Oscar, I agree: the rocket stove is a markedly fascinating concept - with only negligible few downfalls! In that regard, quite similar to the junk rig :)

You are right about those rocket mass heaters! However, there is another way of reducing exhaust temperature and thus increasing fuel efficiency significantly: Use a bell! That's the concept I used, and it works great. This bell even has the added advantage to not only have updraft but also downdraft - which doubles the draft power and let's you get away with no or only very short vertical exhaust pipe height. I tested my rocket stove without any exhaust piping (smoke exiting from the bell at burn chamber level), and it worked like a charm. 

As vertical exhaust height is naturally limited on a boat, that was the point that made me consider this rocket stove concept including a bell.

It's worth a try, and quick to build ;-)

Mark, you asked for the pellet burner. See the fotos attached.

If anyone is still interested in a heater, I stumbled across this one while looking for steering mechanisms: http://www.faymarine.com/plansite/fire_design.htm

The rest is my rambling on one of my ideas for small scale heating.

Having done some experimenting rocket mass heaters it caught my eye. A RMH would have no place on a boat (unless it was much larger than any I can imagine myself in) but some of the ideas are still of interest.

1. Heat the body, not the air.
2. Hot surface temperatures are bad
3. Storing heat is good
4. Burn fast

Radiant heat will feel better than air temperature and transfers heat directly from the heater to the body, but radiation falls off at the inverse square for distance (which means that if you are twice as far from the heater you will get you will get 1/4 the heat not 1/2) so close is important.

Direct heat transfer if even better. Most RMH have a built in heated bench with which to warm one's body directly... may not transfer well to a boat unless one has hot water heating from a running engine, scratch that.

Insulation is like a resister, the warmer the heater is, the better it's heat goes through the walls to heat the outside. So cooler (just shy of being able to damage skin) surface is more effective for the fuel input.

Saving the excess heat in mass is good, except there is not really much room in a boat (particularly a light weight one) for such nice things.

As others have mentioned, burning fast is better for heat extraction and produces less harmful flue gas. So aside from not being as likely to kill you, it uses less fuel. Both of these things are good for a boat... but hotter is not.

Maybe there is somewhere in between. Burning fast has a number of problems besides loosing heat out the flue and through the cabin walls. It requires better shielding, it is easy to get burned and it uses the fuel faster. However, I am thinking of two things (maybe three or four by the time I am done) that could help.

1. Phase change heat storage
2. A Bell heat exchanger

Space is limited, but I have noticed that whatever the size of the heater, there is not anything mounted above it. It is nice to be able to also cook on it, this is true. And while cooking could be achieved at chest height, most would rather not have something hot way up in the air where an errant motor boat wake might fling it on top of someone. However, placing a second chamber on top would allow capturing a good deal of the "waste" flue heat. This is called a bell (in the masonry heater world).

So this is great being able to get more heat before the flue gas and the heat in it leaves, but this would leave to much heat at once. Phase change is the answer I think. Put a second box around the small stove and fill it with tin which melts at about 230C and absorbs much more heat to melt than it does in just heating (lead would be possible too and might hold more heat for volume). This box would have to be welded shut (as in sealed) so that molten metal didn't get all over things. Now as even 230C is very hot the next thing to do would be to put insulation around the outside of this box. (big grin as I wait for sarcastic remarks about keeping the heat in the heater) The insulation is probably not very thick (welders blanket?) but allows the surface temperature to remain relatively cool... maybe we can get it down around 100C or less.

The idea is that the fire burns for a short while till the tin has melted and then let die. The Stove continues to shed heat for hours as the Tin "freezes". The insulation keeps the cabin from over heating with a fast burning fire and also lets the heat keep radiating longer.

A further idea is that the tin boxes could be detachable and placed under bedding or seating... or for that matter placed under a pot to boil water.

I realize that because a boat has a small inside space, most people are not worried about saving fuel so long as it works and keeps them warm and dry. So maybe the world is not ready for such ideas... gotta make one.

I can also recommend the Dickinson Bristol. We had one on our boat Miss Molly I and despite having a week of snow piled up on the coachroof one winter in Vancouver BC, it kept things toasty belowdecks. It got a bit warm for cooking on when we got down to the Sea of Cortez though :-(

Jonathan Snodgrass wrote:

Paul Thompson wrote:

As I said I start the heater when I come on board and it stays on until I leave... however long that may be. To date 3 months has been it's longest continues run. 

Paul, is this only whilst moored or do you or would you use it whilst sailing?

I don't think I'd use it under sail offshore but I have use it under sail in sedentary conditions (and short hops) when coastal sailing and I've had no trouble. Offshore or when it gets bouncy, I think I'd rather not run it.