Add a retractable/foldout solar array for low seas and low wind, add solar capture to the sail fabric, add windmills for side/tailwind conditions too.

Would probably need to be part of a hybrid powerplant though, but if you could get even 10-20% efficiency it might pay for itself in shipping where the boats are in service for a long time usually.

Long range shipping, like air travel, is a major hole in low-carbon strategies right now. Biodiesel or fuel-from-excess-solar/wind might be the only other option.

Or thorium powerplants for ships.