[someweirdperson]

Is the sail-container sufficiently heavy to stay in place, or does it need pull on something that's holding it down?

If the sail is pulling, the container below would see a remaining uneven load from above. Is there a limit to this? Loading/unloading an unbalaned container sounds like a problem, but is it, while sitting on the ship?

[theflyingpigeon]

To me the problem is transmitting the force generated by the sails to the body of the vessel

A small to medium vessel fully loaded is like 150.000 tons. 10% of that is 15000 tons, imagine the force that needs to be transferred to the structure of the ship in a place where it was not designed to handle that force.

[someweirdperson]

Mass of the ship would matter if it was supposed to hover. Trying to google for engine thrust I found numbers that might be in the order of magnitude of 500kN (hp translated to pounds at certain speed). 10% of that through the kite 500kN. That would be enough to lift 50t, which is at the order of magnitude of the maximum mass of a shipping container. Looks like it might work.

[theflyingpigeon]

The mass is actually 50.000 tons, I typed wrong. My point is not the force to move the mass of the ship, but transmitting that force to the structure of the vessel in order to move it forward. It’s a lot of force, the structure is not made for that