I don't think the aim is to replace motor power completely, but to offload some of the effort for long distances to reduce fuel consumption. As regard stays, t

I don't think the aim is to replace motor power completely, but to offload some of the effort for long distances to reduce fuel consumption. As regard stays, the stay needs to be at 90deg to the sail orientation. Presumably mounted on some kind of circular track to rotate with the sail. The engineering is challenging but not inconceivable.