Synthetic fibers have been used on voyaging boats for decades. Egyptian cotton and hemp were replaced long ago by polyester and Dacron — a brand of polyester manufactured by DuPont. As more task-specific high-tensile-strength fibers have been developed, America’s Cup and other racing boats have exploited their uses in a wide variety of ways. Lately, voyaging boats have picked up this trend. It’s important to know how best to use these materials for voyaging.
There are many different kinds of synthetic fiber lines used on cruising boats today, but primarily they include Dacron, Kevlar, Spectra, Vectran and now even PBO (polybenzoxazole) is beginning to make an appearance on some avant-garde voyagers.
The keys to these changes are the high-modulus fibers that are now available and the creative thinking by some of the leading riggers and rope manufacturers of the world. High modulus merely means resistance to stretch. Alex Wadson of Aramid Rigging says that everything is now measured in “stretch equivalency.” Of course, each of the particular fibers has other critical characteristics, but a fiber’s tensile strength is the first consideration when determining how it may be used in a particular application.
Tom Yale of Yale Cordage suggests that there are three components to fiber elongation. In elastic elongation, a line that is loaded with several hundred pounds will stretch a given amount and immediately spring back part of that amount that has been stretched when the load has been removed. Hysteresis is a type of elongation that will give back a little more of the stretched amount over a period of time. In permanent elongation, a line will be stretched permanently. Yale suggests that the best way to break in a new polyester line and make it more stable is to secure one end to a very secure hard point on the boat and winch the line very tight. Have it stay there over a period of time to remove the permanent component of elongation that is characteristic of traditional polyester ropes.
Stretchiness in some lines, such as dock lines, may not only be tolerable, but desirable. But the cutting-edge applications that are taking place now are in the high-tensile-strength, high-modulus lines.
At Aramid Rigging, Kevlar 49 and the higher strength Kevlar 149 are thought of as a baseline fiber. They are showing up in runners and check stays aboard an increasing number of voyaging boats. Only a couple of decades ago, the use of Kevlar in rigging was considered advanced thinking. Now it’s a baseline as far as high-tensile line is concerned. Peter Johnstone’s Gunboat 62, Tribe, employs Kevlar a number of ways. The catamaran’s headstay, martingales and cap shrouds are all made from Kevlar and employ specially designed end fittings and turnbuckles to make sure that the fibers are loaded correctly.
Compared with other higher strength fibers, one of the problems with Kevlar is windage. By using Kevlar over stainless rod, weight is saved, but the windage can still be reduced in some applications by using alternative fibers. Since everything is considered in stretch equivalency, PBO provides another option. With a much higher modulus, PBO is roughly twice the strength of Kevlar 149. Half of the fiber bundle cross-sectional surface area can be used to provide the same strength. For most cruising boats, however, cutting the fine line on windage may not be worth the additional expense.
Weight aloft for cruising boats, however, should be a consideration. Think of it this way: Every pound aloft means that several pounds can be taken out of the keel. A lighter and shorter keel can mean getting into somewhat shallower anchorages. There is less pitching moment with less weight aloft and less force required to sail the boat to its optimum speed. So the ride tends to be more comfortable and efficient.
PBO has its own set of problems, however. Because the modulus is so high, fibers don’t stretch or creep to take up proportional loads. Creep is the elongation of the fibers that occurs over time and is associated with Spectra in some applications. Loading the PBO fibers evenly at the terminal ends is difficult, you must take care to insure even loading at the outset. Because attaining even load distribution at the end fitting is so tricky, PBO failures usually occur at the terminals. As with most high-modulus fibers, knots critically reduce the overall strength of the line, and loading the fibers evenly around corners can be tricky.
Splices and knots can weaken lines depending on the type of the line, the type of the knot and the quality of the splice. Line becomes significantly weaker in a knot because the outer strands in a bend are forced to carry most of the load, while the inner strands take little or none of it. A bowline knot can reduce the strength of the rigging at the knot by 55 percent for high-modulus line and as much as 30 percent for polyester line. The less a fiber can stretch before it fails, the less strength it has in a knot. Because each knot has its own characteristic bends and turns &mdash some more acute than others &mdash each knot also causes more or less loss of strength in a high-modulus line. An inside clove hitch reduces the strength less than a bowline, and a fisherman’s knot less than that. A properly executed splice, meanwhile, can retain up to 90 percent of the line’s strength when spliced around a traditional shackle bale.
Creep was one of the persistent problems with earlier synthetic lines. With some grades of Spectra, however, creep to rupture could occur at 95 percent of a line’s designed maximum load, so there is a relatively low amount of stretch to failure when compared with those earlier synthetics. The application in which a line is intended to be used can be critical to achieving a desired result.
The demands placed on running rigging are different than those on standing rigging. Some lines and fibers are better used in various applications than others. All of the synthetic fibers have their own unique sets of properties, such as chafe resistance, tensile-strength-to-weight ratios, the degree to which they creep or elongate under load, whether they tend to be hydrophobic (retaining less moisture) or hydroscopic. And some fibers, such as PBO, are more prone to UV degradation than others, such as Vectran. Some fibers, such as Technora, another high-tensile fiber, can gain up to 8 percent in weight from the moisture in the air &mdash even on a nice, sunny day with 60 percent relative humidity. Some lines are blends of higher-modulus, high-tensile-strength fibers, such as Spectra or Vectran, combined with other less exotic fibers, such as polyester.
Spectra comes in a few different types, including 900 and 1000. Spectra 1000, while costing somewhat more, also has approximately 20 percent more strength as the same diameter of Spectra 900, and Spectra 1000 also has lower stretch. Adding to the alphabet soup of synthetic lines, Dyneema comes in SK 75 (roughly equivalent to Spectra 1000) and SK 60 (roughly equivalent to Spectra 900).
Johnstone uses Spectra extensively aboard his cruising cat. The sprit stays are Spectra, as are the lashings for the shrouds, the topping lift, the lazy jacks, reef lines and the control lines for the daggerboards. Most of the halyards aboard Tribe are also Spectra. The one notable exception is the jib halyard, which is Vectran. Even the lifelines are Spectra!
The uses of these various types of synthetic are well reasoned. Johnstone believes that he not only saves weight aloft and in other areas of the boat, making the sailing more efficient, he also feels that there is a greater peace of mind that goes along with the high-tensile lines. PBO and Kevlar, he says, should be checked for degradation every 20,000 to 25,000 miles. Spectra can go longer. Of course, the line is visually checked more frequently than that, but a thorough inspection should be made at least at those intervals. The Spectra lifelines, as an example, have been onboard for several years and through at least 20,000 miles of offshore use. Today they look as good as new &mdash far better than stainless-steel wire in the same application.
For voyagers to get the most out of their high-tensile lines and use them appropriately, they need to know the pros and cons of the various fibers. To insure continued reliability, all lines need to be cared for properly, but some are more fragile than others. Chafe is probably the most common cause for premature aging of running rigging in particular. Bad sheet leads or afterguys chafing against wire lifelines, lines running over metal such as the bottom of the boom or against the cap shrouds, or halyards rubbing against the leading edge of the spreaders for extended periods of time are common chafe points.
There are numerous other issues. For example, two characteristic problems of PBO are that it breaks down in visible light, and it is even worse in inter-fiber chafe than Kevlar. Repeated soaking in salt water and drying without rinsing in fresh water can allow salt crystals to build up and chafe the fibers. A high salt content can lead to higher moisture retention and early failure.
One particularly interesting and cutting-edge way in which PBO is being used is in the luff of roller-furling headsails. Johnstone will probably use either PBO or Vectran in this application aboard his next boat. Run as a continuous loop from the roller furler at the tack point to the swivel at the masthead, the sail rolls up around the loop that is all the way up its luff. Because the PBO fiber is so strong, the rod headstay can be eliminated, reducing weight aloft. To make it possible for the sail to roll and unroll smoothly, the distance between the sides of the loop must be sufficiently wide, and the luff must be tensioned properly, thereby delivering the torque from the roller furler at the tack to the swivel at the masthead. The difficult part is to get sufficient torque to the upper swivel to overcome its friction, but proper tensioning and width in the loop are providing the solutions to that problem. The system has been tested and used on racing multihulls, Maxis and a few Farr 60s for some of their sails.
While creep can be thought of as a problem in some ways with lines that have a somewhat lower tensile strength than PBO. The fact that there is virtually no stretch in PBO can cause other problems if you’re not aware of its characteristics. Fibers need to be loaded evenly. If they are not, the ones that are most heavily loaded will take all of the strain, leaving the unloaded fibers ineffective in taking up the tension. Ultimately, the heavily loaded fibers will fail, leaving fewer and fewer fibers to take up the load, and the line will break. For that reason, PBO does not work very efficiently in knots or other situations that place uneven loads on the individual fibers.
Braided PBO lines are being used in halyards, but due to PBO’s weakness in chafe or with uneven fiber loading, it should not be used in conjunction with halyard locks, and special care should be given to the types of sheaves the line runs around. The fiber’s hydrophobic nature, however, means that it repels water and has low moisture regain. So weight won’t be added aloft on rainy or high-humidity days.
Vectran of course is still a competitive choice for halyards carrying higher loads, such as a jib halyard. It has a particularly low moisture-absorption rate. It is better in chafe than either Kevlar or PBO, and it has less creep than Dyneema or Spectra. It can also be used in conjunction with halyard locks, jammers and rope clutches. Aboard Johnstone’s cruising boat, he also uses Vectran for his steering cables. Like any high-modulus line, however, care should be given to the sheaves it goes around, and splices perform significantly better than knots for attachments. Spectra can be used in such applications as outboard reef lines, however. If there is a small amount of creep in the reef line, it can be taken up, and the knots in the line are less likely to be a problem. Because Spectra is good in chafe, it tends to be longer-lasting than Kevlar.
For Kevlar and PBO, Aramid Rigging has created special end fittings for certain applications to help reduce the problems with uneven loading of terminal ends. The fibers are machine-wrapped around a lightweight circular bushing or thimble, and the assembly is encased in a flexible housing. Innovative thinking is getting higher-performance fiber into a wide array of uses.
Aboard voyaging boats, the means of attaching rigging to the deck and to blocks is increasingly with lashings and loops made out of high-modulus fibers. About a year ago, Yale Cordage came out with a product they call Loups. Made from Dyneema, Loups can be used to replace stainless-steel shackles in most applications. A stainless-steel shackle can weigh as much as 11 times more than a Loups in the same application, while the Loups retains the same strength and reliability. To ensure equal loading of all parts in the Loups and increase the efficiency of the Dyneema itself, Yale has developed a proprietary steam heating and loading system. Using this system ensures maximum performance and longevity of the structure.
Visually the loops and lashings appear like a high-tech throwback to an earlier era. Turnbuckles and shackles are vanishing, and lashings and whippings are returning. In some cases the lashings connect carbon-fiber chain plates to high-modulus standing rigging. Hydraulic rams are used to pump down the shrouds while lashings are adjusted and readjusted, and then the hydraulic rams are removed and stored for later use. The lashings are often Vectran or Spectra, which tend to perform better in tight radiuses due to a characteristic to creep slightly.
The voyaging sailor should keep in mind the overall inventory, however, before using high-tensile-strength running rigging and sails. The equipment aboard a sailboat should be matched to the task and to each other. A cruiser who has Dacron sails need not spend the money on high-tensile-strength sheets to reduce the stretch in the line. The sails will stretch to a certain degree, and the line should probably be matched to the sail. In that case, a prestretched polyester sheet would probably be a more cost-effective solution. When considering the option of outfitting a cruising boat with Spectra sails and sheets and a Vectran halyard, give thought to the hardware that supports all of these high-tensile pieces of equipment. Will the sheet lead cars and tracks as well as the foot blocks and masthead sheaves be up to the task and take the surge loading caused by puffs of wind or going over waves? When puffs fill the sails, and the running rigging and sails refuse to budge, make sure that the hardware is sufficiently strong and well-secured to carry the loads.
The key to using the high-modulus lines successfully is in understanding the characteristics of the fibers and having the various elements work in harmony. Apply the correct set of characteristics to the rigging task at hand, properly maintain the lines, and check them frequently for chafe, wear or other causes of fiber breakdown. The results will be stronger, longer lasting, lighter rigging with less windage and fewer breakdowns. With your peace of mind at stake, you’ll be glad to know that things are held together with much more than a mere piece of string.
Bill Biewenga is a record-breaking offshore sailor and navigator, freelance writer and weather-routing specialist.