[Xixi]

I don't see how you could make a "sail" boat powered by reclaimed fuel from air, as there is no fuel in the air. You will need a source of energy to convert the CO2 to fuel. In nature the most common (to my knowledge) CO2 -> fuel process is called photosynthesis, and is powered by the sun.

CO2 -> fuel consumes energy (that is stored in the fuel). Fuel -> CO2 releases it.

[yazaddaruvala]

Sorry, in the case of "sail" boats, I worded it poorly before. Wind turbine-like "sails" could act as a propulsion source when the wind direction is correct, and generate energy and funnel the air into the CO2 -> fuel machine when the direction of wind is non-optimal for propulsion.

I agree in all of the cases I mentioned there would always need to be some energy source.