Can furling jibs or mainsails be used partly unfurled or must they be fully opened or closed?
In the case of most offshore yachts, yes, they can be "reefed"
(furled and tied down) to reduce the amount of surface area exposed
to the wind. However this does depend on the type of furling gear
used, some (mostly jibs) can only be used fully unfurled (In
general a solid forestay is required for a roller-reefing jib.)