History of Sailing Yacht Masts, Rigging and Sails: 1900-Present day.
by James Gilliam
Part II Standing Rigging Materials.
(Continued from Part 1)
Twisted strand stainless steel wire standing rigging:
This is a common type of standing rigging used aboard most yachts and dinghies. It is corrosion resistant, strong, flexible and cheap. As far as I can ascertain this type of standing rigging has been in use for the majority of the latter half of the 20th century. Although there would be small advances in the quality of the steel and the manufacturing process. Its drawbacks are that it is relatively heavy for its given strength and does stretch to some extent. Therefore there have been developments to find stronger, more stretch resistant and smoother standing rigging.
Twisted strand wire comes in a variety of different configurations. The most common forms are seen in Fig 2 & 3. Different arrangements of the individual strands have a large impact on the properties of the rope. Fig 5. shows the relative modulus of elasticity of 2 different types of wire rope. Stainless steel rope 1x19 has around twice the modulus of elasticity of wire rope 7x7. This is an indicator of how properties change with different configurations of strands.
There have been some developments in twisted strand standing rigging to reduce the diameter far a given strength. Such as the Dyform twisted strand wire. (Seen adjacent). It also offers reduced drag akin to the rod rigging. Dyform offers a 30% increase in breaking strength compared to traditional 1x19 wire. The cross section shows how the specially shaped wires fill a greater proportion of the strand cross-section. There is also a smoother surface on the Dyform compared to standard wire rope, this decreases the parasitic drag of the standing rigging.
Rod standing rigging:
It is hard to determine when rod rigging was developed for yachts; I believe that is developed in the 80's as an alternative to twisted wire rigging. The aim of which was to create a stiffer rig to increase performance. This has largely been achieved and rod rigging is increasing in popularity. It is increasingly being seen in large cruising yachts and is used by most medium to high performance racing boats. There are a number of different materials that are used for rod rigging. The most common seems to be Nitronic 50TM, this is a high grade of marine stainless steel. Other types include cobalt, carbon and Kevlar rod rigging.
Comparative analysis of different standing rigging:
The modulus of elasticity is given by the following formula:
S=(P L) / (A E)
Where S = Elongation of a loaded wire
P = Applied load
A = Cross sectional area
E = Modulus of elasticity
Figure 5. Shows the comparative stiffness of the different standing rigging types available. It clearly shows how most rod materials are nearly twice as stiff as wire rope materials. Soft Kevlar rope does not appear to have very good properties compared to rope or metal rods, this is because the soft KevlarTM is primarily used for backstays where high strength and moderate stiffness are the main requirements.
Breaking strength is the first consideration when choosing what type of standing rigging. It is the maximum load the wire/rod can carry without breaking. Generally rod is approximately 20% stronger than wire of the same diameter.
Fatigue is an important consideration. So long as the attachment points are made in such a way as to allow small changes in angle of attack then there will be fewer problems with fatigue failure. Wire is more sensitive to fatigue because the individual strands rub together. Rod however is sensitive to surface damage, which can lead to fatigue cracking. This is where the wire has the advantage, when wire fails through fatigue the strands fail one by one and it is therefore easier to spot and to remedy. Rod on the other hand fails without warning and the signs of initial cracking are almost impossible to detect by visual inspection.
Elongation of the loaded wire/rod increases proportionally to the load and length, and inversely to the Cross sectional area and modulus of elasticity. It has a large impact on the overall efficiency of the rig. As long as the load is within 70% of the ultimate breaking load then there is no plastic deformation. Less elongation has the effect of producing a stiffer straighter rig. This results in a reduction in the weather helm, although this is not much normally around 2-3%. Also the lesser the mast falls of to leeward the straighter it is and the more resistant it is to bending.
The wind resistance of the shrouds and stays increases with increasing diameter. Rod for the same strength as wire has a smaller diameter and also a smoother surface therefore producing less drag.
Continue to Part III - Sail Materials and Technology
History of Sailing Yacht Masts, Rigging and Sails: 1900-Present Day by James Gilliam.
Introduction | Mast Materials and Manufacturing | Standing Rigging Materials | Sail Materials and Technology | Windsurfer Technology | Conclusion & References