After reading about a wood-fired ocean hot tub made by False Profit Labs, I was inspired to create my own. With $100 and the help of a couple friends' minds and muscles, we constructed a working hot tub in a few hours of manly fire building, ditch digging, and water hauling. Below you'll find our construction details and a few points on how we hope to improve on our design.
Things you'll need to buy (all can be obtained at your local hardware store):
20ft. 3/8 in. ID copper tube $37
hose clamp x2 $2
7/8 in. OD x 5/8 in. ID x 20 ft. plastic tubing $16
5 gallon paint bucket x4 $20
10x12 ft. heavy duty tarp $24
It goes without saying that you'll also need plenty of wood to keep the fire going as you heat and use the hot tub. We also brought along a few bricks laying around the yard that turned out to be pretty useful suspending the heating coil over the fire.
Construction:
Carefully bend the copper tubing starting about 18 inches from one end and spiraling outward. Make sure NOT to kink the tubing as this will constrict the flow of water through the tube. You should end up with a pancake shaped coil and the two ends of the tubing coming away from the spiral. These ends should be far enough away from the coil so they will be clear of the fire.
Attach 10 ft. of plastic tubing to each end of the copper tubing using hose clamps. Make sure to tighten them down well to create a good seal. That's it! Now grab your firewood and head off to the beach!
You'll want to begin by excavating your tub next to where your fire will be. The 5 gallon buckets come in handy for hauling out loads of sand. Given the size of heating coil and tarp, your tub should be relatively small (we're brainstorming a larger design). A two person size worked well for us, but I'm confident we could have dug it to fit at least four given the amount of excess tarp and more than sufficient heating.
Line your tub with the heavy duty tarp and begin adding water, making trips to the ocean with the 5 gallon buckets. Before the tub is full, or as your help is filling the tub, you'll want to begin to heat the water. Heating the water can take quite awhile so it's best to start even if the tub is not all the way full.
Begin by elevating one of the 5 gallon buckets about 5 feet above the water level of the hot tub. This can be done by stacking buckets/milk crates/ice chests etc. Fill the top bucket most of the way with water. Submerge one of the free ends of plastic tubing in the filled bucket.
Place the other free end of plastic tube into the hot tub. Start a siphon of water out of the top bucket and into the hot tub. This can be done by sucking on the free end of the tube in the hot tub until the flow of water has started. Careful you don't get a mouthful of salt water! Every minute or so you will need to refill this top bucket to maintain the flow of water. It is IMPERATIVE that this water continue flowing once you place the copper coil in the fire. Failure to maintain the flow of water will allow the copper to overheat and melt.
Start your fire and place the copper coil within the flames. We found bricks useful to keep the coil supported over the fire. Be sure to keep the plastic tubing away from the flames. It will not be able to withstand direct exposure.
Continue filling the tub from the ocean and allow the water to heat. Our roughly 100+ gallon tub took an hour and a half before it was steaming (we'll bring a thermometer next time for better temperature comparison). As the tub heats, the heated water coming from the fire will reach steaming hot temperatures, much hotter than what is fed into a normal hot tub. Though it quickly dissipates to heat the tub, be careful not to burn yourself on the incoming water feed.
Improvements:
- Manual bilge pump for easier circulation of water
- Line outside of tub (between tarp and sand) with cardboard to improve insulation/heating time
- Increase the size of tub and include a bench along the outer edge
- Bring lots of food and beer next time
