Design, Construction and Materials

The design of the FreeWing rig relies on a finite element computer analysis of the structure. Despite the simplicity of the concept, the stress analysis of the individual layers in the composite shell requires considerable computer capacity. The mast is a cantilevered monocoque shell, which tapers from the base to the carbon fibre masthead fitting. The shell is a sandwich structure that is vacuum bagged and heat cured. The skins are made from laminates of unidirectional carbon fibre, pre-impregnated with epoxy resin. The core is tough PVC foam. The result is lightweight yet very strong.

The “Material Properties” table compares carbon fibre with other commonly used mast building materials. The strength and modulus (a measure of how much a material deforms under load) of carbon fibre is impressive. However it’s most important attribute is the combination of high “specific strength”, “specific modulus” and fatigue resistance. By dividing the strength and modulus values by the density, one is able to compare the weight of each material required to carry a load. A high fatigue resistance allows a structure to carry loads close to its stress limit, without the fear of unacceptably shortening it’s working life. Aluminum and stainless steel have poor fatigue properties. Stainless steel rod rigging failure was responsible for 22% of yacht insurance claims in 1999. Insurance companies are considering refusing insurance for rigging over eight years old. Wire rigging is not much better but it at least gives some warning of impending failure, as it is usual for individual strands to break first.

Designers of aluminum and stainless steel rigs have to make a difficult compromise between excessive weight and a short life expectancy. The fatigue resistance of carbon fibre/epoxy resin composite material is far superior to both metals and other composites. It is relatively insensitive to fatigue damage even at very high stress levels. Kevlar (Aramid)/ epoxy composites are very good when subjected to tensile loads, but have poor compression properties. An unstayed mast has to resist bending loads, so the ability to carry compressive loads is vital.

Carbon fibre, despite it’s high cost, is the ideal material for an unstayed yacht mast. Its properties allow the FreeWing rig to be light, strong and durable.

MATERIAL PROPERTIES

CARBON FIBRE EPOXY (UD 60%vf)

ALUMIN-IUM

STAINLESS STEEL
(316)

GLASS FIBRE EPOXY (UD 60%vf)

KEVLAR EPOXY (UD 60%vf)

WOOD (Spruce)

1.6

.71

7.86

2

1.35

0.51

1760

295

690

853

1300

70

1370

295

690

767

260

45

125

69

195

38

76

12.4

SPECIFIC TENSILE STRENGTH

1100

109

88

426

963

137

SPECIFIC MODULUS

78

25

25

19

56

24

FATIGUE
(Tensile Strength retained after 1,000,000 cycles)

1510

83

303

480

480

N/A