UK Sailmakers' Encyclopedia of Sails
9.2 - Applying Spreader Patches
All new Mylar-based genoas either come with spreader patches in place or with spreader patch material to be applied. The patches are sticky-back Dacron material that protect the sail from wear as the sail scrapes across the ends of the spreaders during a tack. Never use a genoa in a race without the patches in place.
To determine the proper location for the patches, put the genoa up in calm conditions and go up the mast in a bosun’s chair to mark where the spreader hits the leech when the sail is sheeted in tightly. The patch should span from eight inches in front of that point all the way back to the leech.
Since the leech of the sail rises up when the sheet is eased, place the patch so that three-quarters of the patch is below the point where the spreader hits the sail when sheeted in. Don’t forget to put patches on both sides of the genoa.
Before you lower the sail, don't forget to mark where the stanchion patches should be. Also mark where the foot touches the shroud bases so that you can put protective patches there too.
If you do poke the spreader through the sail, bring the sail in for repair and ask the oft to patch the tear with two plys of material under the Dacron sticky-back.
To put the patches on a genoa, make sure that the sail is dry and salt free. With a large flat surface to work on, crease the first five inches of leech edge of the patch and peel off the backing paper. Tack that edge in place so that the patch will be parallel to the water and that two or three inches of the patch extend off the leech of the sail. Lay the patch flat against the sail and have another person slowly peel the paper out from under the patch as you smooth it to the sail, avoiding air bubbles or folds in the material.
Hint: The two people doing the job should use their knees to keep the sail under tension so that the sail stays flat in the area you are working.
Next, return to the leech and fold over the first few inches of material that you left loose. After the patch is in place, use a hard plastic squeegee,wooden wallpaper roller or blunt side of a pair of scissors to truly press the patch to the sail.
Repeat the same process on the opposite side of the sail.