Dylan Hoen's Homepage
Electrical Engineer and Computer Programmer for hire in Victoria, BC, Canada
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Things I Have Built
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Things I Have Built
Title Page Camping Hot Tub Tall Bed With Storage Room Underneath
Portable Stareo Storage Bed / Media Couch Camperized Chevy Astro Van
LED Flashlight AUV Sonar Board BedWoofer
Computer Water-Cooling System Potato Cannon

Camping Hot Tub

Built: 2011/06/25 - 2011/09/28
Added to Webpage: 2011/09/27

This is a rough draft. I'll finish it off when I find the time.

Having succeeded in building a reusable camping sweat lodge (camping sauna / steam room) that packed onto the roof of my car, my engineering type camping buddies and I thought, "What do we do next?". The next logical step was to build a camping hot tub. Also, my ex –girlfriend didn’t like steam rooms; she liked hot tubs. I had a coil of copper pipe at home left over from an alcohol still I had built. I had an aquarium water pump left over from a computer water cooling system project that I no-longer used. I bought some plastic pipe and some pipe-clamps and brought a 5 gallon bucket camping. I filled the 5 gallon bucket with water and used a car battery connected to an inverter to power the pump to pump water out of the bucket, through the plastic tube, through the copper coil in the fire, and back through a plastic tube into the bucket. It took 5 minutes to heat the water to hot tub temperature. I estimated it would take a bit over 4 hours to heat up the estimated 1000 liters of a hot tub. The project was on!

I played around with various designs on paper, square and octagonal, and finally settled on a hexagonal design:

It was 2180 liters of water. The height was set to 1/3 the height of a 4’X8’ sheet of plywood. At 2'8" deep, the pressure at the bottom would be 1.16 PSI, which would make the average pressure on the sides 0.5805 PSI. Each side has 1280 square inches of surface area for a total of 743 pounds of force per side. I decided to use 3 2X4's to brace each side and a hole drilled through and ready rod inserted into staggered 2X4's to hold the sides to together, but also to be quickly assemble-able and disassemble-able, like a tent. Since the corner-to-corner-diameter is double the side length, but there are 2 metal rods per corner-to-corner-diameter, each metal rod also supports 743 pounds of force to hold the hot tub together. The whole structure could be constructed out of 2 sheets of plywood, cut into thirds vertically, then to 3’4” wide, 9-8 foot 2X4s cut in half, and 3 pieces of 3/8” ready rod, cut into 6 lengths of just under 2’8”.
I bought pool noodles and sliced them into C shapes on the table saw and used them as cushions on the tops of the sides, so you don't hurt yourself while stepping in or out, and also to use as a pillow.

The ready rods could be removed and the hot tub sides staggered and stacked into a compact shape that could fit into my van.

A friend and I went to Canadian Tire and bought a sump pump. My friend found 3-10 foot lengths of 1/2" copper pipe under his deck and we bought some copper pipe connector pieces at Canadian Tire. We brought everything camping for the Canada Day long weekend. We annealed the pipes in a campfire, then soldered the pipes together into one long pipe, using the copper connector pieces, and then wrapped it around a 5 gallon bucket to coil it. We set up the wooden sides of the hot tub and inserted a tarp to hold the water in. We inserted a tarp in the back of a pickup truck and drove down to the lake. We scooped water out of the lake in a 5 gallon bucket and filled the back of the pickup truck. We then drove the pickup truck back to the hot tub and scooped the water into the hot tub with the 20 liter bucket. After 3 trips with the pickup truck, my friends got tired and refused to help me anymore, but the hot tub was full enough for testing. It takes 100 20 liter bucket full’s to fill the hot tub, and each 20 liter bucket full weighs 44 pounds, so it is a big job to fill the hot tub.
We used an inverter connected to a car battery to power the pump. We pumped water through a garden hose clamped to the coiled copper pipe, which was heating in the fire, then back out through another garden hose, and back into the hot tub. Where the hose was near the fire, we protected it from the radiating heat by wrapping it in aluminum foil. The hot tub heated, but the hose was not designed for high temperatures and the inner surface shrunk and cut off the water flow, then the hose exploded in a cloud of steam. Just by chance, someone had some hose with them in their van and this hose worked. The sump pump was designed for an AC sine wave and didn't work reliably with the square wave that the inverter put out, and it also didn't put out much pressure. The hose became soft when hot and kinked easily, slowing the water flow. The tarp leaked. I layered 3 tarps and hoped that the leaks wouldn't line up, so that the tarps would seal. It helped, but it wasn't 100% leak proof.

After 6 hours of heating, the temperature plateaued at 35 degrees Celsius. I put a tarp over the top to stop evaporation heat loss, and the water temperature plateaued at 38 degrees Celsius. This was just above body temperature, which was perfect, so we decided to hop in. It worked great and we partied in it for a couple hours.

The sump pump wasn't powerful enough and I didn't like having 120 Volts next to water like that, so I bought a 12V boat bilge pump at Canadian Tire, which also turned out to have much more power. I bought a brand new tarp, but it also leaked, so I bought a 15' X 12' X 45 mil pond liner. I could have gone with a cheaper 20 mil pond liner, but I thought that I ought to get something more durable to hold drunk people while camping.
I needed to reduce the heat loss to maintain a higher temperature, so I bought 1" thick Styrofoam to insulate the sides and the bottom. 1" thick Styrofoam + 5/8" thick plywood has 5 times the insulating value as just 5/8" plywood alone. I'm planning to build a tent roof over the top of the hot tub to reduce evaporation heat loss. It is an evolving project.

I outlined the Styrofoam in duct tape to make the edges more durable. I cut the bottom hexagonal piece of Styrofoam into 4 trapezoidal pieces. I glued the Styrofoam sides to the plywood sides using a spray on glue that I had bought for building the bed in the back of my van.

I wraped the transition from copper to hose in aluminum foil to protect it from the heat radiating out of the fire. Further down the hose, I put a foam sleave around the hose to insulate the hose from heat loss.

The 1/2" inner-diamiter hose greatly reduces the flow rate of the water out of the pump. This pump wants to pump water faster and I think that it is a waste of battery power to restrict the water flow of such a powerfull pump like that. I found this 3/4" internal diameter heater hose that should allow a 2.5X faster water flow rate and allow for hotter fires.

Click the image below to view a .PDF file containing the plans to make the hot tub: