High Performance/Low Cost Sails for Small Sailboats
The Effects of Ultra Violet Light and Moisture on the Strength of Sail Materials
By Kyle McMillan
As a young sailor, I have always been advised by my elders to follow certain procedures in caring for my sails when I return from a day of sailing.. “Lay out your sails. Wipe off the dirt and moisture. Flake and roll your sail tightly and carefully. Make certain you keep the sail in its bag. Put the sail away in the shed.” Experienced sailors seemed to feel that the least amounts of dirt, saltwater, rainwater, or excessive sunlight could damage their sails and hinder their boat’s performance. Back in the days of canvas and cotton, I thought, the perceptions of these “old salts” were probably true. But what about modern sail fabrics and technologies? Weren’t Kevlar sails and other modern laminates practically bulletproof? Did I still have to baby my sails? These were some of the questions I set out to answer in this paper..
begin my quest for answers, I needed to know more about the development of sail
fabrics; so I turned first to history and the experts. “In the past, sails had
to be constructed from whatever materials were at hand, such as skins, flax,
cotton, bamboo, coconut fiber, and jute. Whatever the material, all sails
suffered from stretch and shrinkage, and most let the air seep through” (Bond,
1990). Next came cotton. “Cotton, being a natural fiber, has poor resistance to
rot, UV light and water absorption, hence the coating of sailcloth with
varnish, making the sails quite heavy and stiff. These qualities made it
unsuitable sailcloth” (John, 2004). In 1902, Ernest A. Ratsey came from
In the latter half of the Twentieth Century, improvements in design, strength, longevity, and stretch resistance, started occurring all over with smaller companies now joining the competition and making use of newly developed sail materials. The most important fabrics today include Dacron®, woven Nylon, Polyester/Mylar laminates, Kevlar®/Mylar laminates, and Spectra/Mylar laminates. “Prior to laminated sailcloth…the most desirable qualities for sails— lightweight, low stretch, high strength, and durability— could not be combined in one package. Low stretch meant heavy; lightweight meant delicate.” (Whidden and Levitt, 1990). Sailors and sail manufacturers now tend to look more closely at the fabrics and how they move, testing every possible factor.
To create a
successful sail, manufacturers have to test for certain properties that may
reveal weaknesses in their sail fabric. The cloth’s geometry, stretch
resistance, strength, weight, flexibility, tenacity, porosity, water
absorption, and ultraviolet (UV) resistance might all be tested. For example,
Haarsticksails, Co., a sail manufacturing company, first measures initial
strength, or maximum weight capability, on an Instron machine, which pulls a
strip of material until it tears. Next, the same fabric is placed in a unique
Impact Flutter machine, which is essentially a wheel that spins the strip
causing it to forcefully hit the side of a wooden table. Company personnel then
re-test the strip for strength in the Instron machine and compare the two
results as a simulation of how well a sail may hold its shape on a boat. “These
days, few sails actually fail by breaking, but many are flown in more wind than
they are designed for and fail by becoming permanently distorted or blown out”
(Whidden and Levitt, 1990). By determining the material’s yield strength, which
is the point beyond which it can no longer recover to its original length,
having exceeded maximum load capability, and comparing it to the predicted
sail-load that may be experienced, companies can estimate a maximum wind-speed
potential for a particular piece of fabric.
However, I was more interested in learning how well modern sail materials held up to sunlight and moisture. Did the old timers’ advice still hold? If a sail material comes in contact with UV rays and moisture, will its strength gradually decrease, making it more susceptible to tearing?
For my test, I was
going to need three essentials: 1) some modern laminated sail fabrics, 2) an
Instron machine to measure fabric tear strength, and 3) a machine that could
“weather” a sail fabric with UV exposure and moisture. I chose ten different
sail materials representing a wide range of prices and (advertised) quality,
including: DuPont™ Sorona, PolySail International’s PolySail material, DuPont™
Dacron®, North NLT 605XI5 6000 DPI, Dimension-Polyant FLX08A, Contender AKS6,
Bainbridge CL75, Bainbridge Ocean745, Bainbridge DIAX2 70TT, and two strips of
WL Gore & Associates’ architectural fabric, TENARA, a new architectural
fabric that has incredible resistance to moisture and UV exposure. WL Gore
& Associates’ Instron Tensile Tester 3360 Series, Dual Column Testing, was
made available to measure tensile strength/breaking force, which is the load,
in lbs/inch, at which the material tore. WL Gore & Associates also provided
their QUV Accelerated Weathering Tester to weather the sails with ultraviolet
light and moisture for retesting in the Instron machine later. By comparing the
initial and “weathered” points at which the fabrics tore, I expected to learn
whether our modern laminated sail fabrics still needed all the care that mature
To measure the tensile strength of the fabrics, I cut each sample into five 1” x 14” separate strips. The first four strips were used in the Instron machine to measure the initial “breaking point” of each of the fabrics in pounds per square inch. The results of these tests appear in Figure 1 below.
Note: Test results are +/- 20 lbs.
The fifth sample of each type of material went into the QUV Machine on November 22, 2006. Each day this machine emitted 20 hours of UV light intensity and produced a four hour condensation cycle in which the air inside reached 100% relative humidity then condensed on and soaked the fabrics. After 65 days of this intense treatment, I photographed and recorded observations about the “weathered” fabrics recovered from the QUV machine. Finally, I removed the surviving sample strips and sent them back to the Instron machine for final strength measurements. The pictures below show the results of this intensive weathering. The table below the picture reflects my observations.
Visible Observations Post-UV
Not as soft. Discoloration.
Torn across the middle. Very dry.
Discoloration. Stiff. Curling under from the sides.
Coating is peeling and chipping off. Stiff
Lost all color. Shrunk. Very fragile.
Discoloration. Sides are curling under. Stiff
Little change except stiff and some wrinkling.
Coating is coming off.Stiff.
Coating is coming off and has air bubbles. Stiff. Discoloration
Some stiffness. Discoloration.
No visible changes.
As the table makes clear, only the TENARA fabric showed no visible changes and still felt the same, having an average initial strength of 502.5 lbs/in. The TENARA fabric is (barely) visible at the top of the photo above.
Finally, all fabrics, except for the nylon ripstop and PolySail material both of which had deteriorated too much to test, were re-tested on the Instron for its post-UV and moisture strength. The results appear in the following chart:
The results of this experiment appear to justify experienced sailors’ concern for sail care. Nearly all the sail fabrics weakened and deteriorated substantially after intense exposure to UV and moisture. Only the TENARA archtechtural fabric, which is not currently used as a sail fabric because of its thickness, weight, and expense, showed no deterioration. In fact, a couple of my samples surprised me with the amount of deterioration they suffered. The ripstop nylon practically crumbled in my hand after the UV/moisture testing. In addition, I showed the Post-UV PolySail sample to David Gray, who markets sailmaking kits made from the PolySail material, and he was shocked at the extent to which his material had failed. He emailed this response:
“One of the reasons that the polyethylene and ripstop nylon materials failed so miserably in this test could have something to do with the thickness and/or weight of the materials being compared. The 5.5 oz. /sq. yd. white Polytarp, might look pretty good compared to 2.5 oz. blue Polytarp under the same conditions. From experience in using both materials as boat covers, I know that the blue tarps disintegrate much faster than the white ones under harsh winter conditions. Ripstop nylon is also a very thin, lightweight synthetic which might account for its poor performance under the UV/moisture test.” D. Gray, of Polysails (Personal Communication, January 26, 2007).
The fabrics with the clear coating on them, like the Contender AKS6 and the Dimension- Polyant fabrics seemed to have more resistance to strength loss during the test.
Although my experiment tested multiple variables, there were many more variables that I could have tested. If I were to run my tests again, one thing I would certainly do would be to make certain that all fabrics weighed the same. I would also test these samples for more properties to figure out their overall efficiency. That way, I could compare one fabric’s overall performance level to another. A second test might also explore the economics of sail materials to see if less efficient ones are being sold for higher prices. The results could be released to customers as more information for making the smarter buy.
After talking with David Gray about his PolySails, I also learned that the color of a fabric can make a difference in its resistance to UV light. Using that knowledge, I would like to conduct another similar test using only one color for different fabrics. Another alteration I would make in a future experiment would be to test each sample in more than one direction. Each fabric has multiple types of fibers running in separate directions, whether in warp, fill, or bias. By testing all three directions, I would most likely get three strength measurements that differ considerably. A three-direction test would be a much more reliable test of overall strength because the sail has to hold shape and strength in every way possible.
For maximum performance, a sailor selects the sail for his or her boat depending on how it will be used. Manufacturers sell cruiser sails, racing sails, spinnakers, etc. In another test, I would use only one type of sail. If more variables were controlled, the results could have been much more significant and valuable to both manufacturers and sailors.
The results of this experiment made me realize how much I am harming my sail by leaving it out in the sun too much. Also, after seeing the extent of Nylon’s degradation due to moisture, I am going to start wrapping up my sail even tighter and putting a cover on it in the shed in order to help protect the fabric from mold, moisture, and all light. “Treat your sails like a newborn baby, and you will sail faster and extend their life. A little tender, loving care goes a long way” (eHow, 2000). I guess I have to concede that the “old salts” were right.
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Science of Sails.New York: St.Martin's Press.
Email email@example.com or call Dave Gray at 317 385-3444
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This page updated on 4/3/2009