Fiber rigging comes of ageSep 4, 2008
Metal rigging is increasingly becoming a thing of the past on racing boats and large yachts. During the past decade, four companies — Future Fibres, Navtec, SmartRigging and Composite Rigging — as well as several emerging smaller players, have succeeded in overcoming the considerable challenges that underlie adapting polybenzoxazole (PBO), carbon fiber and other space-age fibers to sailboat rigging.
The greatest technical hurdle in adapting the new fibers to rigging has been finding mechanisms to attach end fittings without compromising the strength of a rig. With traditional rigging in mind, initial attempts focused on bonding or clamping a fitting to the ends of the fibers. There are two considerable challenges: the first is to ensure that all the fibers in a bundle are adequately gripped; and the second is to ensure that they are all equally loaded. Because of the incredibly low stretch of these fibers, if loading is unequal, the most loaded fibers will try to take all the load, resulting in overload and failure, and then the next most loaded will load up and fail, and so on, producing a sequential failure.
Early on, Navtec developed a bi-conic socket-and-cone end fitting for Kevlar rigging. This fitting traps the fibers between a tapered plug inside a cone. The plug is partially tightened. The fitting is loaded up to stretch the fibers, allowing them to slip between the plug and cone until all are equally loaded, and then the tapered plug is fully tightened. A traditional threaded terminal screws into the base of the cone. Navtec later successfully adapted the bi-conic fitting for PBO, but Future Fibres simultaneously did an end run around Navtec with the continuous winding process (also known as the sling process), which has come to predominate.
Continuous winding uses two titanium or stainless steel thimbles placed apart at a distance equal to the length of the finished piece of rigging. Fibers are then pulled off a spool and wrapped around and around the thimbles in one continuous run until sufficient fiber has been added to achieve the desired strength and stretch properties for this piece of rigging. There is now no risk of the fibers pulling out of the end fittings, but the challenge of maintaining an even load on the fibers remains. In fact, the loading challenge is greater.
Consider a bundle of fibers wrapped around a thimble. If, when under load, the outermost fibers work their way in towards the thimble, the tension on these fibers will reduce, transferring the load to the other fibers, with a risk of the kind of sequential failure described above. Future Fibres, Navtec and SmartRigging have all developed very expensive, highly customized, proprietary machines and processes to wind the fibers with equal tension and then to maintain this tension over the life of the rigging, which requires a mechanism to maintain the relative position of the fibers on the thimbles.
Carbon-fiber rigging has evolved in a different fashion. In the past, carbon-fiber rods have been extruded in whatever diameter is necessary to achieve the desired strength and stiffness. However, not only is it expensive to extrude custom rod sizes, but it is also difficult to attach end fittings because the fittings are only gripping, or glued to, the fibers on the outside of the rod. Problems are compounded by the fact that conventional carbon rod pultrusion techniques require a mold-release agent that inhibits the bond between end fittings and rods. Composite Rigging’s breakthrough lay in finding a way to both reduce the cost of making the rods and at the same time improve the security of the end fittings. We have to go back to the fiber optic industry to see how this came about.
Fiber optic cables are pulled in bundles. The cables are not strong enough in and of themselves to withstand the stresses of being pulled. Another high-strength fiber is added to the bundle to take the pulling loads. Composite Rigging created a mechanism for continuously and economically extruding small diameter (1 mm) carbon-fiber rods at relatively high speeds for use as strength elements (Composite Rigging can produce up to 48 rods at a time per process line at a speed of 40 feet per minute; there are 12,000 carbon fibers in each 1-mm rod).
When applied to rigging, instead of changing the diameter of the extruded carbon rods to match the loads on a rig, Composite Rigging decided to bundle up however many of the 1-mm rods were necessary to achieve the desired properties. This substantially drops the cost of the rig over custom-extruded carbon while greatly accelerating the pace of rigging construction. One of Composite’s engineers, Rob Sjostedt, was a sailor and devised a mechanism for preloading all the rods in a bundle to an equal tension and then splaying them out inside a cone where they are glued in place with epoxy. Once set, the glue bonds to the surfaces of all the rods, creating a large bonded surface area to absorb the loads. The incompressible glue plug cannot be pulled out of the cone.
To keep weight out of these terminals, the cones are machined from stainless steel or titanium that is pared down to a minimum thickness and then bound in pre-preg carbon fiber. This provides the hoop strength necessary to keep the load on the epoxy plug from splitting the cone. The net effect is a very effective, high-strength terminal that places a balanced load on all the rods. Clever stuff!
Protecting the fibers
At this point in the process, whether it’s Aramid, PBO or carbon, we have bundles of parallel fibers with end fittings in place. These fibers need to be packaged up to minimize their cross-sectional area (to reduce windage) and must be protected from chafe. That’s about it for carbon, but PBO and other fibers also need to be protected from various environmental conditions. PBO, for example, is damaged by ultraviolet rays (UV) and moisture intrusion, with the effect of moisture being exacerbated at higher temperatures. In practice, at temperatures below 100° F, which is what is normally found in rigging applications, damage from moisture intrusion is generally limited. For any reasonable life expectancy, PBO fibers must be kept in the dark and sealed inside a watertight sheathing.
All four manufacturers have devised mechanisms for wrapping the fiber bundles in a manner that holds the fibers tightly together. For example, Future Fibres and Navtec spiral wrap a high-strength debulking tape around the fibers to compress them, then spiral wrap self-amalgamating (heat shrink) tape around this and shrink it down. SmartRigging has a mechanism to slide a continuous heat-shrink tube over the fibers. This is then shrunk down. In all three cases, the heat shrink provides both UV protection and waterproofing.
Composite Rigging pulls the carbon rods together with Kevlar thread. As noted, the carbon needs no additional protection against the environment.
For the three PBO users, so long as the heat shrink is not damaged, it does a good job of keeping moisture out along the length of the rigging. Waterproofing the cable itself is thus relatively easy.
What is not so easy is making the terminals watertight, and ensuring the watertight integrity of the transition from the terminal to the heat shrink on the rigging itself. Each manufacturer has a somewhat different approach to this. Navtec, for example, has a carbon shell glued around the terminal and to the thimble, and sealed to the heat shrink on the rigging with additional heat shrink. Whereas SmartRigging has devised a mechanism to create a poured terminal in which the PBO and thimble are fully potted in a dense, hard, rubber-like substance. Future Fibres clamps a mold over the terminals and injects resin. Naturally, each thinks it has the best approach to this rather critical issue!
Finally, whether PBO or carbon, the rigging needs protecting against chafe. This is provided by a braided sheath. Future Fibres and Navtec have a weaving machine that allows the density of the weave to be varied along the length of the piece of rigging according to the anticipated chafe. SmartRigging has devised a mechanism to slip the sheath over the rigging and then tension it. Given the inherent durability of its pultruded carbon rods, Composite Rigging only applies chafe protection in areas where chafe is anticipated. In all cases, the sheath is invariably some exotic material such as one of the Aramids (e.g., Kevlar or Technora), or high-density polyethylene (e.g., Spectra or Dyneema), chosen for such properties as its moisture, UV and chafe resistance.
Aramid versus PBO versus carbon
The finished pieces of rigging have different properties. Typically, when comparing one technology to another, the baseline that is used in any given application is the stretch of the Nitronic 50-rod rigging that would have been used prior to the advent of fiber rigging. Because Kevlar stretches more than PBO, it takes more to achieve an equivalent stretch and as a result it ends up being larger and heavier. This is why PBO has largely supplanted it in recent years. Today, when money is no object (for example, many high-profile racing boats) the primary comparison is between PBO and carbon. Here we see, at least on the surface of things, that for a given amount of stretch, carbon is significantly heavier. Then there are the end fittings and spreader fittings that must also be taken into account. Given the fact that the thimble-style fittings used in continuously wound PBO rigging tend to be lighter than the socket-and-cone approach used by Composite Rigging, once again, PBO would appear to have the edge. However, it’s not this simple.
Almost all modern high-tech rigs have discontinuous rigging, which is to say that any part of the rig that comes to a spreader terminates at that spreader. This is in contrast to traditional rigging, which is continuous from the masthead to the deck, bending around the spreader tip. Discontinuous rigging adds more fittings than continuous rigging, but whereas, for example, a two-spreader continuous rig would have cap shrouds and intermediate shrouds both coming over the lower spreaders and down to the deck, the discontinuous rig will have a single shroud running to the deck from the lower spreader, reducing the overall weight.
Composite Rigging has devised a mechanism to build prestressed continuous rigging in which the various shrouds and their associated diagonals (from the mast to a spreader tip) are tapered down and bonded together such that the carbon bundle at any given point is only as large as is needed to support the load at that point. This results in the weight efficiencies of discontinuous rigging without any of the multiple spreader fittings normally needed. Composite Rigging claims that the net effect is to cancel out the normal weight advantage of PBO with the added benefit of significantly reducing windage. Future Fibres counters that in a recent independent study of a maxi project, their discontinuous rigging was still found to be 5 percent lighter. Finally, in 2006, SmartRigging launched its own version of continuous PBO rigging, with five boats already rigged, and claims to have restored a 15 percent weight advantage over continuous carbon!
Two types of costs
This leaves the cost issues. There are two aspects to this: the upfront cost and the lifetime cost, which is not the same. If two different approaches cost the same up front, but one lasts half as long as the other, then it is twice as expensive over its lifetime.
Up front, Kevlar is about twice the price of rod rigging, PBO is about one-and-a-half times the price of Kevlar (which makes it about three times the price of rod), and carbon is about one-third more expensive than PBO (i.e., four times the price of rod). If you start adding titanium end fittings, costs can go considerably higher.
But then the rigs are 70 to 80 percent lighter than rod. In the case of superyachts, this can literally translate into tons of weight saved aloft. A recent refit of a Baltic 147-foot sloop by Future Fibres reduced the rig weight from 2,869 lbs (1,304 kg) to 769 lbs (350 kg) for a saving of 2,100 lbs (955 kg). The schooner Meteor, built at the Royal Huisman yard in Holland using Composite Rigging’s carbon rigging saved an estimated 1,600 kg (3,520 lbs) as compared to rod rigging. These savings translate into improved motion and stability, improved performance to windward, reduced fuel bills and/or a substantial reduction in the keel weight (in some superyacht examples, by several tons).
As we have seen, for the same stretch as rod, PBO and carbon are approximately 100 percent stronger. Let’s assume rod rigging that would be loaded to 25 percent of its ultimate breaking strength. The load on the replacement PBO or carbon rig will be the same, which means a rig with the same stretch will be loaded to 12.5 percent of its ultimate breaking strength. In theory, there will be reduced fatigue degradation and enhanced life expectancy for the fiber rigs.
In practice, no one knows what is the reasonable life of these rigs. The longest that any of them have been in service is eight or nine years. As the years go by, the manufacturers are becoming increasingly confident about life expectancy issues.
When it comes to maintenance, the key requirement is to inspect the rig for damage to its sheathing. With carbon, the only concern is that the rods are not damaged through abrasion, shock loading and impact. With PBO, there is the additional critical need to maintain the protection from UV and moisture intrusion. In general, damage to the sheathing is easier to detect than the kind of incipient cracks that threaten failure with rod rigging (this is because most rod rigging cracks are hidden inside terminals).
The other inspection point for PBO rigging is the connection between the terminals and the heat-shrink sheathing. Over time, there may be a tendency for the heat shrink to pull away from the terminal. There is an inner glue layer that provides additional protection, but clearly if the PBO is exposed it will lead to problems. Navtec terminal covers also need to be inspected for signs of cracking or opening at the glue seams (unlike Future Fibres and SmartRigging, the terminals are not filled with any kind of sealing compound).
It’s easy to forget that PBO and carbon rigging is less than a decade old. In that time, it has been thoroughly tested on the racecourse, including many of the world’s toughest events such as the Volvo round-the-world race and the Vendee Globe. It has emerged with an enhanced reputation, and as a result the pace of adoption is accelerating rapidly, especially in the superyacht arena. New players are jumping in with lower cost solutions (e.g., Powerlite Rigging and EasyRigging). As volume goes up and price comes down, we can expect fiber rigging to increasingly work its way down into more price-conscious markets. We will likely test a fiber rig on our next boat.
PBO and carbon rigs have truly come of age and are likely to see continuing rapid growth … until the next revolutionary fiber supplants them!
Nigel Calder is the author of numerous books, including Nigel Calder’s Cruising Handbook.