Benefits of carbon fibre vs. aluminium for manufacturing spars
A key benefit of carbon over aluminium is that it is significantly
lighter than aluminium for the same strength. The weight of a carbon
fibre spar all depends on section size and the type of carbon modulus.
For a typical tube that is the same size and stiffness using standard
modulus fibre, a weight saving in the order 45-50% is expected. Using
high modulus fibre, for a mast of the same size and stiffness, a weight
saving in the order 60-65% is expected.
The advantage of carbon is not only in weight. Dynamically the spars
behave differently. This has led to the use of stiffer spars that in
turn has resulted in totally new rig/sail combinations that are superior
to older aluminium rigs.
The result of the dynamic difference is clearly illustrated in one
design classes where the introduction of carbon has led to stiffer
spars. When the Finn and European classes introduced carbon, the spars
got gradually stiffer and stiffer until an optimum was found.
A sailor was unable to handle this same stiffness in aluminium but
now finds that the dynamic response in gusts is easier to handle so they can carry the stiffer rig through the range when they need
power. In New Zealand the optimum competitive weight range for sailors
was lowered by over 5kg when carbon was introduced to our junior
training boat, the P Class.
In the Finn class, this led to a different sailing style in some
conditions. With aluminium masts, the sail was flattened by sheeting
harder, thereby bending the mast more and tightening the leech. With the
stiffer carbon masts, the mast is raked back to flatten the sail
section through twist.
The reduction in weight also allows the designer to use smaller
sections if they choose and take a gain in windage as well as weight.
The fourth advantage is that the spar can be engineered to varying
stiffness up the length of the spar. An alloy extrusion has the same
mechanical properties all the way up to the taper. The carbon tube can
be engineered such that the laminate varies as needed up the spar to
give the correct properties in each panel of the spar. Carbon fibre
laminates also generally have much higher strain to ultimate failure
than aluminium, which means they are ultimately stronger than aluminium.