[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.