[jasonwatkinspdx]

This. Anachronism is not the answer. These newly built tall ships will never be anything more than in essence a vanity project for brands that want to highlight their environmentalism.

If you want an efficient modern cargo sail ship, you should start with freestanding self trimming wingsails. These have roughly 3x more lift vs drag at the same sail area. But far more importantly: they can be controlled entirely by deflecting a small control surface, similar to the rudder on an aircraft. This can be automated nearly trivially with commodity hydroelectric actuators, redundant electronics, etc.

Making those out of a sustainable material vs the common composites is an open project. But I strongly suspect even using modern materials, over the lifetime of the ship, such a ship would have better net carbon footprint than modern tall ships, if you're honest about the staggering amount of labor that goes into building and operating tall ships, and the footprint that has.

[hinkley]

We are going to have to think of a different way of accessing the cargo.

Cargo ships as currently built cede the entire middle section of the boat to cranes. Cranes they won't see until the end of their voyage, or the beginning of the next. The whole rest of the time you can't use the vertical axis for anything else.

Things that slide around tend to break loose in heavy weather, and boats don't stay afloat if the center of gravity moves too far, so sideways seems like a bad idea. I don't know what the solution is, exactly.

[jasonwatkinspdx]

Depends on the ship. Container ships are just one type. Bulk cargo and stuff like car transporters remain relevant too.

But in the case of container ships, they have frames every so often to stabilize and secure the load. Those are the natural tie in point for wing sails. This is a relatively straightforward naval engineering problem. CG vs Center of Lift has been well understood since the 1800s.

[nradov]

Fixed sails won't be viable for container and bulk cargo ships due to the need for unobstructed access. They might be able to make limited use of kite type sails for running downwind.

Sails could be more viable for tankers and car carriers. Those don't rely on cranes for loading and have more free space on deck.

[hinkley]

You could rotate it out of the way in port, assuming you can keep other ships from running into them (I've seen one fixed sale design that collapses like an antenna), but there are wear and tear problems with a mast that is not fixed, aren't there? Especially given the direction and magnitude of the forces we're talking about.