Description

Wire/rod terminations and tensioning devices have been developed from historical designs. This makes any development work very difficult. For example, new designs have not proven themselves ‘in the field’; this leads to a perceived risk. The perceived risk is reduced if a new design is similar (even if just aesthetically) to an existing design. This has the problematic result that people will not use new products that differ greatly from existing designs. If a design attitude is adopted whereby new designs are simply a slight modification to an existing system a lack of scientific underpinning means that there is no guarantee that a new design will be any better than the original, if not worse.

This has led to designers using a combination of various ‘rules of thumb’ and experience. A natural result of this style of design process is redundancy in some areas and inadequacies in others, neither of which are acceptable. One of the rules of thumb is that a rig on a sea-going yacht should be replaced every ten years, this is intended to mitigate against failure due to fatigue and corrosion. If a rig is designed to withstand a ‘once in a hundred years’ storm then it is massively over-engineered for the conditions it will operate in for most of its working life. Would a better solution not be to correctly assess the actual rather than perceived risk to the rig and then design a new component to protect against atypical conditions? Using better analysis and design therefore it would be possible to specify the correct rig for the job and still provide an acceptable level of cover against the most extreme of conditions.

It is important for yacht rigs not to deflect too much during use as deflection leads to changes in sail shape with a corresponding reduction in performance it also leads to increased fatigue damage in the structural members which deflect such as the mast and deck. To this end there has been a recent trend toward the use of rod instead of cable for the rigging of yachts. However, the rigidity of rod can cause problems. When a rig is subject to loads in excess of those it is designed for cable has the ability to stretch elastically absorbing excess energy that could otherwise cause damage to one or more of the components of the rig. The rods used for yacht rigging tend to be of high-grade steel such as Nitronic50 chosen specifically for their high modulus of elasticity. Under large transient loads therefore the excess energy is not taken up by the rod but instead causes damage to the terminations and tensioning devices or to the mast and deck. One of the key questions to be answered is how would the system perform when operating outside its design parameters, how would it be damaged, how would it fail? The project intends not only to answer this question but also to correctly specify and design a safety device to limit the damage to the system by absorbing the excess energy and alert the user to the fact that damage has occurred.

The project will initially look specifically at the HiMod range of rigging components as manufactured by Petersen Stainless Rigging Limited for which full access to all design data and all technical drawings is available. By restricting the project to a single project range in this way the influence of differing manufacturing techniques can be ignored from the point of view of comparing various test pieces. The scope of the project can then be widened to other product ranges from other manufacturers at a later date.