Into the Wind
by Jeff Williams

Which Sailcloth is Right for My Boat?

Most people don't spend a lot of time thinking about the type of cloth their sails are made of. I do, but that makes me a bit of a geek. Most sail lofts say that their cloth is the best or the most high tech or some rubbish like that. The fact is, different types of cloth are used for different types of sails, boats and the type of sailing expected.

In a pinch, you can use almost anything to make a sail. In the past sailors used woven palm mats, bark, skins, flax and more recently, cotton. There are limitations to using these materials. They rot, mildew, stretch out of shape and don't have the strength of newer synthetic materials. Even replica yachts usually sport "Egyptian cotton" or "tanbark" coloured Dacron sails.

The usual choices today for sails are nylon, Dacron and laminated fabrics. There is a lot of variation within these categories, but we'll leave the boring chemistry stuff to guys like... well, me.

Nylon cloth is used for spinnakers, which are a big, somewhat baggy sail that is only used to pull the boat along with the wind. Spinnaker fabric is available in a wide range of colours and is woven in a zip-stop pattern; that is, there is a grid of heavier threads running through the cloth to prevent tears from spreading.

Nylon is inherently stretchy, which makes it unsuitable for a sail that must hold an airfoil shape for upwind work. Instead, the spinnaker's main job is to obstruct as much airflow as possible. These sails are supported only by three ropes, along with the sail's geometry, that support their shape. Because of this, they sometimes collapse and then suddenly fill with wind again with a vicious snap. The stretchy nylon absorbs these shock loads which would otherwise damage the sail or the boat. Nylon's biggest weakness is the sun's ultraviolet (UV) radiation. If left in the sun, it will fade and lose its strength.

Dacron sailcloth has relatively little stretch and will retain the shape that the designer created for the sail over a long period of time and wind ranges. For these reasons, as well as its mildew resistance, Dacron replaced cotton as the material of choice for sails. Dacron sailcloth is made from polyester fibres. The raw fabric is heat shrunk and coated with a resin that is bonded into the cloth using both high heat and pressure. The result is a very stiff and airtight fabric. The resin helps to keep the cloth from stretching on its bias. (To understand bias, take a dishtowel and stretch it along its long edge. It will stretch a little bit. Now grab the towel at opposite corners and pull. You will be able to stretch the cloth quite a lot. Untreated cloth can be stretched much more at an angle to the weave of the fibres. This is the bias.)

The weave of the Dacron usually has more fibres running across the width of the roll than it does along the length. This is called an unbalanced construction and helps the sailmaker to orient the cloth to minimize the amount of stretch. There are several different weaves to choose from when making a sail.

The resin coating can also be different to change the characteristics of the sail. Cloth that is meant for a racing sail will have a high resin content because stretch will be kept to a minimum. A cruising sail will have less resin to make it softer, and therefore easier to handle and stow. A sail made from low resin sailcloth will absorb more shock loading and generally last longer. Dacron is also vulnerable to UV exposure.

Finally, laminated sails consist of two layers of Mylar that envelop a reinforcing fibre. If you haven't seen this stuff, picture the packing tape that has glass fibres imbedded in it. The fibre in laminated sailcloth can be Dacron, kevlar, carbon or a host of other exotic fibres. The advantage here is that the fibres don't have to go under and over each other as in woven products. This fibre "crimping" both weakens the threads and allows more stretch. Laminated fabrics don't have this limitation, making them lighter, stronger and having less stretch. A few of the large sailmakers have the capability of laminating the fibres directly into a sail, orienting the strands to handle the predicted stresses on the sail, eliminating seams and speeding their production. Laminated sails are significantly more expensive than Dacron sails, making them the choice of racing sailors who are very competitive.

Your sailmaker should ask you questions about how and where you sail your boat. This will help them to narrow the above options to just a few choices of what is the right material for your boat's sails.

Jeff Williams