Jane Hermanson

Honestly, that didn't sound like the seam would be watertight. I'm familiar with the liquid gaskets used in car engines. I haven't taken apart a lot of engines, but from those I've been working on, I've noticed that any place the liquid gasket is used, there is no standing water or oil. In this dual drum seam the seam will be under at least 80cm (30in) of water - while the water expands.
In a location where water leak will cause problems, I wouldn't trust this solution. It could be worth trying to weld the seam. Or the safer way would be to wrap a pipe a number of turns around the barrel. It will take a lot of piping though, and be way less efficient heat transfer than the dual barrel solution.

Great idea with barrel inside barrel, @Glenn. How did you connect them? Did you cut a huge hole in the bottom of the outer barrel to put the inner barrel inside and weld them together around the hole? I've never tried welding anything watertight before. Will welding hold up over time with the heat? How hot does the outer barrel get during fire? Is there a risk the water will boil?

All the fire bricks I've seen are heavier than the regular bricks. Where do you find the insulating fire bricks?

The plan is to move the caravan every 3-6 months. If the road is rough, the speed will be low.
Would it be enough the secure the outside of the heater with steel? What about the riser? Any way to secure it from falling over?

Thanks for the links. I've read them before. I feel like I've read all rmh build threads here and my questions are a result of that. I haven't seen any rmh that aims for low weight and water heat storage. (Those aiming for low weight use rocket stoves.)

The caravan is about 10m (33ft) long, with an side tent, insulated. If it gets too hot inside, we could leave the door open to let the tent be heated as well. I know the rmh is powerful, but this is also a cold climate. We typically reach -25C (-13F) for a couple of weeks. I'm thinking, if there is enough mass to heat, it will help to average out the temperature. Maybe it's ok to fire up less than once a day. If the mass radiate its heat too fast, there is always the option to slow it down by insulating around it.

I dry stacked some bricks (not firebricks) like shown in a drawing for a 6" system in one of the threads here. I did this just to get a feel for the size. It felt really heavy. I think firebricks are even heavier. To drive this caravan legally on roads, there can't be much weight added. That's why I'm looking for ways to make this as light as possible.

Would it be possible to build a low weight rocket mass heater? The obvious answer would be no, as even the name includes the word MASS. But my think is, could the mass be avoided during transportation? If the mass was tanks of water, they could be emptied before transport, and refilled from any water source when parked.

Whats the preferred way to transport the heat from the rocket to the water mass? Long air ducts through water tanks has two issues - it has to be watertight, and the ducts takes up quite a bit of volume that should have been water. Could this be solved by winding a coil of piping around the steel drum and let a small pump flow water from the tanks, through that coil and back to the tanks?

I've read some posts here where people have used superwool for casting riser. That sound like a quite low weight construction. Could the batchbox be built in similar way?

Hi folks
I'm considering buying a boat for liveaboard in nordic climate and are facing the same challenges as Mike. I'm looking at boats size 35-40', hence a bit larger than Mikes, and a bit more area to heat. I've never had issues finding wood to make a campfire on islands when I've been boatcamping with the 14' boat, without using driftwood, so I guess that salty firewood will not be an issue.
When it comes to mass, I'm planning to have an inboard engine and hot and cold freshwater tanks. I'm thinking that if there could be a way to circulate the engine coolant through the rocket (mass) heater, the engine mass would heat up and work well as a mass. After all the engine is mounted near the center of boat and far down in all the boats I've been looking at, so it does have the perfect placement for radiating heat. I'm thinking of placing insulation around the engine compartment for slowing down the heat radiation. Not sure how much thermal energy an engine would store but assumed it has 200kg of various materials, I guess it would hold quite a bit.
Water tanks... It seems to be normal for boats of this size to have 4-600liter fresh water, and 40-60 liter of hot water. In the heating season, these tanks could swap, so that there is 4-600liter of hot water and 40-60 liter of cold water. Maybe add another 100liter of cold water. I guess in most cases it's possible to put insulation around the water tank as well.
With this setup, the issue of making a light weight rmh in a boat is no longer the missing mass, but rather to find a way to circulate some liquid through pipework through the heater and trough the water tank and the engine. This should be able to run with a solar powered pump, or maybe a TEG mounted on the rocket. If the pump runs too slow, the heater will heat the living space to 90C before the water tank reaches 30C.

How could a rocket be built so that most of the heat is transferred to the tank/engine?
Could this all be combined in such way that the rocket also could be used for cooking food?

Also, grey water  collection, particularly from the shower, would contribute to preserving heat.