Are they? How is maintenance? How sensitive are they in changes to wind? How much training does the crew need to operate either? https://en.wikipedia.org/wiki/

Are they? How is maintenance? How sensitive are they in changes to wind? How much training does the crew need to operate either? https://en.wikipedia.org/wiki/Wind-assisted_propulsion https://en.wikipedia.org/wiki/Wind-assisted_propulsion says > In comparison to kite sails, Flettner rotors often offer considerable efficiency gains when compared to the size of a sail or kite vs. size of the rotor and prevailing wind conditions. But I'm not sure if this size comparison makes all that much sense (maybe the performance per dollar invested would be more interesting?) I'm genuinely curious about a comparison from an operator's perspective, and I also hope both approaches are being developed further.