Traditionally, rod rigging has been sized to the break load (BL) of Nitronic50, in lbs and represented as ‘dash.’ Since the properties of Nitronic50 vary greatly with the manufacturer, the performance of rod rigging can vary significantly even if the ‘dash’ reference is the same. Hence, careful comparison may have to be done. Moreover, since ‘dash’ sizing is defined in stepped increments of BL, the ‘dash’ reference may not be precise enough for composite specification.
Composite Cable Design
The use of lightweight composite materials provides a new rigging solution at one-third of the weight of rod. Designing with composites offers more options in terms of diameters, strength and stiffness. The dash reference on its own is not relevant for specifying a composite cable, which simply needs a different approach. Gaining insight into the strength and stiffness of a composite cable will be helpful in optimizing the design of composite cables. It is a known fact that composites are up to 50% stronger and up to 75% lighter. In order for composite cables to go with existing marine industry design values, it is necessary to design them to perform in the same manner as rod in terms of stiffness. The following factors need to be considered to design a composite cable.
- The expected Maximum Working Load (MWL) in the cable is the optimum expected load a cable will experience in normal use and is estimated by mast designers.
- The stiffness of the cable, which is revealed in the elongation of the cable when subjected to two different loads. The cable reverts to its original size once the load is released.
The elongation of a cable under load is depicted in Figure 1. The ‘EA’ value of a cable defines the elongation behavior of the cable under load. The cable is stiffer if the ‘EA’ value is higher.
EA (MN) = Modulus of Elasticity (GPa) x Area (mm2)
E (Gpa) is the modulus of elasticity of a material or 'Young’s Modulus.'
A (mm2) is the measure of a cable cross sectional area and driven by the style of construction.
Rigging matched on break strength but not on stiffness is shown in Figure 2. The cable is not rupturing, but instead elongating in a different manner.
Designing Cables with Composites
Rod performance factors are actually better with a 100% packing ratio. However, solid rod is more in weight, thus handling will be difficult at larger sizes. With a wide choice of composites, material selection and style of construction significantly affect the finished cable properties. The addition of more fibers will increase the A value of Kevlar, which has a lower modulus E. This, in turn, helps achieving the same EA value (stiffness) used for rod. Hence, the diameter of Kevlar cables is much larger when compared to PBO for the same stiffness.
PBO and carbon have very similar modulus and comparable physical consolidation (packing ratio). Nevertheless, carbon fibers are significantly heavier than PBO fibers even if both have the same stiffness and cross sectional area. This is due to the fact that carbon fibers trap resin, whereas PBO fibers trap air pockets. When compared to rigid carbon composite rod, the key benefit of the bundled pultured carbon rods is their coilability. The bundling of small rods increases flexibility for the same weight but reduces the packing ratio, which, in turn, results in much larger overall diameter.
Composite rigging technology provides great variations in construction techniques and modulus, which, in turn, result in many different finished cable properties. Decisions need to be taken by considering the relative advantages of weight, durability, coilability, windage and cost, which will vary by individual cable and by project. Composites provide the opportunity to develop an improved rigging package that is customized specifically to meet the objectives of a project.
This information has been sourced, reviewed and adapted from materials provided by Future Fibres.
For more information on this source, please visit Future Fibres.