Rocket Stove Water Heater Design

Hi Permies! First time making an account and posting, but long time reader.

I have a bit of an odd idea: A semi-portable water heater rocket stove/oven. We have a nice river near us with naturally cut pools that hold stagnant water, and I'm hoping to bring a wood fired water heater to the site to heat up a pool of standing water. (I'll obviously drain the standing water and refill with fresh!)

This would be a 2 pipe design. Interior pipe would be 4", exterior is 6". 3/4" copper pipe wrapped in the open secondary chamber.

The idea is that top lid is removed, and the internal pipe is loaded with kindling and a fire is started at the base above the blower fan. After placing the top cap back on the flame will be fed by the blower fan at the base, and the heat will flow down the external chamber and out the base holes.

I've seen the dangers in using metal pipes, but I'm not looking to get these to extremely high temperatures. Was looking to use Stainless Steel 316 SCH40 for these pipes.

The water pipe will have an external pump for water movement, and the fan is an all metal fan made to withstand high temps.

Would love any thoughts you have on the design!

Wow, Nick interesting idea.  You are not showing your water connections or your pumping arrangement.
If I understand correctly you want to drain a pothole of stagnant water and then use your heater to pump fresh clean (hopefully hot) water in for a homemade hot tub?    
I see some possible issues.  
1) That metal skin is going to be very hot.
2) your pump will need to be just the right flow so your water is at the perfect temp.
3) Not sure you will have a sufficient wood quantity to heat and fill a rock hot tub.
4) The rock will steal the heat from the water very quickly unless the sun has heated it up.
5) you're running an extension cord to the river for the fan.  Water-electricity...

Defiantly a cool idea or a hot one depending on your point of view!

Wow, Nick interesting idea.  You are not showing your water connections or your pumping arrangement.

Thanks! The outlets of the water connections would be to compression fittings and high temp rubber/silicone hoses that can be dropped into the pool. A bit annoying to model but I have that covered. They'd exit at the top and bottom of the external pipe

If I understand correctly you want to drain a pothole of stagnant water and then use your heater to pump fresh clean (hopefully hot) water in for a homemade hot tub?    
I see some possible issues.  

1) That metal skin is going to be very hot.

yep! I have plans to add silicone grips to the top lid and have fire gloves to move in case of an emergency. It'd stay put in a drained pothole. There is an idea to have it suspended with wire cable to a rock so there is air flow to the fan and it's out of the way.

2) your pump will need to be just the right flow so your water is at the perfect temp.

Great point. That'll be something to test. A simple 12v submersible pump on the inlet would help prevent any vapor lock/steam.

3) Not sure you will have a sufficient wood quantity to heat and fill a rock hot tub.

Do you mean from just one filling? I imagine I'd need to refill many times depending on the temp of the pool to start.

4) The rock will steal the heat from the water very quickly unless the sun has heated it up.

Fair point, would be something to test with smaller/larger pools since there are so many unknowns.

5) you're running an extension cord to the river for the fan.  Water-electricity...

Haha no problem there, right!? This would be a 12v fan that is rated for ip67. Small waterproof battery pack to run it, so no extension cords down by the river.

Ultimately I could code up a little Arduino to optimize the fan speed, water pump speed and a temp sensor on the water outlet/inlet to get the optimum setup going, but that's for later!

The hot tub industry has wood fired water heaters you just place in the tub.
One of those style may work well, no added bits no pumps.
Almost like a reverse locomotive boiler.
A firebox with tabe tubes running through the fire.