The top of the mast

boat-tripsThe top of the mast is tapered cutting a strip of material from the side of the profile of increasing width. Then the two edges are welded together obtaining a decreasing section towards the masthead. This simple procedure allows a reduction in weight and makes the top of the mast more flexible.

Carbon masts began to be used in the early 1980's, initially in racing dinghies, and then the America's Cup and Admirals Cup yachts. In two decades since their first use carbon fibres are not as widely used as one might think; in fact they are only considered when weight is critical and are therefore limited to racing yachts or performance oriented cruising yachts. This is an area which has evolved greatly in recent years, as innovative materials and designs have been explored, especially for long boat trips on the yacht. Monolithic and sandwich structures have been used. Dimensioning of composite masts is complex and requires analysis of global and local buckling, aerodynamic considerations and evaluation of the strength reduction due to many attachments and geometrical variations. High modulus carbon fibres including M55 and Pitch have been used but the most popular choices are intermediate modulus fibres such as M46 for racing yachts or standard modulus fibres such as T300 for cruising yachts. Software now exists to assist in material selection like, as an example, SIMSPAR code (Pallu, 2008). Carbon masts consist of mainly longitudinal unidirectional fibres (over 80%) with some at ±45° and 90°, in an epoxy resin matrix.

Most composite masts are manufactured in two half shells with the primary shell reinforced with local buildups at hardware attachment points. Preimpregnated fibres are laid up by hand in a female mould and cured at 120°C in an oven or autoclave. The two parts are then bonded together. An alternative fabrication process involving braiding of fibres around a mandrel produces a single part mast. A large number of finishing operations are then required, including machining of holes to fix the mainsail track, rigging attachments and spreader features. Note that the two part masts must also require detail attachment work in addition to the work involved in the bonding of the two sections. Therefore a carbon mast can be built with increased strength in the direction of the principal loads. For optimum sail shape the bend of the mast is very important, as the bend, along with other factors, directly contributes to the sail's draft depth. As the vessel becomes overpowered greater mast bend flattens the sail, and since a carbon mast can be manufactured with precisely controlled orientation of fibres it is possible to create a mast which has the correct bending characteristics. Additionally the inherently easier shape tailoring of a laminated structure provides for optimized aerodynamic or structural shaping throughout the length. This is an important advance in technology, complement this with new sail technology and they form a superior aerodynamic shape that could ever be achieved with an aluminium mast and polyester sails. A review of carbon masts construction is presented in Hall, 2002. A top example of this technology is represented by the mast of Mirabella V, the largest sloop of the world. Her carbon epoxy mast is 100 meters long, with five sets of spreaders, a section of 1600 mm in the longitudinal plane and a maximum thickness at the step of 40 mm.

For America's Cup boat masts, high strength intermediate-modulus type carbon fibre (Fibre Modulus=295GPa, Tensile Strength=4400MPa) is used in accordance with the appropriate property limits of the America's Cup Rules. As an example, the mast for the Nippon Challenger 1995 was formed in two pieces, front side and back side, then bonded into a unique piece (Figure 12). The 2000 challenger mast was built by an integral moulding with a female mould and a pressure bag. This method requires a very high strength of the mould as a good quality can be attained just by applying a high pressure by a vacuum bag; it was very effective and it does not need any auto-clave or assembly procedure. The female mould was built from aluminium alloy with a similar technology of aluminium mast building.

Figure 12: Two pieces carbon mast (1995 America's Cup Nippon Challenger).
Further developments in masts could come from the use of new matrix materials and new fibres, such as PBO (para-phenylene-benzobisoxazole), which could be used to increase the properties of the mast. Standing rigging, traditionally in ropes from natural fibres such as hemp, manila or sisal, is today generally in steel wire rope (1x19) on small yachts. For racing yachts and superyachts Nitronic 50 stainless steel is being replaced by high performance synthetic fibres, notably PBO and aramid. The use of continuous fibre slings results in lighter cables. Carbon fibre rigging is also under development. For running rigging polyester is the standard choice, more expensive fibres such as HMPE (Dyneema), aramid or Vectran are used for halyards.

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