Sailboat Rigging - Highly Strung
Time and again sailing has been revolutionized by the introduction of new fibers. Traditional wood hulls have been supplanted by glass fiber, Kevlar and carbon. Canvas sails have given way first to Dacron and then to laminated sails utilizing various high tech fibers. Running rigging today is entirely synthetic. And now stainless steel standing rigging is being replaced by fiber rigging, resulting in significant reductions in rig weights with less stretch and improved performance.
Kevlar was the first high-tech fiber to be used as standing rigging. It is still in use, but has been supplanted by carbon and PBO, which are both lighter and require smaller diameter sections for equivalent strength and modulus.
Unfortunately, carbon and PBO (and, to a lesser extent, Kevlar) have exotic price tags that put them out of reach for most sailors. Now, however, we are beginning to see cheaper alternatives such as Dynex Dux (a variant of Spectra and Dyneema). Although Dynex Dux stretches more than the other fibers, it still stretches less than traditional stainless steel rigging. However, it suffers from permanent elongation (known as "creep," which also occurs with Kevlar) if loaded beyond a certain point, so it must be engineered with this in mind.
The greatest technical hurdle in using the new fibers as standing rigging is finding ways to attach end fittings without compromising rig strength. Initial attempts focused on bonding or clamping a fitting to the ends of the fibers, but it is difficult to ensure that all the fibers in a bundle are adequately gripped and equally loaded.
Because these fibers stretch so little, unequal loading leads to sequential failure in which the most loaded fibers are overloaded and fail, and then the next most loaded can fail, and so on.
Early on, Navtec developed a socket-and-cone fitting for Kevlar rigging where fibers were trapped against a tapered plug inside a conical socket. 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 seated. Navtec (and Ocean Yacht Systems) successfully adapted this fitting for PBO at about the same time as another company, Future Fibres, introduced their "continuous winding" process.
A piece of continuous-winding rigging has two titanium or stainless steel thimbles around which fiber is wrapped in one continuous run until the desired strength and modulus is achieved. This eliminates the risk of the fibers pulling out of the end fittings, but the challenge of maintaining an even load on the fibers remains.
Unlike all the other forms of fiber rigging discussed, which have parallel fibers, Dynex Dux is a braided line. Braided fibers tend to straighten out somewhat under load, and Dynex Dux thus stretches more than parallel-lay fiber rigging. However, Dynex Dux rigging can be cut from a roll with the end fittings spliced into the rope, thus lowering costs. The braided structure also allows fibers to shift in relation to one another so the load stays evenly distributed, preventing the sequential failure that makes end fittings for parallel fibers difficult to engineer.
Consider a bundle of fibers wrapped around a thimble. If the outermost fibers work their way in toward the thimble when loaded, the load on these fibers is transfered to the other fibers, with a risk of a sequential failure as described above. The principal fiber-rigging manufacturers have developed proprietary machines and processes to wind the fibers with equal tension and maintain this tension over the life of the rigging.
There is also now hybrid rigging, used extensively by Navtec, in which continuous winding is used to create an overlength piece of rigging. One thimble is incorporated into the final piece of rigging, while the other is cut off and replaced with a socket-and-cone fitting.
End fittings for carbon fiber rigging have evolved in a different fashion. In the past, solid carbon-fiber rods were pultruded in whatever diameter was necessary to achieve the desired strength and stiffness. However, it is expensive to create custom rod sizes, and it is difficult to attach end fittings. Typically the fittings only grip, or are glued to, the fibers on the outside of the rod, although Navtec and others have experimented with splitting and splaying the end of the rod and gluing in a wedge.
The latest breakthrough involves bundling several small diameter (1 mm) carbon-fiber rods together to match the loads on a rig. This is substantially cheaper than custom-extruding one-piece rods. Composite Rigging (a division of Southern Spars) has a mechanism for pre-loading all the rods in a bundle to an equal tension and then splaying them out inside a conical socket, where they are glued in place with epoxy ("Element C6" rigging). Once set, the glue bonds to the surfaces of all the rods, creating a large bonded surface area to absorb loads. The incompressible glue plug cannot be pulled out of the cone.
These various termination methods solve the problem of gripping and evenly loading the slippery fibers used in most fiber rigging, but do so in a labor-intensive fashion that increases cost, especially with continuous-winding and carbon rigging.