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.)
(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.)