Instantaneous efficiency is well and good, but a sail only redirects power; it doesn’t capture it. To me, the goal of adding wind power to a cargo ship would be the same as adding solar power: to recharge the batteries asynchronously to demand, to then later use that power when there is demand but no active supply. Wind→electric gives you the ability to “save up” power in a tailwind, and then later “spend” that power against a headwind.

(You’d think pure wind power would do this as well—the engines can in theory work less hard if the sails are full—but wind is too precarious to match the slow ramp-up/ramp-down times of the giant motors used to power boat propellers. Those things are what power plant designers would call “base load” — mostly you don’t even turn them off, you just engage a giant clutch to put the boat into neutral when you don’t want to be moving. The master “transmission” of a boat is essentially a cylindrical steel flywheel; and you don’t want to lose its momentum. This is why boats a good match for bunker-fuel furnaces — or, on subs, nuclear — which are power sources that also ramp up/down slowly.)