There is prior art for most useful and static trade routes.

However, you could probably patent a system that uses real time satellite imagery to discover the ___location and magnitude of ocean currents, predict what they will be for the next week, and then calculate the the ideal routes for a fleet of cargo ships in real time. But, the patent is only going to last 20 years and shipping companies would probably just wait you out. Then again, I expect that patent probably exists and expired a long time ago.

Yeah, but assume I've patented the system for calculating etc, etc the route, and the patent had not expired yet.

If somebody else used the same equations I've used, but made the calculation on his abakus, and steered his ship by himself, he wouldn't infringe my patent, I think.