Thread is the twisted or bonded synthetic yarn a sailmaker uses to sew panels, tablings, patches and hardware into a finished sail. On a modern sail it is almost always polyester, sometimes with a PTFE or high-tenacity alternative for the most demanding jobs, chosen for its strength, low stretch and resistance to sunlight and salt. A beginner sees thread as the zigzag lines across the seams. A sailmaker knows that thread is usually the first part of a sail to fail, that the choice of thread weight and stitch type is a balance between strength and cloth damage, and that ninety percent of the sail repairs a loft sees are stitching, not cloth.
What sail thread is made of
For most of sailing history thread was linen or cotton, and sails were resewn constantly. Modern sail thread is almost entirely polyester, for three reasons. It is strong for its size, it stretches very little so seams do not work loose under load, and it is highly resistant to ultraviolet light, which is what kills fibre on a sail exposed to years of sunshine. Nylon, though stronger and more elastic, absorbs water and degrades faster in UV, which is why it is used for sewing spinnakers – where a little stretch is welcome – but rarely for working sails.
Sailmakers select thread size to match the cloth: a light spinnaker or a 4-ounce Dacron jib takes a fine thread, a heavy cruising mainsail or a storm jib takes a much heavier one. The thread is usually bonded so the plies do not untwist, and often treated with a UV inhibitor.
Where the stakes are highest, sailmakers reach for specialist threads. PTFE thread, sold under several brand names, is virtually immune to UV and chemicals and is used on the exposed stitching of furling headsails and sail covers that will live in the sun for years. It is expensive and slippery to sew, so it is applied where it earns its keep rather than throughout the sail.
Stitching and seam construction
Sailmakers sew almost everything with a zigzag stitch rather than a straight one. A zigzag lets the seam stretch a little with the cloth under load without breaking the thread, and if one stitch fails the seam does not unzip along its length the way a straight stitch can. Most seams carry two or three rows of zigzag, and heavily loaded areas such as leech tablings and corner patches carry more.
On laminate sails, the thread is doing a different job. The panels are first bonded with adhesive seam tape or glue, which carries much of the load and stops the film from being cut by the needle holes. The stitching is then a safeguard and a way of clamping the bond. Because film does not heal around needle holes the way a woven cloth does, sailmakers use finer thread and fewer stitches on laminates than on Dacron, and they take care not to run a row of stitches straight across a highly loaded area where the holes would act like a line of perforations.
Why thread matters for sail shape and life
Thread is the weak link in the sail because it is the only part that sits proud of the cloth, exposed to UV on both sides and to every bit of chafe from spreaders, shrouds, lifelines and the boom. Cloth is protected by its own bulk and its finish; thread is not. On a sail that lives on a furler or on the boom under a cover, the exposed stitching along the leech and foot is the first thing to go. When it does, tablings lift, patches peel and the sail loses shape long before the cloth itself has stretched.
UK Sailmakers matches thread size, needle size and stitch density to the cloth and the load in each area of the sail, using heavier thread on patches and tablings and finer on the body seams, and specifies UV-treated or PTFE thread on the exposed edges of cruising sails. To check your own, run a fingernail across the stitching along the leech and foot: sound thread is firm and springs back, while degraded thread is chalky and powders when rubbed. Restitching before it fails is a quick loft job; waiting until the tabling flogs off doubles the cost.
Frequently Asked Questions about Thread
Why does my sail stitching fail before the cloth?
Because thread is exposed to sunlight on both sides and sits proud of the cloth where chafe reaches it. Polyester thread is UV resistant but not immune, and after several seasons it becomes brittle. This is normal and restitching is routine maintenance.
What thread should I use for a hand repair at sea?
A waxed polyester sail twine and a sailmaker’s needle, sewn with a herringbone or round stitch. Use a palm to push the needle. Keep the stitches spaced and avoid pulling so tight that you pucker the cloth.
Thread is a small thing that carries the whole sail. Learn to read the stitching on your own sails, get the exposed edges restitched before they fail, and ask your sailmaker about UV-treated or PTFE thread on any sail that lives on a furler. A well-sewn sail is a sail that keeps its shape, and that starts with the humble zigzag.