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The wind from the fan on the boat was just bouncing off the sail. It would be more efficient to just take down the sail, point the fan aft and turn it on.

For propellent-less drives, I would be looking at light-sails or mag-sails, rather than trying to collect and unburn the trillions of water molecules your engine is scattering across the vacuum of space.

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The thought was to re-collect the propellant for reuse in an environment where you cannot easily replenish it, once you have harnessed the energy from burning the components of that propellant.

I don't think that the boat experiment is a good analogue, though, as it seems the engine would be pushing with more force in the backward direction than could be harnessed in the forward direction.

It still seems like there could be a possible way to re-collect the water for further use as propellant in a vacuum once you have harnessed the energy from it's combustion. Essentially using solar to propel your craft and then reclaiming the raw propellant (water) so that it can be separated again.

But what happens when the boat speed matches the propeller airspeed? Wouldn’t sucking the air from the ground and deflecting to a variable angle allow for higher top speed until you are power-limited (as opposed to gearing limited)?

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