Hi.
You could look at some plots and things I said on this page: Peter Lynn's Can kites be economically viable (for propelling commercial shipping in mainstream applications)
Not to reply to a few of your concerns:
I believe in hybrid onboard generation / traction for ships. This provides maximum flexibility and allows the ship to stay still on sea (unanchored in wind and moderate currents), powered by wind, or even go directly upwind. I think this “autopark” feature could be quite useful/neat, fixing a yacht to shore is not easy.
An efficient kite can easily be useful even with headwind. Take the apparent wind speed and then find the maximum upwind angle of the give, given by its combined kite and tether lift to drag ratio. I believe one would be looking at rigid kites with medium to short tethers for this purpose. As the scale (wingspan of the kite) is smaller than utility scale electricity production, the mass scaling issues would be less dominant.
The requirement of the kite system would be that the pull must not be acting in any way against the speed of travel. In the end, a hybrid system would continuously optimize electric power generation and direct pull relative to the desired path/movement.
Another limitation is that the kite does not generate a lot of power if it is at the edge of the apparent wind window. I think you are correct to assume that the minmax problem is dominant (the minmax problem as coined by Peter Lynn, related to the maximum pull being much higher than the minimum pull). I think these issues may be somewhat fixed by digital control, but this leaves you vulnerable to the system actually working at all times…
The real problem is that modern expectations require the ship to sail fast, while average winds are not always very fast. So even if the wind is facing directly dross to the path of the ship (expectedly optimal), the apparent wind may face mostly directly as headwind.
The hybrid electric/pull ship may alleviate some of these effects if you could run for a while only on batteries. “A while” could be a long time if running on foils, which may be possible in “yacht” scale.
There are a few things that such a design would benefit from that electricity production have problems with: First, you could use the electric propulsion to generate nice apparent wind for launching the kites. Second you have lots of open space, and probably no other kites to worry about. Third, the wind gradient is pretty sharp at sea, so you don’t need long tethers to get the higher winds.