Would my idea of a fire powered boat work?

Question

Recently my girlfriend's dad asked me if it was possible to make a fire powered boat. What he suggested was that a fire be made on a (thick) plank of balsa wood. The fire would be then incased around a chimney (with an air opening obviously).
The chimney's hole would then not be directed upwards but rather sidewards.

I am aware that it is fire on wood, but if the surface is properly treated there would be little issue of burning damage. I am more concerned about whether is is possible to create thrust using just the sidewards re-directed updraft from a fire and not using any steam engine etc whatsoever.

Does anyone also have any other simple ideas on how to make a fire powered boat? ie: not a steam engine, rocket motor, combustion engine, etc.

Thanks
H

Answer 1

the thrust from the convection of open-fire heated air would be tiny indeed

with a really huge fire and the tiniest possible boat you might be able to get detectable motion in the boat

remember that the thrust in this case is approximately equal to the mass of the air time the velocity of the air

air is very light, and with an open fire, the exhaust will not have much velocity

the reason "fire" powered vehicles use steam engines, or internal combustion engines is becuase it takes that type of cycle to make an effective conversion of heat into thrust

they had fire and wagons and boats for hundreds of years but nobody built a useful powered boat or wagon until the invention of the steam engine

Answer 2

I don't think you can sustain enough draft to get usable thrust out of such a scheme. Ramjet engines work somewhat as you describe, but they need to be moving at supersonic speeds in order to develop thrust. Without a shock wave to compress the incoming air, the flame will go out rather than force its way out the exit nozzle.

For subsonic speeds, the addition of a turbine to maintain airflow makes the idea work, and you have a turbojet.

The key element in both these engines is pressure. Converting thermal energy directly to mechanical necessitates developing sufficient pressure to exert a usable amount of force. Fire alone doesn't involve much pressure, and if you try to force it to develop pressure, it just goes out.

The other known way to develop pressure with fire is to heat a fluid in an enclosed chamber, for example, the steam engine.

Answer 3

A jet engine squirts small volumes of air and fuel into a combustion chamber, where the heat of combustion causes it to expand, and the large volume of exhaust pushes it forward. Here in Invercargill NZ recently a similar idea to yours was used to propel a motorbike with no moving parts. The motor consisted of two combustion chambers, one on each side. A bottle of propane gas fuelled it with a series of explosions in each chamber alternately. The noise was deafening, the chambers were bright red hot, one propane bottle propelled it for about 7 minutes at 20 km/hr.

Answer 4

The amount of gravity draft created by a fire would not be enough to generate propulsion for even a small boat and adding the weight of the combustion chamber would make the method even less feasible.

Answer 5

There are fire powered boats. The apparatus is called an internal combustion engine which is a fire powered engine.

Answer 6

You would never get enough power to move the boat... that is why there is no such thing as a fire-powered anything...