Dyneema, while being incredibly strong, light, unaffected by water, and UV resistant, it is still a material of this mortal world. As such, it has various physical properties that can not be ignored.
One of these physical properties that can not be overlooked is the way Dyneema will change in length as it heats or cools. Sure, all normal materials experience this phenomenon, so what makes Dyneema special?
If you look at a bridge, you will see those nifty expansion joints in the roadway. These allow the bridge to expand and contract without breaking the bridge. If you look at these joints during the hot days of the summer, you will notice how the finger joints are fully interdigitated. Yet, in the winter on a very cold day, the finger joints will be pulled apart.
Concrete behaves as most materials do, expanding as they heat up and contracting as they cool down.
This expansion and contraction actually happens at a fixed rate that is known (by experimentation) for each material. The rate is called the Coefficient of Thermal Expansion and for all things that expand when they heat, this coefficient is a Positive Number.
Dyneema is a special material because it actually behaves backwards to this common convention. It will contract as it heats and expand as it cools, resulting in a Negative Coefficient of Thermal Expansion. This rare trait means that it will change length on you throughout the year and probably in an opposite direction to everything else on your boat.
In the summer, as your mast grows slightly longer, your stays will become slightly shorter. In the winter as your mast grows ever shorter your stays will become even longer!
Dyneema has a Coefficient of Thermal Expansion of -12 x 10^-6 m / K. This means that for every change temperature equivalent to 1 Kelvin (also equivalent to 1 degree Celsius) a meter of Dyneema will change its length by 0.000012 m, or 12 μm.
A little more clearly, for every degree change in Celsius, Dyneema will expand or contract by 12μm for every meter of length of the line.
This might not sound like much, but the temperature fluctuations throughout the year on a yacht can easily be 40 Kelvin (or 40*C) and stays on a yacht are very long. Each meter of Dyneema is now fluctuating by 480μm. That’s almost half a millimeter per meter!
This all adds up and in the dead of winter, your rigging can be significantly longer than you expected and thus very slack, or by inverse it could be very tight as it contracts on hot days.
Being how Dyneema expands as it cools and contracts as it warms, it is imperative to always tune your rigging on a warm day that way, worst case, your rigging is a little loose. If you tune your rigging to perfection on a frigid day, by Spring your rigging will have contracted so much that it will break something else on your yacht. Let’s face it, the synthetic standing rigging is going to be the strongest part of your rigging so something else is going to break when the stays all contract!
By being mindful of this physical property, you can safely enjoy the ease of inspection and reduced weight aloft that come with synthetic standing rigging.
