Tentering is the finishing process in which woven sailcloth is stretched to a controlled width and heated so that the yarns lock into position and the fabric becomes dimensionally stable. The cloth passes through a tenter frame – a long oven with rails on either side that grip the selvedges – where it is pulled taut in the fill direction, held under tension in the warp, and heated to a temperature at which the polyester yarns shrink and set. For a beginner, tentering is why a sail does not bag out of shape the first time it gets wet or warm. For a sailmaker, the quality of the tentering is one of the biggest differences between a cheap Dacron and a premium one.
What happens on the tenter frame
Woven polyester comes off the loom loose. The warp yarns run along the roll and the fill yarns across it, and there is a certain amount of slack, or crimp, in the weave. If a sail were cut straight from loom-state cloth it would stretch every time it was loaded, and it would shrink unevenly the first time it was heated in the sun. Tentering resolves this in a few stages.
- Heat setting: After scouring to remove spinning oils, the cloth is fed into the tenter frame, gripped along both edges by pins or clips, and drawn to a precise width while it is heated close to the glass transition temperature of the polyester. The yarns shrink slightly, the crimp is pulled out, and the weave tightens and locks.
- Resin impregnation: Most sailcloth is then passed through a resin bath. The resin fills the gaps between yarns, stiffens the cloth and greatly improves its bias stability. The cloth goes back through heat to cure it.
The tension applied in each direction during tentering, and the temperature and time in the oven, are set by the mill to suit the intended weave. A cloth meant for a mainsail is finished to be strong along the fill so that panels can be cut cross-cut with the fill running parallel to the leech; a warp-oriented cloth is finished the other way round for radial-cut sails.
Why tentering matters for sail shape
Sail shape lives or dies on stretch. A sailmaker builds the draft into a sail by shaping the seams, and every bit of stretch in the cloth moves that draft aft and makes the sail fuller. Tentering is what gives the cloth its baseline stability before any resin is applied, and it is also what determines how well the cloth accepts and holds the resin. A well-tentered cloth has a tight, even weave with the yarns locked against one another, so the resin does not have to do all the work. A poorly tentered cloth relies on resin, and when the resin cracks and breaks down after a season or two of flogging and folding, the underlying weave is free to stretch.
Tentering also controls shrinkage. A cloth that has not been properly heat set will go on shrinking in service, most noticeably where the tabling and the body cloth shrink at different rates, which is the origin of many of the wrinkles that appear in older sails.
The sailmaker’s view: choosing cloth
At UK Sailmakers, the conversation about cloth starts with the finish as much as the weight. Two cloths of the same nominal weight and weave can behave very differently depending on how tightly they were tentered and how much resin they carry. A firm, heavily resinated finish holds shape well when new but can become brittle and crack; a softer finish is easier to handle and lasts longer but stretches more. For a cruising mainsail that will be flaked on the boom a thousand times, a softer, more durable finish often makes sense. For a one-design racing jib that must hold its designed shape for a season of regattas, a firm finish with tight tentering is worth the trade in handling.
Laminated sailcloth is different. Film laminates and scrim cloths derive their stability from the polyester film and the fibre layout rather than from a heat-set weave, so tentering in the woven sense does not apply, though the woven taffeta layers used in cruising laminates are themselves tentered before they are bonded to the film.
Frequently Asked Questions about Tentering
Is tentering the same as heat setting?
Heat setting is the key step within tentering. Tentering describes the whole process of stretching the cloth to width on the tenter frame while heat setting it; the two terms are often used interchangeably in sailmaking.
Can a stretched-out Dacron sail be re-tentered?
No. Tentering happens at the mill before the cloth is cut. Once the resin finish has broken down and the weave has stretched, the only remedies are recutting to remove the worst of the shape loss or replacing the sail.
Tentering happens long before you ever see a sail, but it is one of the reasons a good Dacron sail holds its shape and a cheap one does not. Ask your sailmaker about the finish on the cloth they are proposing, keep your sails away from heat and flogging so the finish lasts, and the work done on the tenter frame will keep paying off for years.