Obviously I realize there has to be some fuel in case the vessel would be becalmed, but apart from that, what are the limits?
A sailboat. The word you are looking for is a sailboat.
Sailing cargo vessels have a number of issues with economics, which is why we stopped using them 75 years ago. There are some people using them in limited capacities, but for the most part we can't replace our global shipping with them.
Speed is one. Even the fastest sailing cargo ships, which carried very limited cargo averaged 15ish knots on favourable routes. Modern motor cargo can travel at 20+ knots on almost any route.
Even the largest metal hulled sailing ships that lasted until the 1950s carried an order of magnitude less cargo than a small container ship today. And take more crew to handle.
The other issue is that sailing to windward is still not solved by this, and just from looking at the design, I would guess that it actually decreases fuel efficiency on any route that requires you to go into wind.
The other problem with stacking more of these onto a ship is that you have to counterbalance both the weight and the lateral forces. Sailboats do this with a keel. Adding a keel to a large bulker like these is expensive, and will limit the ports you can visit due to draught, which will already be limited by the enormous sail keeping you from transiting under bridges.
Realistically, a sail cargo ship is going to move along at a rate less than 10 knots.
And of course this is all on a route with favorable winds. If the winds are against you, all bets are off.
The same is true of these wingsails, there are limited routes where they will be efficient. Anything going up-wind is actually going to be a loss.
So they can clear bridges.
There appears to be at least one company working on this: https://www.cnn.com/2023/06/30/travel/airseas-giant-kites-sh...
What's not covered in the article is the weight of the devices and the amount of cargo that must be removed from the load to support them. Without that you can't project forward at all.
And even so, you're never going to get to 100%. Cargo ships need electricity to keep cargo refrigerated. They need to be anchored for long periods of time. They need to navigate harbors and other channels.
According to the article it is 12 tons/day on the best days and 3.3 ton/day average. 14% average with 37% max for reduction of greenhouse gases emissions.
AFAIK there are a few companies that have been adding electric drive systems to sailboats that actually do work this way, not only can it be used to maneuver around when the wind is uncooperative but it can generate power when the wind is good, basically acting as a "hybrid" of sorts where hypothetically given enough battery storage and maybe some solar it could be possible to never have to plug in or run a combustion generator.