Mylar is a trade name for biaxially oriented polyester film, chemically the same polyethylene terephthalate that is spun into Dacron® fibre, but extruded and stretched as a thin, clear sheet rather than woven into cloth. In sailmaking the word has become the generic label for the film layer in a laminated sailcloth, the smooth, crackly component that gives a racing sail its distinctive look and sound. For the newcomer, the essential point is that film does not stretch on the bias the way a woven fabric does, so laminated sails hold their designed shape far better than woven ones of the same weight. The experienced sailor adds that film is measured in mils, thousandths of an inch, typically half a mil to two mils in a sail, and that this thin film is why a laminate must be handled and stored with more care than a Dacron cruising sail.
Why a Film and Not a Weave
A woven cloth is strongest along its warp and fill yarns and weakest at 45 degrees to them, on the bias, because the yarns simply rotate and the weave distorts under load. Sailmakers fight this with tight weaves, heavy resin finishes and panel layouts that run the primary loads along the yarns. Film has no yarns to rotate; it is a continuous sheet with roughly equal stiffness in every direction. Bond a film to a grid of high-strength yarns and the yarns carry the main loads along their length while the film carries everything off-axis, including the bias and shear loads that would otherwise let the sail stretch and the draft blow aft.
The result is a sail that stays flatter and keeps its draft where the designer put it as the wind builds, which translates directly into pointing ability and a wider groove upwind. The cost is toughness: film tears once nicked, dislikes hard creasing, and is degraded by ultraviolet light and by the constant flexing of a flogging sail.
How Mylar Is Used in Sail Laminates
A laminated sailcloth is a sandwich. In the simplest version a scrim of load-bearing yarns – polyester, aramid such as Kevlar®, carbon, Vectran® or ultra-high-molecular-weight polyethylene such as Spectra® or Dyneema® – is glued between two layers of film. In others, a single film is bonded to a light woven taffeta that adds abrasion and tear resistance for cruising use. The adhesive and the laminating process matter as much as the ingredients.
UK Sailmakers built its Tape-Drive® construction on exactly this principle. A lightweight base laminate of Mylar film provides the membrane, and high-modulus tapes are laid along the computed load lines to carry the primary loads from the corners. The film holds the shape between the tapes and resists off-axis stretch; the tapes do the heavy lifting. Today’s Titanium™ and X-Drive® sails are direct descendants, with film and fibre laid up as a single engineered structure rather than panels cut from a roll.
Living With a Mylar Sail
Laminates reward good habits. Keep them out of the sun when not in use, because UV attacks both film and adhesive: a furling headsail needs a UV cover strip and a mainsail wants its cover on the moment the boat is tied up. Roll or flake in soft, loose folds rather than stuffing the sail into a bag, and vary the fold lines. Do not let a laminate flog; a headsail shaking through a long tack is accumulating fatigue in the film with every flap.
Stow the sail dry, because mildew can grow between the layers where you cannot scrub it, and tape small tears immediately, because a tear in film propagates quickly under load.
Cruising and Racing Perspectives
The racing sailor accepts a shorter life in exchange for shape retention. A light film genoa may be at its best for a couple of seasons, but during them it holds a fast shape through a far wider wind range than a woven sail could. The cruising sailor values years of service and tolerance of abuse. For cruisers who still want laminate performance, the answer is usually a taffeta-protected construction in which the film is shielded on one or both faces, or a load-path sail built on a more durable base. See also scrim cloth, laminated sail cloth and warp-oriented cloth.
Frequently Asked Questions about Mylar
Is Mylar the same as Dacron?
Chemically, yes: both are polyester. Dacron is polyester spun into fibre and woven into cloth; Mylar is the same polymer extruded into a clear film. Their behaviour in a sail is completely different because of that form.
Why do laminated sails sound crackly?
That is the film. The stiffness that prevents bias stretch also makes the material crinkle audibly when handled. A new laminate is noisiest; the sound softens with use as the film becomes more pliable.
How long does a Mylar sail last?
A racing sail may hold a competitive shape for two or three seasons; a taffeta-protected cruising laminate, kept covered and dry, can serve for many years. UV exposure and flogging shorten the life, not sailing miles as such.
Mylar changed sailmaking by breaking the link between weight and stretch that had always governed woven cloth. Understand what the film does in a laminate, and what it cannot tolerate, and you can choose the right sail for the way you sail and get every season of performance it has to give.