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,
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
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.