Stretch is the elongation of a material under load. In sailmaking it describes how far a sailcloth, a yarn or a piece of rope lengthens when it is pulled, and how much of that lengthening comes back when the load is removed. A beginner learns that stretchy sails go baggy; an experienced sailor and sailmaker knows that stretch has a direction, a time element and a temperature dependence, and that almost every advance in sail construction over the past fifty years has been a campaign against it.
Elastic, permanent and creep
Three kinds of elongation matter. Elastic stretch is recoverable: pull the cloth and it lengthens, release it and it returns to its original dimension. Every fibre shows some elastic stretch, and a sail designer accounts for it. Permanent stretch or set is elongation that does not return; a woven cloth whose yarns straighten and shift under load, or a fibre that has been loaded beyond its yield point, stays longer. Creep is slow, continuing elongation under a sustained load, and is the particular weakness of some otherwise excellent fibres such as ultra-high-molecular-weight polyethylene.
Direction: warp, fill and bias
Woven cloth is not equally stiff in every direction. The warp yarns run the length of the roll, the fill or weft yarns run across it, and each is fairly straight, so the cloth resists stretch reasonably well along those two axes. On the bias, at 45 degrees to the yarns, there is nothing but the weave’s friction and the resin finish holding the cloth together, so bias stretch can be many times higher than warp or fill stretch. This is why panel layout matters. A cross-cut sail places the low-stretch fill yarn along the leech, where loads are highest; a radial sail aligns warp-oriented cloth with the load lines fanning out from the corners.
Fibres and how much they stretch
- Nylon: high stretch and high elasticity, which is exactly what a spinnaker needs to survive a collapse and refill, and exactly what a headsail does not.
- Polyester (Dacron): moderate stretch, excellent durability and ultraviolet resistance. The standard for cruising sails, made low-stretch by tight weaving and resin impregnation.
- Aramid (Kevlar, Twaron, Technora): very low stretch, several times stiffer than polyester, but weakened by ultraviolet light and by flexing and folding.
- Carbon: the lowest stretch of the common sail fibres and unaffected by ultraviolet light, but brittle if sharply creased.
- UHMWPE (Dyneema, Spectra): very strong and light, with low initial stretch but noticeable creep under sustained load.
What stretch does to a sail
As cloth stretches the sail becomes fuller and the point of maximum draft moves aft. A mainsail that was designed with its draft at 45 per cent of the chord ends up with it at 55 or 60 per cent, the leech hooks to windward, and the boat heels more, points lower and develops heavy weather helm. A genoa whose luff cloth has stretched develops a round, powerful entry that cannot be flattened with halyard tension. Stretch is why an old sail cannot be re-cut back to new: the cloth itself has changed.
How sailmakers fight stretch
Woven cloth is tentered and heat-set on the loom, then impregnated with resin, to lock the yarns and reduce bias stretch. Laminated sailcloth goes further by bonding straight, uncrimped yarns or a scrim between films of Mylar polyester, which carries the bias load. UK Sailmakers’ Tape-Drive® construction took the next step in the late 1980s: high-modulus tapes, originally Kevlar and later carbon and aramid blends, are laid directly along the computed load lines of the sail over a light base laminate, so the fibres carry the load in exactly the direction it acts and the base cloth only has to hold the shape between them. Titanium™, X-Drive® and Uni-Titanium sails continue that approach, producing sails that hold their designed shape across the wind range and for far longer than a woven sail of the same weight.
Practical advice
Match the cloth to the load: a light-air genoa flown in 25 knots will stretch permanently in an afternoon. Reef or change down before the sail is overloaded. Keep aramid and laminate sails out of the sun and roll or flake them rather than stuffing them. Have your sailmaker check the draft position every few seasons; a modest re-cut can recover shape from a cloth that has stretched a little, but only before the fibres have taken a permanent set.
Frequently Asked Questions about Stretch
Is all stretch bad?
No. Spinnakers rely on nylon’s elasticity to absorb shock loads, and a little elastic stretch in a cruising sail makes it forgiving. Permanent stretch and creep are the enemies because they change the sail’s shape for good.
Why does my old Dacron sail look fuller than it used to?
Years of loading have caused permanent stretch, particularly on the bias, so the draft has moved aft and increased. A sailmaker can sometimes re-cut it, but past a certain point the cloth has simply lost its ability to hold shape.
Stretch is the slow thief of sail shape. Choose cloth and construction that keep the fibres straight and aligned with the load, sail the sail within its range, and it will keep the shape your sailmaker gave it for many more seasons than a sail that is asked to do the wrong job.