Practical Sailor Test Boat Gets Half a Refit With Powerlite PBO Rigging

Although stainless steel wire and rod rigging are still the traditional, most popular, and economical material of choice for mast rigging, metal is rapidly giving way to a new generation of synthetic rigging material. There are a handful of companies that manufacture standing rigging using ultra lightweight high-modulus synthetic fibers such as Kevlar, PBO (polybenzoxazole), and carbon.

There are many advantages of synthetic rigging. Weight savings aloft is the primary benefit, and high-modulus fiber can be up to 50 percent stronger than similar diameter rod or wire rigging, and up to 90 percent lighter. Another touted benefit is greater fatigue resistance. For the average racer-cruiser, there are three big drawbacks to PBO: It is four times as expensive as stainless steel wire, although roughly on par with rod rigging; should its cover fail, it drastically loses strength in direct sunlight; visual inspection and DIY repair is currently impossible. Until recently, the technology for high-modulus, synthetic fiber rigging was only available to deep-pocketed mega-yacht owners. As development advances and competitive pricing is sparked, this type of rigging may one day be an option for average sailors. Under the current state of technology, we can only recommend this product for the serious racer with very deep pockets.

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Steel wire rigging dates way back. When small-diameter stranded wire rope began replacing large-diameter tarred hemp rope and heavy iron chain, it revolutionized a wide range of industries, from ship-building to bridge construction to railroads. By the 1880s, steel wire rope was sufficiently established to be offered by major marine suppliers in at least 23 different sizes.

In 1902, the tall ship

Preussen was launched. At 408 feet overall, with five steel masts carrying more than 60,000 square feet of canvas, the Preussen was the most powerful sailing vessel ever built. Preussens steel wire rope rigging measured an astonishing 15 miles and ushered in a new century where wire rigging had become the choice for supporting masts on all ships, big and small.

In 1909, aerodynamically streamlined rod rigging was invented in Scotland. “Lenticular” shaped rod became highly sought after by the fledgling aircraft industry for streamlining aircraft tie rods, bracing, and controls. In 1917, England adopted rod rigging for its Royal Air Force, and trickle-down technological developments from the aircraft and other industries began to reach sailors, a trend that continues to this day.

In the late 1930s, rod rigging began to be found on board racing boats. Preparing for the ultimately successful defense of the 1937 Americas Cup, rod rigging was incorporated aboard the Starling Burgess/Olin Stephens-designed “super J Boat”

Ranger. A year later, rod rigging debuted on Stephens quintessential 12-meter, Vim. And in 1939, Stephens first 6-Meter design, Djinn, was also rigged with lenticular rod.

Just as the newer, smaller-diameter, wire rope was an improvement over older hemp rigging, the advantages of less stretch, weight, and windage made rod rigging desirable over wire rope.

Stainless steel rod as a rigging material saw limited availability after World War II, and it wasnt until the late 1960s that a new company entered the marine field and began offering stainless steel rod rigging for the general sailing public. Navtec Corp. began manufacturing round rod rigging from a high-grade stainless steel called Nitronic 50. Nitronic 50 was 20 percent stronger than wire of the same diameter.

Despite rod riggings initial advantages over stainless steel wire rope, it had the drawback of being sensitive to surface damage, and it failed without warning. Rigging failures, usually caused by pitting and crevice corrosion or end-fitting fatigue, caused some high-profile dismastings, and initially dampened public enthusiasm for rod. But by the mid-1970s, rod rigging quietly and rapidly had become accepted as reliable by racers and cruisers alike.

Although stainless steel wire and rod rigging are still the traditional, most popular, and economical material of choice for mast rigging, metal is rapidly giving way to a new generation of synthetic rigging material. High-modulus running rigging, such as Spectra, Dyneema, and Vectran, has been in use for sheets and halyards for at least two decades. And now there are a handful of companies that manufacture standing rigging using ultra lightweight high-modulus synthetic fibers such as Kevlar, PBO (“polybenzoxazole”), and carbon.

“Modulus” describes the particular stiffness of a fiber component and that particular fibers resistance to deformation under load. If the material deforms very little, the modulus is said to be “high.” Trade-offs exist when you venture into fibers with high-modulus numbers. The fiber becomes more brittle, less robust, and expensive.

FIBER RIGGING TYPES

PBO, “the worlds strongest fiber,” was developed by Dow Chemical in the 1990s. It has the highest strength and modulus of any synthetic fiber and is made by mixing the PBO polymer while forcing it through a spinning machine. PBO is currently the gold standard of super fibers for rigging. Used in standing rigging applications, a PBO thread only
1 millimeter (1/25 inch) thick can hold 272 pounds (123 kilograms) of strain.

PBO offers good impact resistance, excellent coil-ability, and non-conductivity. Last year, continuous PBO standing rigging was developed to accommodate those rigging situations where diagonal and vertical shrouds are conjoined above the spreader tips.

Kevlar (Aramid) is made by spinning a solid fiber from a liquid chemical blend. It is similar to PBO in flexibility characteristics, is about 40 percent less expensive than PBO, but not nearly as strong or lightweight. Kevlar has excellent impact resistance and is used as fabric in bulletproof vests as well as in yacht rigging and hull armoring for high-speed catamarans and around-the-world racers.

Carbon is similar in weight, stretch, stiffness, and strength to PBO, but substantially more expensive. Carbon, unlike PBO, will conduct electricity in a lightning strike. Carbon rigging is not subject to UV degradation or moisture, and is said to have greater longevity, higher fatigue resistance, and durability than either PBO or Kevlar. Carbon rigging such as “Element C6” from Southern Spars is a bundle of carbon fibers bonded together in an epoxy matrix. As with PBO, carbon rigging can be continuously wound rigging from masthead to deck, with no spreader end fittings or turnbuckles in between. When building custom carbon rigging, the mast dimensions are “jigged up” on a design floor so as to ensure an exact fit.

ADVANTAGES

There are many advantages of high-modulus synthetic rigging. Weight savings aloft is the primary benefit. High-modulus fiber can be up to 50 percent stronger than similar diameter rod or wire rigging, and up to 90 percent lighter. Weight savings aloft translates directly into less pitching and rolling in waves, more righting moment with less heel angle, increased performance, and ultimately, more sailing comfort. However, the actual impact of the weight savings varies greatly between boats, and is less pronounced in smaller average racer-cruisers (30 feet or less). Certainly the trade-offs you make when switching to PBO seem to make little sense for a cruising sailor (see “Cutting Through the High-modulus Hype,” above).

Another touted benefit of high-modulus rigging is greater fatigue resistance. Tests in lab conditions show that high modulus has up to three times the fatigue resistance of wire, and six times that of rod. In practice, however, high-modulus standing rigging is replaced at relatively short intervals, so, currently, there isn’t a lot of data supporting its practical lifespan. A four-year warranty is the most that manufacturers are offering. Wire rigging, on the other hand, can last twice that long or longer&emdash;although a typical warranty only runs for one year.

While the high-modulus standing rigging makes much more sense for the bottomless pockets of the Grand Prix set, PBO offers advantages for small boats. Trailer sailors&emdash;who are regularly unstepping and stepping the mast&emdash;will find it easier to coil, uncoil, and stow fiber rigging. There is also less chafe to the hull, deck, and mast when towing.

A promoted, but yet unproven benefit of PBO and Kevlar is that, when dry, they are non-conductors. However,

Practical Sailor would strongly advise against holding onto any rigging, even PBO or Kevlar, when thunderstorms are near. When a lightning strike occurs, the charge ionizes an 8-inch diameter column of air. The ambient salt that builds up on the skin of dielectric rigging is enough to act as a lightning leader, leaving a big question mark as to how toasty the PBO or carbon will become.

DISADVANTAGES

For the average racer-cruiser, there are several drawbacks to PBO rigging, but three very big minuses stand out. These handicaps make PBO even less appealing for the long-distance voyager. (See “Cutting Through the High-modulus Hype”)

It is four times as expensive as stainless steel wire, although roughly on par with rod rigging. Should its cover fail, it drastically loses strength in direct sunlight. Visual inspection and DIY repair under current construction technique is impossible.

If these more serious problems are resolved, the other issues will be minor.

An initial problem with the manufacture of high-modulus rigging, as with wire and rod, was the application of terminal end fittings. Early end fittings were chemically or mechanically bonded, and problems with bending and fatigue could compromise strength and longevity. The Powerlite system uses epoxy resin-infused end fittings, which are, for the most part, adaptable to existing spars and turnbuckles.

Another approach to end-fittings involves thimbles. Using this technique, the shroud can be manufactured in any diameter and length by winding fibers around thimbles until the bundle reaches the desired size. This eliminates hard spots at terminal connections, but may require more modifications for retrofitting. Both Navtecs Z-System and the Dutch company Smart Rigging (which also has a small boat line called Easy Rigging) manufacture rigging using this approach. Navtec also makes PBO rigging with biconic end (mechanical, spike and cone) fittings. These studs, T-balls, and stem balls that are readily adaptable to existing spars and turnbuckles.

UV sensitivity and degradation, water absorption, abrasion, and chafe are serious problems for high-modulus material. Lab tests have shown that PBO can lose up to 98 percent of its strength after three weeks of direct exposure to sunlight UV. But rigging manufacturers claim to have all but eliminated potential damage to high-modulus fiber by sheathing the fibers in a variety of materials.

These sheathings come at a potential cost in windage, as they increase the diameter of most high-modulus rigging products to one and one-half times the diameter of equivalent rod rigging. Thus, a trade off comes between reducing weight and increasing windage.

In terms of big-boat racing, the reduction of rigging weight seems to be offsetting the disadvantage of the slight increase in windage, which is only a disadvantage when sailing close to the wind.

Other shortcomings deal with rating certificates and ordering. Rigging with high modulus needs to be reported on rating certificates, and PHRF and other handicap systems may ding a yachts rating when rigged with high modulus standing rigging. Lead time for building high modulus rigging can be quite long, often measured in weeks, and particular attention needs to be given to ordering parameters.

The Test Boat

To get a better handle on the use of PBO standing rigging in a real-world application,

Practical Sailoris currently sail-testing PBO for stretch, flexibility, UV and chafe resistance, and longevity. How long does this stuff really last?

For the test, we replaced the starboard side standing rigging on board the 28-foot performance cruiser

Wildflower&emdash;owned by PScontributor Skip Allan&emdash;with Powerlite PBO rigging and terminals developed by Applied Fiber. Generic 1×19 stainless steel wire rigging has been fitted on the port side of the boat.

The wire was installed onWildflower in early December 2006. So far, it has been in use for 30 hours and 120 miles of ocean sailing off Santa Cruz, Calif. To date, Allan reported, he can ascertain no difference in tuning or performance of the boat. Some problems with installation did arise, and were complicated by the fact the swap was done without removing the mast. This required going aloft and removing one shroud at a time, a time-consuming and tedious effort, in which not dropping pins and bushings while working aloft with cold fingers became a priority.

In addition, the marine eye end fittings were 1/32 inch wider than both the female mast tang and chainplate turnbuckle forks, and needed to be filed down. Another problem arose where the Powerlite PBO passes over the spreader tip. The diameter of the spreader tip rigging groove was 1/4 inch and the 1/2-inch-wide plastic protection “spreader bend” would not fit into the spreader tip without modification to the spreader end.

Comparing costs, stainless 7/32-inch wire rigging for a 16-foot port lower shroud and a 32-foot upper shroud, with end fittings, was $250. The PBO upper and lower shroud replacement, including the required endfittings, retailed for $1,110,&emdash;more than four times the cost of wire, but about the same as rod.

What did

Wildflower gain? According to our estimates, the refit saved a little over one pound of weight. Clearly, there are cheaper ways to shed ounces. The experience suggests that anyone who is serious about shaving weight aloft should also look at a carbon-fiber spar, which might actually come out ahead in the cost-benefit analysis over the long-term. Nevertheless, manufacturers are quickly resolving the issues surrounding high-modulus synthetic rigging.

Conclusion

Until recently, the technology for high-modulus, synthetic fiber rigging was only available to deep-pocketed mega-yacht owners, Americas Cup, and Grand Prix syndicates. As research, development and testing advances, and the need for these advanced fibers in various industries fosters competitive pricing, this type of rigging may one day be an option for average sailors.

Under the current state of technology, we can only recommend this product for the serious racer with very deep pockets, one who is looking for the maximum weight savings regardless of cost. Depending on the boat, the edge gained may be nominal, so one shouldnt expect miracles.

However, the state of development in this arena is changing fast. Whenever retrofitting an existing racing or cruising rig, or designing a new rig, the use of high-modulus synthetic fiber rigging bears a fresh and unbiased look. Only time will tell whether rod and wire-rope will go the way of hemp and chain.