Halyard wrap usually occurs when the sail is a little on the short side and doesn't run all the way up the length of the foil. The problem is in the geometry of the foil and halyard relationship. In other words, the halyard is running parallel to the foil and wants to rotate with the foil. The result, unfortunately, is the halyard will bind tight on the foil preventing it from rotating.
There are two solutions most commonly used prevent the problem of furler
Restrainer / Diverter:
The halyard restrainer (diverter) simply mounts to the mast
and pulls the halyard away from the foils so that it doesn't wrap around the foils. The diverter is installed on boats where the sheave is too close to the headstay.
The diverter is mounted on the mast
below the genoa
halyard exit, thus increasing the angle of the halyard.
If you frequently change furling headsails (say between a 100 and 150), and the sheave is low enough, the solution is to add a pennant to the top of each sail. This raises the halyard swivel up to the sheave box, so the halyard isn't quite parallel to the foils, thus keeping the halyard from wrapping.
It’s important to make sure the pennant is at the top of the sail and not on the bottom. If the pendant is mounted at the bottom, there is a danger
of blowing out the luff of the sail.
This is what 1x19 wire looks like at the upper headstay terminal fitting after it has been twisted back and forth a few times from "halyard wrap". Even slight damage from minor episodes warrants replacing the wire.
Goto Seaworthy Magazine: Inspecting Your Boat's Mast and Rigging Surveying Your Rig:
At: BoatUS.com Seaworthy magazine