[imoverclocked]

> it needs to take on another 100 tons of new cargo (or ballast) ... in order to maintain the correct buoyancy.

Sounds perfect for a cargo situation. Add new cargo as old cargo is removed.

[knowaveragejoe]

As I understand it, it's common for a cargo ship to unload full containers and take on empties for the return voyage. Are cargo vessels holds always partially full vs. empty containers, bouncing between ports?

Would this issue even be a problem for fixed routes? If you're always taking full containers between two points, that is.