Obviously, slapping a sail on a diesel powered ship straight up doesn't work. We would have done it. We need something more.
WE have microprocessors, textiles, steels, servo motors, FEA. We are up against the tyranny of 200 years of incremental efficiency gains, fluid mechanics, and essentially thermodynamics.
Not that excited about the shipping container or even the sail. That is an arbitrary gut feel design constraint that may have gotten you into YC, but we can feel free to toss all of it by the wayside now that we are getting serious, no?
Engineers need a good gut feel for torque, impact, and dynamics, and the big picture. We have to think about more than just static forces and how to make our pet square peg ideas fit into real world corkscrew holes.
We should try to make a servo operated kite that pulls straight upwards and sometimes forwards or a little sideways, (much more than it pulls backwards), that we put on a very slow ship, in very windy conditions.
Why do all of this work? We should just develop some sort of golf ball coating or fairing and coat or cover the tops and sides of some containers and do some wind tunnel studies, patent it and sell it for $$$, and call it a day. It might even be an actual efficiency improvement.