There are two reasons for leaving a scrap of genoa unfurled when you’re not sailing. One is the mistaken belief that by tightening up both genoa sheets and applying tension to the clew you’re somehow stabilizing the forestay and stopping it from vibrating in the wind, and thereby being kind to your rig. This is not so: a properly set-up rig doesn’t need to be “stabilized.”
The other—and much more likely—reason is that you’ve been sailing in strong winds and the furling line has stretched enough that it won’t take the last couple of turns of the drum needed to completely roll away the sail.
In this case, back on the mooring or in your slip, untie the sheets, and then spin the furling extrusion to take up some more of the furling line. You want enough line left over to take three or four turns around the sail once it is furled. What you do not want is a triangle of sailcloth that can catch the wind and flog, which often leads to the shameful scenario of a completely unrolled genoa flapping away on an unattended boat.
To be completely sure the sail can’t unroll, cleat the furling line off as securely as possible, lash a sail tie around the rolled-up sail, or take a line from the tack shackle above the furling drum to the pulpit.
Photo by Peter Nielsen