Consider however that a modern sailship needs a thermal engine for practical and safety reasons: you want to be able to avoid storms, calm periods or inconvenie

Consider however that a modern sailship needs a thermal engine for practical and safety reasons: you want to be able to avoid storms, calm periods or inconvenient winds that can make you drift hundreds of miles from your original destination, all of these big problems during the Age of Sail. This is in fact why we abandoned sailing ships for merchant fleets: they run on the Sun's free nuclear fusion energy, but they are very inconvenient! So a modern sail ship will need to use its engine, and therefore I'm not sure they would be cleaner than a nuclear fission ship.