[jameshart]

Without seeing your back-of-napkin numbers it’s hard to know where you might be going with this, but the idea that sail efficiency has something to do with the amount of energy in the wind in a particular cross section seems in need of justification. You’re probably better off considering air/sail interactions as a momentum transfer - the goal of the sail is not to extract all of the energy from a packet of air (presumably leaving blocks of frozen nitrogen in the sail’s wake), but rather to extract as much forward momentum as possible out of the interaction, leaving the air with more rearward momentum than it started with.