Pocket Rocket/snorkel stove/pool heater fail

I have a 6'x6'x5' deep, 1700 gallon, plunge pool made from cement filled cinder block, with a shell of 2 1/2 foam on the outside. I am working on trying to heat it. The last attempt was a Pocket Rocket/Snorkel Stove made from a beer keg with 6"x2' feeder tube and 4"x5' vent. I sunk the keg about 2/3. I thought the fire burned good, but it didnt heat up the pool like I thought it would. 3 days of 8 to 10 hour heated the top foot of water from about 42 to 70. I'm looking for suggestions how to make it work a little better.

Possible problem areas
Lack of water Circulation. I didnt have the pump working at the time.
Insulation on the water surface. I covered the pool with tarps and loose foam scraps.
Not taking advantage of the heat going up the vent.
thickness of the keg metal.

Hopefully my second revision will be better with some suggestions. Thanks.

Welcome to Permies!  I am afraid I am not of much help, but I am sure there are many here who will be.

Pete:

Welcome...

A couple of things about this ..

1) that's a LOT of water to heat.  The largest snorkel branded hot tub is about 130 cu ft of water, you've got something like 180 cu ft.
2) in addition to the water, you've also got the mass of the cinderblocks and concrete.  You've got a LOT of mass to heat up.
3) the air feed and exhaust aren't matched.  I'm not sure how important this is, but the smaller exhaust creates back pressure which affects the burn quality.
4) Snorkel stoves are indeed almost totally immersed in the water - you've got an air radiator above the surface!  So you're losing some heat there.
5) I can't find the link for it, but there is a real issue with inserting the burn chamber into water.  Take a step back to consider how an RMH works... the burn tube mixes oxygen and the gases, and the riser and bell provide a very hot place to ensure all the fuel is converted into heat.  The riser tube is insulated to help accelerate the pumping action, and the bell gets very hot  (and there is lots of heat remaining to go into a mass).  If you've got the body of this in water, it struggles to get hot enough to do its thing.  And really, you don't want it to get that hot in the water - a piece of steel that is hot enough to glow really doesn't get along with water!

So ... we'd like to see a sketch of what you're doing to be sure.  I'm worried that most of the heat is just going out the exhaust.

 We just had a similar thread here (https://permies.com/f/260/rocket-mass-heaters) where some similar issues are discussed - basically I'd like to see an exterior burn chamber and riser, going into a submerged stratification chamber.  And the bigger the chamber, the better.

I think your results were pretty impressive.  40* water, 40* sidewalls and 40* earth immediately around the pool.  YOu have a lot of water and a great deal of mass to heat and 28* increase over 3 days is pretty good results.

I use 1/2" black irrigation tubing as a solar heater for our 15'x4' swimming pool each spring.  Just during the day in 70* temps it raised the water from about 60* to 80* in about 3 days.  Just a wild guess but your results do not sound bad.

Suggestions:
1)  Keep going, it will work but you need to be patient.
2)  Buy a solar pool cover and cut it to fit your pool.  If it were me I would take the cut off scraps and. using spray contact adhesive, glue the scraps on to the piece you cut to fit the pool.  That will give you twice the insulation value at night or on cold days and it will still allow the sun to warm the top of the water during the day.  (worked well for me)
3)Use your pump to circulate the water, even if only for a few hours during the day.  Don't run the pump at night, unless the temps will drop below freezing.  Night time use will actually cool the water.

Update I ordered 1/2 neoprene for the cover directly on the water. I do have 2 solar panels, but had a couple of freeze cracks I had to fix. I'm going to cut off the bottom foot of copper tubing and replace with pex. Hopefully that will be a little extra insurance against frozen pipes. I have plans on a mass fireplace/rocket stove and have plumbing for that. The keg is just another extraheat source.