I have a dim recollection of exactly this as a VC proposal likely put to YC in the past 24 months or so.

I think the notion was to fit masts to existing container ship and stacks, and I gsve it scant attention as my intuition (I once studied actual civil/mech engineering prior to jumping ship for applied math) suggested masts are better as integral parts of ships rather than bolt on after thoughts.

EDIT: Wingsails to reduce cargo ship fuel consumption (April 2023)

https://news.ycombinator.com/item?id=35426482

  I’m Joseph, and along with Arpan and Bailey we are the founders of OutSail Shipping. We’re building a sail the size of a 747 that rolls up into a shipping container.

https://outsailshipping.com/ https://www.linkedin.com/company/OutSail-Shipping/

  When deployed, it will generate thrust from the wind to reduce the fuel consumption of a cargo ship. An array of these devices will reduce fuel consumption on ships by up to 20%. These sails are easily stowed and removed to cause no interference with cargo operations.

  Here’s a short video showing our prototype:

https://www.youtube.com/watch?v=VUpVqzpym54

Not quite as I remembered .. kite sails, et al. are a good idea, I'm still a bit torn by the physics of a container deployed boom extension sail and the thrust transmission to the ship. Still, I haven't modeled it, so take my thoughts with a grain of salt.

EDIT2: Both links appear dead, so I guess that was a swing and a miss. Still, good to see such ideas pursued.

EDIT3: Also related

350 tons of of chocolate and wine arrive on world’s largest cargo sailboat (April 2024)

https://news.ycombinator.com/item?id=40022801

Largest cargo sailboat completes first Atlantic crossing