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?