From what I’ve seen it’s kites acting as sails so you aren’t bound to using a mast which takes a lot of space and limits your sail area
Some sails are basically kites, symmetrical spinnakers specifically are basically just big kites.
Ed: to be clear they're using these for lift more like a wing than a sail.
I don’t think it’s about limiting the sail area, we got really really good at filling a mast with a shitload of sails. I’d say it’s more likely they’d get in the way of cargo un/loading. Or heaven forbid, take up space for containers.
I think they are actually proper kites flying rather high (at least compared to regular sails) in approximate 300 m AGL.
The big difference is that at this hight there is significantly stronger wind.
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Wanted to see what that would look like and found more info here: https://www.euronews.com/green/2021/12/20/this-huge-kite-is-dragging-cargo-ships-across-the-atlantic-ocean
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.
It's kind of interesting that sail-powered cargo ships remained commercially viable up until the start of WWII (at least for non-time-sensitive cargoes like coal and grain etc.). Eric Newby wrote a book called The Last Grain Race about sailing from Britain to Australia and back on the Moshulu in 1938/1939. Moshulu ended up with an acting role in The Godfather Part II (as the ship that carried young Vito Corleone to the US) and is now of all things a floating restaurant in Philadelphia.
This is like a whole thing with people looking for more environmentally friendly ways to ship stuff - tho rn it looks more like a hobby for rich people hauling their bougie wine and chocolate or whatever. I for one think that having sailing ships come back with modern features would be cool!