One answer is to wait for conditions to change. Another is to pole -- we intend to set the boat up with outboard planks for maximum poling efficiency. Our research suggests that a flatboat of our tonnage can be poled at a slow speed (2 mph) in a relatively calm adverse current so long as bottom depth is fairly consistent. Ceres will be equipped with an outboard but we intend to keep usage of it to a minimum.
We estimate that including time under sail, time poling, and time waiting out adverse conditions we can make the trip in about 10 days. Our overall model is viable with a voyage of about this duration.