It’s actually proper kites, at least the pioneering tech is kites. They’re computer-controlled, deploying and retracting on the push of a button and navigating themselves into and out of winds to complement the main drive and controls. It’s been on the market since the early 2000s and has always made economical sense, but:

There’s a structural problem slowing down the process: ship owners (who have to make the investment) often don’t pay for the fuel – that’s the charterer’s duty. The charterer on the other side doesn’t charter the ship for long enough a period to make low-carbon technologies pay back.