North Sails NEWS

January 27, 2020


UV Is Not Your Friend


We all know that ultraviolet (UV) light breaks down traditional fabrics, which is why sunshine is bad for your sails. (FYI 3Di is composite technology, not a sail laminate. For info on 3Di sail care, read Sail Maintenance.) Even the lower sun angles of winter can wreak havoc on our sails. Here’s what you need to know about UV damage on traditional fabrics. Fortunately, there’s a way to protect your sails and extend their life expectancy: be diligent about covering them up when they are not in use. Once fabric is rotted from UV exposure, the only remedy is replacement. Read on for other factors that make a difference.

Fiber type

Some fibers have better UV resistance than others. For two fibers of the same type, a smaller diameter fiber will degrade more rapidly than a larger diameter fiber. Woven polyester fabrics are made with warp yarns that are almost always smaller than the fill yarn. So when dacron has been rotted by the sun, it will rip more easily across the warp (parallel to the fill).

The simple test of your sail’s fiber integrity is to lightly scrape the surface of the fabric with a dull metal object, like the edge of a spoon or the thick side of a knife blade. If the fibers are still in good shape, the fabric will become shiny and smoother where you rubbed it. If the fibers are rotted, the filaments on the surface will fuzz up or sluff off of the sail. If the UV damage is very advanced, the fiber might rub away completely—the sign of a sail that is going to fail.

Most UV exposure occurs when the boat is sitting with the sails down. Accordingly, some areas of the sail will get more exposure than others. The mainsail leech lying on top will rot long before the parts of the sail hidden beneath it. The same is true for a roller-furled headsail. The fabric on the inside of the roll will last much longer than the leech, which forms the outside of the roll.

Sometimes the degraded area of a sail can be economically replaced. But more often, when the sail fabric is degraded enough to easily rub the fibers away, or to tear a Dacron fabric along the fill yarns, it is time to replace the sail.


You may be surprised to learn that your sail will degrade right through its sun cover. As the cover ages, it becomes less effective at blocking the rays of the sun. Heavier fabric provides a more effective barrier than lighter fabric, and dark colors provide better protection than light colors. If you spend time in the tropics, consider a multiple layer cover. It will be much more bulky, but your mainsail will last longer.

Last but not least, the deck of your boat is an excellent UV barrier. Keep your sails below decks whenever possible.

sail suncover
A suncover can help protect your sail from UV rays, but step one is making sure your sail is furled correctly so the suncover is on the outside. This image shows how the sun has damaged the sail material because the sail was furled incorrectly.


Sewing thread will rot in the sun much sooner than a fiber of the same size woven into sailcloth, because it sits on top of the fabric and is more exposed. If resistance to UV rot were the only criteria for selecting the thread weight, bigger would always be better. But selecting thread too heavy for the application will result in a hopelessly puckered up seam and eventually the fabric will tear along the dotted line. A needle that is too small for the fabric weight can bend and deflect around large fibers, adversely affecting the timing tolerances of the machine. With these considerations in mind, the sailmaker typically chooses the lightest thread and smallest needle that the fabric will allow.

Thumb Test

Anyone with a thumb can test for rotten thread. If you can scrape the thread away with your thumbnail, it’s time to restitch your sail. Check each seam in several locations, especially along the leech tabling or on the sail cover of a roller furling genoa. Some areas will rot sooner because of the way the sail is rolled or flaked.

If you can scrape away any of the thread, circle the area with a pencil and then start looking for more rotten thread. Restitching is an easy, time-consuming job, but less time-consuming if you catch the rotted thread before the seams come apart.

winter sun damage sail repair
Sun degrades stitching. It’s important to get scheduled sail service to address these types of issues to avoid disaster on the water.

Straps and webbing

When you inspect your sails, take a close look at the outside of any web straps. If you see any broken fibers, start planning to get the sail into a loft to have new straps sewn over the old ones. It is not a good idea to attempt to sew through an old
corner strap with an onboard sewing machine, or any other light duty machine. The corners of the sail get harder as the sail is used, and you will be exploding lots of needles.

Excerpted from The Complete Guide to Sail Care and Repair, by Dan Neri

sail strap webbing
Webbing damage due to long-term sun exposure.
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