Inspect your sails: how to find UV damage in stitching

sail evalCruisers flock towards the tropics, where all that sun exposure can be tough on sails. Short of alien ships on a bad landing approach, UV damage is the biggest culprit in ending the useful life of a sail. Jamie often checks sails on the boats we’re with, like Papa Djo next to us in the shipyard: in the last few months, a spate of them had no idea their sails suffered from moderate to severe damage.

It’s not difficult for cruisers to inspect their own sails and have a good pulse on the condition, so compromised integrity doesn’t unexpectedly turn a nice day on the water into a mess. Jamie shares his perspective in this first of two posts on evaluating UV damage to sails.

Sailmaker says:

As with Totem’s resident gecko repairing its damaged tail (it fell off), it’s possible to revive sails from minor and even severe damage. With the right cloth, know how, and machine a (oops, didn’t see that squall) shredded sail can fly again with reconstructive surgery. Recutting a blown out sail is also possible. The nip and tuck of a skilled sailmaker can remove some sag and stretch of a well used sail. The bigger question for cruising sailors is: is it worth it?

I’m all for extending the life of cruising sails and anyone that seen Totem’s current headsail can attest to that! Still, as frugal as cruisers tend to be, there comes a time when the underlying structure of a sail begins failing. The culprit is usually damage from the sun. Just as UV rays can cause skin cancer, it also can rob strength from the strongest sailcloth and thickest thread. No fixed exposure time signifies when a sail becomes a rag. Sailcloth, thread, and protective materials degrade at different rates. So every time “the bloody sun, at noon” beats down upon the sails, a clock ticks time away from their lifespan.

Checking the Derm

First, step back and look at the big picture of the sail in question. Just as sailors should check their skin for potential UV related problems (especially old farts, defined by my children as anyone over 40) they should also check sails for UV damage. It easy enough to do and like many cancers, early diagnosis goes a long way toward mitigating the problem. Stitching or sailcloth with some UV damage is repairable. A sail weakened by UV damage and flogged while reefing or tacking can quickly become a shredded mess. Shredded sails are much harder to fix. With inspection you can also get a good sense of when UV damage is bad enough that repairs are a waste of money. Fixing one failed seam may not make sense if pervasive rotted thread means all the other seams are on the edge of failure.



Testing stitching is simple: just scrape a thumbnail across stitching in various areas of the sail. You have to put some force into it! Look for two things:

  1. Do the stitches break? Bad news, the thread is toast.
  2. Do the stitches fray? Rotting: the more fraying you have, the  more damaged it is.

Location matters. Stitching on the protective UV strip along a leech and foot is the first to go. Second is the mainsail leech area, because after dropping the main, the cover doesn’t always go on right away – does it? Test stitching on webbing reinforcement, seams, leech tapes, and batten pockets. You can also use this test for thread on dodger, bimini, leecloth, and mainsail covers, etc.

Stitching is the weakest part of any sail. Most sails and covers are sewn with UV stabilized Polyester (Dacron) thread, of varying thicknesses. In the tropics, two years of UV exposure on “UV stabilized” thread degrades strength by about 50%. That’s not a lab test figure, but drawn much experience. Higher latitudes may not have the same UV intensity (with exceptions like the ozone layer hole over New Zealand), but longer hours of sun in summer don’t help.

There’s a relatively new and supremely awesome thread that is little affected by UV: it’s PTFE (Teflon) thread. Tenara is a brand of PTFE thread made by Gore that sailmakers will know by name. PTFE thread is a little weaker than Polyester when both are new, though not so much as to compromise a sail. After time in the sun, PTFE retains its original strength while Polyester weakens significantly. PTFE thread is also very slippery, so I suspect a little more chafe resistant. Some sailmakers shy away from PTFE thread because is it more expensive and harder work with (read: a royal pain for sewers). Still, it’s a must-use on UV strips, mainsail and exterior canvas or covers. Long term tropical sailors should also consider PTFE thread for seams as well. It should only adds about 3% to the price.

This covers stitching; Part II will address sailcloth. And of course, if that sail is beyond salvaging, Jamie is an active sailmaker and would love to provide a quote for a new sail! And whether you are interested in a new sail from him or not, he’s happy to just answer questions to try and combat the reams of misinformation he sees online about sails.

Coming soon- Part II: evaluating sailcloth. Savvy sailors know we love it when you read this on the Sailfeed website.

11 Responses

  1. Or you can take your new-to-you boat out on it’s first major shakedown and have the UV cover on your jib sacrifice itself and get shredded to clue you in that the stitching is rotted. So we’ve added a new UV strip project to the list and your timing on this blog couldn’t be better because now I know how to check the stitching on the sails themselves. Thanks!

  2. I had intended to use Tenara on our sails but Sailrite told me this: “Tenara is not typically used in sails. The manufacturer until recently has not recommended it for sails. It is about the same as v-92 with regards to chafe but gives no warning prior to breaking. When Tenara breaks in a sail application, it tends to ‘unravel’ more than polyester thread. You can use it but it would not be the norm for sails.”
    Now I’m just confused! 🙂 any thoughts? Thanks!

    1. Alissa – that is rubbish.

      Tenara is more expensive and finicky to sew with, so gets a bad rap from some sailmakers. As Sailrite sells to DIYers that often struggle enough to make friends with their sewing machine, I’m not surprised to hear what they said.

      Still, it’s dumb to promote polyester thread that will fail with UV exposure, when Tenara does not. True, polyester thread shows signs when failing (frowning face start to appear in your sail!), but those signs mean the stitching is bad and needs new sewing. It means the sail is UV DAMAGED -great feature! And to say Tenara thread would not be the norm for sails is simply naïve.

      Tenara is the way to go on UV covers, and on seams for sails expected to last many years/miles in tropics.

  3. After being one of the first sailmakers to adopt Tenara as a standard, we’re still using it with the following caveats.

    If the sail would normally require a stronger thread than V-92, don’t use Tenara.

    Understand that Tenara doesn’t stretch the way regular thread does. Time will tell how this effects smoothness at the panel seams. Some sailmakers have made the argument that if the thread doesn’t stretch, there will be some puckering. Using high quality cloth obviously reduces this possibility.

    If Tenara isn’t feasible, consider going to larger thread than required (larger thread takes longer to deteriorate) and using a darker thread. One advantage of darker thread is you can see flaws quite readily. Most people don’t like the aesthetics of that approach though.

  4. I own Sailrite and have been a sailmaker for decades. I also have a huge amount of experience with thread. I agree with Dave (possibly with the exception of the benefits of darker thread – a different topic). Modern Dacron and Laminate sailcloth is quite stretch resistant. As a result Tenara is perfectly acceptable as a thread to make or repair sails. It will most certainly outlast the alternatives. I would not suggest it for use in Nylon sails where stretch and elasticity of the thread are more important. Use the Tenara, it is great stuff!

    1. Hi Matt, I am 3 months shy of 30 years since I first learned sailmaking. Nice to hear you say that about Tenara; and I know from your website that you sell Tenara thread. Agreed on not using it with Nylon.

      To clarify Dave’s comments about Tenara strength and stretch – Tenara is appropriate for UV covers on any sized sail. As for seams, well, start with quality cloth or why bother. Then using triple step stitching and more rows produces stronger and lower stretch seams than increasing thread size above Tenara (or v92). Going to V138 makes far bigger hole sizes which have a negative affect on seam strength. V138 is appropriate for reinforcement webbing, corner quilting, etc, – robust v138 is still polyester, so rots in the sun. Any exposed areas that use v138, such as clew ring webbing on furling sail, must be covered.

      1. Agreed regarding use for UV covers on sails of any size. It should also be specified for any exterior canvas, especially sail covers and dodgers.

        Interesting idea about increasing seam width to accommodate extra rows of Tenara as opposed to moving up in thread size. I just spec’d out a main for a 64′ fast cruiser and planned 4 rows of triple step. Be interesting to do some actual testing of seams sewn with 4 rows of V-138 or V-192 versus say 5 rows of Tenara. Have you seen any done like that seaming multiple layers of 12 or 13oz cloth?

        We have facilities for testing ideas like this. Now you’ve got me thinking. I certainly wouldn’t want to do it on a client’s boat until we proved the concept. I’d certainly be willing to publish test data. I wish more sailmakers would adopt Tenara but from a practical standpoint, you have to have dedicated machines that only sew Tenara due to the time lost going back and forth between thread types. I understand the resistance from the smaller operations.

        1. Dave and Jamie are spot on. I am not sure what lofts you two represent but given your knowledge I bet you produce some beautiful sails. I did manage to come across a quote from Mark Taylor (at North Sails), “We are currently using Gore Tenara thread for a 12 sail Classic Superyacht. Our sewing machines have not required any additional adjustments when switching from Polyester thread. With the high melting point we have far fewer instances of thread breaking. We can now run the machines at full speed which saves production time.”

          Here is a link to this fine yacht under sail:

          1. Matt,

            Thank you for the kind words. I own and manage Island Planet Sails, a company I started in 2004. I’m really intrigued my Matt’s quote. I wonder what kind of machines they’re using. I’ve never heard of being able to go back and forth between polyester and Tenara without adjusting tension. But since you’re the guy who has written books on the subject, I’m curious about your opinion. I don’t know Mark, but is he a marketing guy or production? Not seeing anything about him in a Google search.

            I’d like to have an offline conversation sometime. If you have a chance, could you drop me an email?

          2. You would still need to make machine adjustments to sew the Tenara (tension and needle change). I think the point is that if the project is large enough that the switchover cost is easily absorbed given the gain of sewing speed (we have all likely experienced Polyester thread melting and breaking at the needle due to heat buildup).

            Mark is on the design team I am told. But like most sailmakers I bet he deals with some production too.

            I am sending you my contact information as requested.

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