Black rod

The ubiquitous use of carbon fibre rods in place of steel cable has failed to materialise because of difficulties inherent in fastening them – until now that is. Roger Brownlie investigates.

Carbon fibre has long been admired for its tensile qualities, its strength being greater than steel. But the irony of carbon fibre is that it is stronger than the fastening system required to secure it. A good example of this problem is in yacht rigging. To support the mast, normally stainless steel cable is used in tension. This offers an adequate solution but the use of carbon fibre pultruded rod would be the ideal solution. Both lighter and stronger, a single strand of fibre is oriented 100% longitudinally so the tensile strength of the rod is optimised.

For yachts, carbon fibre cables offer many advantages. By reducing mass high up the mast, the lighter cable allows ballast to be removed from the keel. For every 0.4kg removed at the top of a mast, 3.6kg can be removed from the keel. This improves acceleration and hydrodynamic drag as the hull can float higher in the water. Aerodynamic drag is also reduced and corrosion becomes less of a concern. But the problem of securing the carbon rod at deck level has remained a problem.

‘Some time ago,’ says Rob Sjostedt of Air Logistics, ‘several firms attempted to make carbon composite yacht rigging using a single pultruded composite rod but there is difficulty in attaching a reliable terminus to a single large pultruded rod. There have been embarrassing failures using this approach. The available surface area at the end of the rod available to glue a terminus fitting to the rod is not adequate relative to the strength of the rod.’

However, a technique has been developed by California-based Air Logistics Corp. which cleverly solves this problem with encouraging consequences for many ‘rigging’ type problems.

‘Air Logistics approached the problem of making a carbon composite cable in a different manner,’ explains Sjostedt. ‘By making the cable out of a bundle of very small pultruded rods, it is possible to pot each rod in a conical terminus wedge. The retention strength of each rod is nearly equal to the strength of a larger single rod simply embedded in a resin block. However, the wedge compressive forces of the conical fitting approximately doubles the retention strength of each rod such that the rods break before pulling out of the terminus wedge. This approach also produces a fitting that is comparable in size to conventional wire rope fittings currently used on sailboats. Additionally, almost any size cable can be economically made with our bundled rod concept versus the expense of dedicated pultrusion dies.’

Air Log currently has five pultrusion machines, each capable of producing up to 48 streams of 1mm rods simultaneously at line speeds of 12m/minute. Since each rod represents approximately 227kg of tensile strength, the Air Logistics cables can be customised to deliver the specific performance required, unlike steel cables which are limited to standard sizes.

While Air Logistics has initially focused on the application of this technology for sailboats, the technology should be applicable to any tension member requirement where lightweight, high strength and corrosion resistance is required. Other possible applications are helicopter lift cables, bridge cables, offshore oil well tethers, tower guy wires, and building structure stabilising wires.

‘There are other ongoing development efforts to make large offshore oil well tethers out of pultruded carbon composite rods. These efforts utilise larger carbon composite rods than the Air Log concept.

Air Logistics envisions manufacture of offshore oil well tethers and cables by first making cables in the 1 to 2 inch diameter range (from a bundle of 1mm rods like rigging) and then bundling those cable elements into larger cable bundles.’