem happy in our soil; I definitely see the potential to grow an absurd amount by refining practices. Mark Fulford has done some experiments with intense organic cultivation of carrots that seem pretty impressive, yielding 54-65,000lb/acre in 70 days – equivalent to a feeding power of approximately 15 people/acre.
• Beets: got a couple meals from an early planting, but goldfinches have taken a liking to them and skeletonize the leaves (likewise of chard). Plant small successions of beets under floating row cover next year.
• Chard: Grew some; goldfinches ate most of it. Kelsey feeds it to her family and they seem to like it. One of those things where by the time it’s ready to eat, there are a lot of other tasty things to eat.
• Spinach: overwintered some and it was great to have in the spring. Got a good long row planted early, and ate heavily of it (plus froze some) despite heavy predation by cutworms. Later plantings bolted in the heat; Kelsey got in another good long row for fall that’s doing well.
• Alliums: not a fantastic year. I planted some yellow onion sets, and they got heavily munched by cutworms; the ones that made it looked good but started to rot quickly after harvest. Kelsey transplanted a small patch from seed that seemed to do better though they also got cutwormed. Maybe BT spray would help. We also planted leeks that she started from seed; it too got cutworms and struggled to get going. Definitely not as good as last year.
• Green beans: Kelsey planted a couple rows and they kept going all summer – seems like they liked the rainy weather.
• Peas: snap peas did OK as usual – great fresh eating right in the garden; I haven’t figured a way to freeze them that’s actually tasty. Also grew a couple types of shell peas, but they are a lot of work – seems marginal for to grow and shell them for storage. I did look online and it appears that one can make a homemade tumbler out of wood and hardware cloth to shell them automatically.
• Potatoes: did fine; crop seemed not-especially-heavy but quality was good. Picked a lot of beetles, plus one whack of spinosad when the picking got behind.