rocket stove water tanks?

I have been heating my bath tub by burning wood under it. Making way too much smoke and wasting wood. I want a 30gallon? Gravity feed vertical tank with 304 stainless steel for the rocket stove, up into the center of the tank. So has any one built such a tank?  
I want to build this into a gas water heater tank..


I think it could be a .065inch walled double tube, 3 inch inside a 2 inch inside the other. That is less than a half inch (.435”) between the inner tube and the outer.
1. I need to know how tall it can be and still have the fumes come back down and out?
2. Does the downward out flow need to be more roomy??  Like maybe I need a 4”tube for the outer down-tube? (3.5”is not available)?

I'd like to offer advice but all I can do is bump your questions.  I'd like to see an answer to this as well.  You may have to begin a "Lindsey Labs" kinda thing and let us know how it turns out.  I'm bookmarking to watch (hopeful) progress.  Good luck!

After reading this thread "Rocket Stove Hot Water Heater for Outdoor Wood Fired Hot Tub”... I think I should have two large pipes a the top with pressure release valves. And a drain pipe at the bottom. And if the tank is not open-able I will have to keep it dry most of the time. So I should start with a electric hot water tank. Is that enough ventilation?

If the 3inch tube is not large enough they do have 4 inch tubes and I could put a 2.5” tube inside.

http://www.aprovecho.org/lab/rad/rl/stove-design/category/1

https://permies.com/t/38318/Rocket-Stove-Hot-Water-Heater