View Full Version : Sewing thread for lightweight cotton sails

05-03-2013, 06:31 PM
What type of sewing thread is best for a lightweight sail in cotton?

I am building a small wooden dinghy (10ft Atkins Vintage) and want to have the look and feel as traditional as possible. I had some nylon upholstery thread I was going to use on the sails but have become spooked by the idea that it may be weakened by UV light.

The plans are for a 53 sq. ft. spritsail and I have some really nice downproof cotton ticking fabric (the plans had called for 4 oz. balloon fabric). I am mostly following the advice given here by the excellent Todd Bradshaw (I was delighted to find Todd had written a book - Canoe Rig - which I bought and find to be most excellent, and I am also using The Sailmaker's Apprentice by Emiliano Marino).

Also what size thread is needed? And do I need different size thread for seams and patches?

Thank you,
Richard Phillips

05-03-2013, 06:50 PM
Todd Bradshaw is the chap to answer this question, but ignoramus that I am, I have used one of the stronger polyester threads for this purpose. I figure that if it nearly cuts my hands if I try to break it by pulling, then it is probably stronger than the sail material. Let's see what the more experienced folks say...

Todd Bradshaw
05-03-2013, 08:57 PM
Polyester thread is the best bet - good strength, best UV life and it runs through the machine pretty smoothly. As you noted, nylon has poor UV life and also stretches too much. We measure sail thread by industrial standards in the business and either V-46 or V-69 would work with a size 16-18 needle. These are not particularly heavy, and for a sail that small from material that light, regular fabric store thread and a sturdy needle that fits your machine should be fine. The strength is much more about the seam type and construction than it is about the actual thread used. Seams need to be lap-feld (French-feld) and should be double stitched and about 5/8" wide (even triple stitched if they are wider joining large sections). You want a straight stitch with 8-10 stitches per inch. All raw edges need to be tucked under into the seams (the lap-feld seam does this) and any hems along edges like the leech should be rolled (folded over twice to hide the cut edge). Cloth stretch is a major problem to deal with on light cotton sails and wide panel sections are usually not very stable. They can be improved and much more stable and stretch resistant if you divide the area up into narrower panels. The same needles and thread should work fine for corner reinforcement patches.

David G
05-03-2013, 09:20 PM

I'm not a canvas guy. I've read enough about sailmaking, and talked to enough sailmakers - and watched them work - to know that it's a feck of a lot of work... even for a small sail.

I'm wondering about investing that sort of effort into a sail of the material the OP describes. Reasonable? Fun little first project... with material that is easier to manage than dacron? Mildly bad idea? Colossal waste of time? Or?????

Todd Bradshaw
05-04-2013, 12:00 AM
Can be good, can be bad. The first thing that is great about making a small cotton sail is that you are free from the danger of putting ugly creases in the fabric at every stage during the process with cotton. If you need to turn a corner while sewing, for example, you just do it and let the cotton hang off the table or temporarily get wadded up near the area you are sewing. With Dacron, you have to roll the fabric in a scroll, run it through in one direction being careful to support all the cloth, pull it all out, re-roll it parallel to your next seam, sew that and repeat as needed. A little job like sewing around a diamond-shaped reef patch will take multiple re-rollings, just to avoid putting permanent creases in the cloth.

On the other side of the coin, having to use more complex seams just to tuck all the raw edges with cotton is a lot more work. For Dacron, we can just make simple fabric overlaps, pre-baste them with seam tape so that nothing moves and run them through the machine for the stitching. The tucking, folding, pinning, pre-sewing work on a cotton seam takes much longer and is a lot more tedious. Since Dacron is much more dimensionally stable, it's also much easier to get the shape you are after in Dacron and maintain it in use. There is no doubt that the Dacron sail will outperform the cotton sail due to the Dacron's superior stability. This stability does mean, however, that the stiffer Dacron sail will need the shape to be designed and cut into the panels - and to do that well, you need to know how to do so - at least the basics of edge rounding and broadseaming to generate non-flat shapes from flat fabric. A lot of small old cotton sails have little or no shape built into them and would basically rely on the stretch of the cloth itself to create at least most of their 3-D shape. Like most of the other projects we do though, the outcome in either material (or even in others, like polytarp or Tyvek) is going to depend most on how much you know about sails and sailmaking, and the availability and quality of the materials used. If the finished sail is going to set well and work nicely, it still needs to be designed and constructed properly, no matter what it is made of and to do this, the builder will need to do a bit of study work and learn some of the basic principles of sailmaking before diving in.

Is it fun? I'd say yes - in a rather tedious fashion. We all enjoy making our own stuff and the satisfaction of watching it perform out on the water. When done well though, there is a lot of picky little detail work that needs attention, and most of it shows, so it's probably not something that some folks would enjoy. If someone is seriously interested in learning sailmaking, I still think the best first step is to put a Sairite kit together. The shape is already programmed into the kit, so you can concentrate on learning the assembly part of the process first, generate a good sail and if you enjoy it, then you can start studying the design and shaping aspects.

David G
05-04-2013, 09:30 AM
Todd - appreciate the insight. Hope it helps the OP as well.

05-04-2013, 10:19 AM
I was inspired, so I just bought Todd's book. Can't wait for it to arrive!

David G
05-04-2013, 10:45 AM
I was inspired, so I just bought Todd's book. Can't wait for it to arrive!

Definitely a book worth having. Even if you don't plan on building sails, but only want to understand how they work, and how to use them.

05-04-2013, 11:12 AM
Thank you Todd. That's just what I needed to know. Glad you got maybe a "sale" or two out of this thread too! It really is a very good book (imho) and is actually inspiring me to think about making an Edwardian sailing canoe for my next project!

Thank you for spending so much time on this forum sharing of your experience in sailmaking.

Now if only Bob Smalser would write a short monograph on wood for boats! He's my man too.

Great forum, great advice all.

Richard Phillips