rocket stove and hot water/weight

Hello i'm new here but have found myself reading permies/mother earth news often
i have a few basic questions on rocket stoves
i live in a 20x30 cabin its currently heated with a small wood stove 6 inch pipe
the cabin is built on a slope part of the cabin is near the ground and other part is on cinder block stacks about 5 feet high joist are 2x12's 2 foot between joist and rather thick osb sub floor
what im interested in doing is heating water and home with a rocket stove my main question is would a rocket stove be ran long enough to really heat water and would it be two heavy for a wood built floor?
should i just do a coil around stove pipe and do a thermo syphon setup with current wood stove?
i have seen where a rocket stove acts as a heat battery and dont need to run all day im in eastern kentucky so i'm thinking a rocket stove wouldn’t run long enough to get water very hot
the current wood stove nearly runs you out i have very little trouble heating the place up it can reach 90+ F inside with the stove going

Eastern Kentucky sounds like you would have relatively mild winters, with probably changeable weather. This would indicate a smaller mass system that would probably be safe to put on your floor joists, especially if it can be arranged to run over a number of joists rather than be concentrated on one or two. You would need to space the mass up off of the subfloor and allow ventilation between floor and mass, to avoid overheating the wood.

The fact that you might not run the fire for long doesn't mean that it would not heat water well; if the mass is warmed up to a couple hundred degrees internally and has a copper coil running through it, it will give even and safe heat for a long time, with little risk of the coil overheating and bursting from steam (as a coil around a wood stove pipe would risk.)

A cob bench or even bed would be possible
Would you happen to have or know of media or reading material on such a setup?
im very curious on the heat/hot water exchange the J tube would get way to hot for a coil there maybe coil inside the drum or would it cut down on airflow/draft?

I've been heating water with my RMH for several years now

about 120 feet of 1/2" copper around the 55 gallon metal drum

A thermostat and circulating pump is set up with it now since my water tank is about 10 feet away from the stove- i tried a thermo siphon, but the water tank and heater coil would need to be within a couple feet of each other for a really effective transfer. I have a thermosiphon set up on an old coal fired water heater, and that works great, but there's only 3 feet between stove and tank and the connecting pipes are larger diameter. The circulating pump that sends water back and forth between tank and coil  also pumps water through the radiant floor, and hopefully will also work once i get solar hot water in place--The plumbing is quite creative, but the pump is small and cheap and reliable

With my setup there is considerable cooling of the exhaust gas which boosts the push of exhaust out of the stove. It also reduces available heat for the mass/bench.  In my setup it doesn't matter because my main heat is radiant floor, so in effect I have turned my concrete floor into the mass of the rmh.

My bench is about 8 feet long, with an 8" exhaust that doubles back on itself before going up about 8 feet and then sideways outside the building. It takes a long time to heat up even though it is relatively small I would caution that 6 inch stoves require lots of attention with frequent loading, heating water requires incrementally more fuel (still not much compared to a wood stove) and an 8 inch exhaust with correspondingly larger firebox is much easier to operate

The stove i have i modified because the air controls where crap it was an air tight with a glass of course the glass broke replaced  with metal plate and gasket I removed the oem baffle it had tiny holes for cleaning the glass replaced with a different baffle
built a thermostatic coil (opens as temperature drops closes when to hot) aka auto damper before the mods it would be to hot or to cold now it works great
I know with rocket stoves you can get away from the firewood splitting,use less fuel and much more
the gears in my head are trying to figure out how to implement it into this small wood frame cabin im wondering if i could put the J tube under the house and the business ends come though the floor keeping the burn chamber away from combustibles as im thinking heat rises so i would get the heat + exhaust/thermal mass inside  
but I dunno how long the J tube can be maybe dumb idea?
I know with the current stove it can run you out it can get very hot inside I dont want to overkill with too big of a stove its  only a 20-30 cabin

The first response given to someone new to RMH and wanting to heat water with one is "BOOM-SQUISH", that being the sound of the water supply exploding due to a sudden pressure change, and your body absorbing the brunt of the force. Like, "wasn't there a cabin out here last week?" kind of force.

There's a 4 DVD set sold on this site that includes one specifically on water heating, and you might be able to buy that one solo too. Check the digital market button on the side for that info.

You need the J tube to be accessible to feed it wood and clean it so keeping it in the main space would be wise, along with the barrel. It also needs to be insulated to improve burn performance, and the barrel/bell needs to give off sufficient heat for draft. If you can place the RMH on the side which is closer to the soil, you could install additional supports under the mass to brace it to avoid any chance of the floor sagging. A 20x30 space is plenty of room for one.

What is the plan for the heated water? Taking a shower, cooking, etc? Keeping a tank full of water hot enough 24/7 for on-demand hot water, and too hot for legionella to grow, might be a hassle. Pressurized hot water heated with a variable heat input like wood fire, can be dangerous. But if you just need hot water now and then a bypass to prevent heat storage in the bench when it's warm, and heating a pot on the barrel could work, or having a rocket stove or pocket rocket located outside the cabin could work too. Heat water indoors in the winter, and outdoors in the summer. I've seen several folks who built a summer kitchen outside to keep heat out of the house.