Does steady flow exist in sailing conditions

It depends how you define "efficiency", as used by most people it does not meet the engineering meaning. That is in the most basic sense, energy in/energy out. Or for a wing or sail that generates thrust or lift it would be lift/drag. The total drag is the "Cost" of creating the lift. Drag comes from three components; there is parasitic, form and induced drag. To reduce parasitic drag the rig must be clean with minimal (or no) stays, spreaders, smooth surfaces, etc, to reduce form drag you need to have smooth streamline shapes without bluff bodies or broken surfaces (such as where the mast meets the fabric), to reduce induced drag you need high aspect ratio (or good end plate effects). All conventional sails fail in all three areas compared to even the simplest of aircraft. The only type of aircraft that uses cables and spreaders are very low speed hang gliders and ultralights (and few acrobatic aircraft also use cable stays, they need to be strong, but are not efficient). There is no way a crab claw rig can be better than a foil shaped high aspect wing sail. If you want simple to operate and light (and lower heeling moment), the crab claw is not a bad way to go.

It is easy to measure efficiency of a sail by comparing the maximum true hull speed achieved as compared to the wind speed. The faster the true hull speed in the same wind, the more efficient the sail (this is also true with hull and keel design). Also the higher you can point into the wind (true course) the more efficient the sail design.

 

Kites

The priority on the kites is generating lift at virtually zero airspeed.