Thermal Expansion is the phenomenon where objects become bigger as temperature changes. In general, with most objects in the world, as things heat up, they also expand. Negative Thermal Expansion is reserved for those rare occasions where materials actually contract as they heat up; Dyneema is one such material.
On a yacht, the rigging must be perfectly tuned to hold the mast in column while withstanding the forces of the wind placed upon the sails and spars. This perfect tune revolves around the lengths of both the spar and the standing rigging. If one of them were to change drastically, so would the tune!
Unfortunately, all materials used in rigging have slightly different coefficients of thermal expansion. Where thermal expansion was the phenomenon of changing size, the coefficient is the rate of such change.
The issue comes down to mixing materials for the spar and standing rigging that will be changing size at various rates.
The most common spar material at the moment: Aluminum, has a coefficient of 23.1 x 10^-6 per Kelvin (or for practical applications “per *C"). This is to say that 1 meter (39 inches) of aluminum will expand or contract 0.000023m for every degree change in Celsius. This might not sound like much, but if you think about a yacht that will endure summers and winters, the temperature change can be rather drastic.
Imagine a yacht that sails in temperatures from cold winter days of 0*C all the way up to hot summer days of 40*C. That is suddenly a 40 K change (Each degree of Celsius is equivalent to 1 Kelvin). On a mast that is 19m tall (62 feet), that means that the change in length of the mast will be:
( 23.1 x 10^-6 / K ) ( 19 m ) ( 40 K ) = 0.017556 m = 17.56 mm ( 0.69 inches )
That is a pretty drastic change in size of your mast!
The next material to think about for a spar is wood, and while Sitka Spruce is the ideal wood for a spar, it is becoming ever harder to find good clear wood for the purpose. The next best wood for a spar, and the one that is becoming ever more popular as a wooden spar is Douglas Fir with it’s coefficient of 3.5 x 10^-6 per Kelvin (when parallel to the grain). The same spar now becomes:
( 3.5 x 10^-6 / K) ( 19 m ) ( 40 K ) = 0.00266 m = 2.66 mm ( 0.10 inches )
Significantly less change in length.
The last common spar material these days is also a very modern material: Carbon Fiber (Carbon Fiber Reinforced Polymers) with a coefficient of -0.8 x 10^-6 per Kelvin. The negative is an important part in this because that means that as the carbon fiber spar heats up, it also contracts!
( -0.8 x 10^-6 / K ) ( 19 m ) ( 40 K ) = -0.000608 m = -0.608 mm ( -0.024 inches )
This material is incredibly stable and barely changes size during the whole year, with its longest being on the coldest days and the shortest on the hottest of hot days, but the difference is less than 1 mm!
A changing spar length means very little if this change is not relative to something else, something like your standing rigging!
The most common material for standing rigging is Stainless Steel with a coefficient of 16.5 x 10^-6. Grade 304 and 316 both have the same coefficient which is why you don’t have to worry about which type is being used in your rigging.
On a spar that is 19 m tall, the cap shrouds will be roughly about 20m long (the beam of the boat is the only additional length in the stay, and this is run at an angle). Lets see how much the length will change over the same temperature variation:
( 16.5 x 10^-6 / K ) ( 20 m ) ( 40 K ) = 0.01254 m = 12.54 mm ( 0.49 inches )
This means that the steel rigging will expand almost half an inch over the years temperatures.
When you combine an aluminum spar with steel rigging, the variation is about 17.5 mm while the rigging is about 12.5 mm. This means that they will expand and contract together and only at the extremes be off by a few millimeters.
On a wooden spar with steel rigging, the difference would be 2.66 mm for the spar and 12.5 mm for the rigging. This means that on the really hot days, the rigging will be about 1 cm longer than the spar if the rigging was setup on the coldest of days.
On a carbon spar with steel rigging, the difference is a bit more drastic. The spar will contract by 0.6 mm while the rigging will expand by 12.5 mm. This means that if the rigging were tuned on the coldest of days, the rigging would be 1.25 cm too long on the hottest of days. If a boat has a carbon spar, then you can assume that the owner of the yacht is interested in performance and therefore would notice the horrible state of the slack rigging!
A newer material for standing rigging is UHMWPE, or Dyneema. This plastic fiber has a coefficient of linear thermal expansion of -12 x 10^-6 per Kelvin. Just like with the Carbon Spar, Dyneema also contracts as it heats up and expands as it cools.
( -12 x 10^-6 / K ) ( 20 m ) ( 40 K ) = -0.00912 m = -9.12 mm ( -0.35 inches )
The change in rigging length is rather dramatic, very close to the change in length of stainless steel rigging, except in the opposite direction. As steel expands, Dyneema contracts and as steel contracts, Dyneema expands.
When we pair these with spars, we see a rather drastic difference emerge!
With an aluminum spar: 17.56 mm expansion of spar and 9.12 mm contraction of rigging as it heats. This means that the difference between the two will be 26.68 mm ( 1.05 inches ) of difference!
With a wooden spar: 2.66 mm of expansion of spar and 9.12 mm contraction of rigging as it heats, with a difference of 11.78 mm ( 0.46 inches ).
With a carbon spar: 0.61 mm of contraction of spar and 9.12 mm contraction of rigging as it heats, with a difference of 8.51 mm ( 0.33 inches ) but going in the same direction.
The take away message here is that the components of your standing rigging will change as temperatures fluctuate. Some materials do not change much while other materials change drastically! Knowing which material combinations you have is imperative to properly setting up your rigging and having it perform the best that it can under most conditions.
If you fail to take into account the temperature fluctuations, you risk serious damage to your yacht. Think about it, if Dyneema rigging on an aluminum spar have almost a full inch of variance between the two, if you setup your rigging on a cold day everything will become too tight during the rest of the year! As spring comes, the mast will get longer and the rigging will get shorter. By summer, your chainplates will rip through your deck or the tangs on your mast will crack!
To prevent such a catastrophe, you simply need to take this change in length into consideration and setup your rigging on a hot day. Not necessarily the hottest day, but a hot day none the less. As winter approaches, your rigging will go slack and no damage will befall your yacht. If you wish to sail in these conditions, you will need to adjust your rigging, and then adjust it back in case you don’t revisit your yacht before a warm day appears.
If you have an aluminum spar and steel rigging, the two materials change length in the same direction and almost at the same rate, this means that you probably will never notice any issues with temperature affecting your rig tune.
If you have a carbon spar, you should have Dyneema rigging for the exact same reason as an aluminum spar and steel rigging. The change will be in the same direction and roughly the same rate so that the temperature range of proper tune can be wider than it ever could be on an aluminum spar.
