Simple and effective outdoor wood fire hot tub bath design

Thought I'd share this setup here, as it's served well now for a few years and one of the best overall ultra low tech designs I've seen for this.

Photo should be pretty explanatory, it's just two parallel 'stem' walls made of stone mortared with mud/cob up to about 16" above ground, with an opening in the front to feed the fire and one out the back to mount a 6' smoke stack, all done such that a flat metal panel cap (heavier gauge ribbed roofing in this case) can mount evenly end to end to enclose the fire box below. The addition of an inch or so of loose sand or fine aggregate on top of the metal creates an even base for the tub, and really helps to both distribute and hold heat in between the fire and tub.

Takes an hour plus or minus to heat a half full volume of water, and will hold temp for several hours easily. Quite delightful, and I'm sure with some creativity same principles could be adapted to larger tub sizes, but this is quick and easy enough that it sees regular use!

Thanks for passing along your ideas, it is a lesson in simplicity and efficiency.  In my past ramblings, I soaked in a few tubs like the venerable Snorkel model.  They are adequate but wooden barrels don't last forever, and were cost prohibitive to a vagabond lifestyle.  

Your setup is durable and worth every shilling, sir

Ben, may I ask what the length and width of your tank is?

I admit that I've been spoiled by the Japanese style deep tub. I'm wondering if the volume would be close to what you have - just a different shape.

That said, not sure how much the efficiency would change with the shape. Your tank has a lot of surface area lengthwise. My idea would have more heightwise. (Discrimination happening - why does spell checker allow "lengthwise" but not "heightwise"?)
You would have a longer fire, which might speed up the warming.

Thanks for posting this Ben! I was imagining something similar with dry stacked cinder blocks but I hadn't thought of capping the walls with roofing tin. I was just going to use the stock tank to contain the fire but this seems smarter.

How hot is the bottom of the tank to sit on?  

Tank is right about 6' x 2' Jay, it's a standard size stock watering tank, that I happened to find used locally. Surface area to depth ratio probably does matter for heating efficiency, but you might be able to rig a taller one to have one upright side of the tank heated by a wide chimney flue?

Mike, I think this approach helps on a couple levels versus direct fire-under-tank. I think durability of both the metal itself and the seam glue that seals around the base perimeter is a consideration with very high temperature epxosure. And yes, the metal/sand sandwich really helps even out the temperature conduction. I generally have heating dialed in with practice, tapering off fuel feed rate toward the end to be able to get right in at ideal temperature without hot spots on bottom. Some kind of thin wood slat buffer inside could also be used as needed.

Thanks Ben, I didn't think about the possibility of glue in the seam but that makes a ton of sense.

My plan is to have two of these side by side with a central run of blocks to support where they meet.  The fire would be at one end, the heat would run to the other end, do a U turn and come back under the second tank and end up at a chimney next to the fire opening.  Any thoughts on why this would or wouldn't work?  That way I can do a 2 or 4 person hot tub experience...