Aluminium mast

Aluminium masts

During the 1960's wood was eclipsed by aluminium due to its greater durability, higher specific properties and lower cost. Aluminium alloys, generally 6000 series, are commonly utilised, with magnesium and silicium that give to the material high mechanic characteristics and good resistance to corrosion in the marine environment. For short, economical masts, 6063 alloy is used, 6061 type for high quality masts and6082 type, which is the most expensive one, for racing yacht masts.

Aluminium alloy

σ0.2%
[MPa]

σu
[MPa]

εu HB

E
[MPa]

AA 6063 150 195 12% 80 69 000
AA 6061 235 255 18% 80 69 000
AA 6082 255 305 10% 80 69 000

Table 4 Aluminium alloys adopted for sailing yacht spars.

An imperative requirement for a mast section is to provide adequate inertia with minimum dimensions in order to assure good buckling resistance and low interference with the mainsail. Mast profiles are obtained by extrusion in a wide variety of section shapes, with longitudinal inertia Jyy much greater than the transverse one Jxx. Some of them, as an example, are listed below (see in Figure 11).

  • oval sections, used for small-medium size cruising yachts without particular performance requirements. The ratio between the two diameters is about 1.5 while the ratio JYY/JXX ranges between 1.8 and 1.9.
  • bullet sections (or "D" sections) are employed for high efficiency rigs. The ratio between the two diameters is about 1.6÷1.9 and the JYY /JXX ratio for these types ranges from 2.5 up to 3.
  • open sections are used when a mainsail reefing system is to be set up. The ratio between the two diameters is about 1.8÷2.0 while the JYY /JXX ratio ranges between 2.5 and 2.8.

Aluminium-masts

Figure 11: Aluminium mast section shapes: oval, bullet, rectangular and open (Claughton et.al., 1998).

The most part of aluminium masts have constant section along its length; in the case of big and/or high performance yachts it is a common practice to reinforce the mast base and to taper the top. The first action is performed by bolting aluminium strips inside the fore and aft part of the section to increase longitudinal inertia; the more effective alternative consists in introducing a sleeve inside the mast and to bolt or rivet them together. The same method is employed to create masts longer than 18 metres jointing two profiles. In this case a coupling profile is introduced in the mast for two - three diameters in length and the two parts are bolted together.
The 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.

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