> Why stop there? Add more than 15 sails and you could be generating fuel!

AFAIK there are a few companies that have been adding electric drive systems to sailboats that actually do work this way, not only can it be used to maneuver around when the wind is uncooperative but it can generate power when the wind is good, basically acting as a "hybrid" of sorts where hypothetically given enough battery storage and maybe some solar it could be possible to never have to plug in or run a combustion generator.

Why not put a windmill on the ship and generate electricity, turn the screw, and drive the ship? Not as efficient?

If it works what prevented ancient people from building windmill ships?

likely lots of moving parts with a less-than-ideal place to be conducting regular repairs

My guess is it's a "equal and opposite reaction" sort of thing.