Thermal Expansion and Rigging Tension

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.

Bronze vs. Stainless Steel, The Solution!

Bronze is just as strong and a little softer than stainless steel. This means that components can be manufactured to the same specifications in either material which also means that they are interchangeable on the same boat. Their working loads and breaking loads for the same size of component will be close enough that they are still interchangeable from a structural standpoint as well. What’s the difference? Softness.

Isn’t softness kind of a downside? While bronze isn’t soft enough to cushion the blow when you stub your toe on a component on your deck, it is soft enough to avoid cracking when overloaded. Under normal loads, both materials operate on your yacht in an identical fashion. The difference only emerges when the two materials are pushed to their breaking point.

Stainless Steel is harder than Bronze, and this means that it will hold its shape all the way until the end when it cracks. Once the metal has cracked, it looses all of its strength and the component will fail in a catastrophic manner! Bronze on the other hand is softer than Stainless Steel and as a result will not crack and fail all at once. Instead, it will deform, bend, or stretch a little. It is now weaker but it is still functioning at a reduced capacity. This means that when a bronze component is pushed to its breaking point, it will not fail in a catastrophic manner, but instead will deform and wait to be replaced while still performing its duties as a necessary component of the yacht.

If bronze is so great, why do new yachts all use Stainless Steel? Well, that is more of an aesthetic choice. Since the components are interchangeable on a yacht, there is no real difference in having a yacht finished with Stainless Steel components compared to one finished with Bronze components. This means that when a yacht is new, either material works just the same. Does the term “Brightwork” ring a bell? Well it should because that is where the difference comes into play.

Brightwork on a yacht are all the components that shine and are pretty, but as the name implies, it also takes work. Lots of work! Brightwork refers to wood that is then oiled and varnished, and it also refers to all the shiny bits of metal on the yacht. Wood can be left natural, where it will dry and turn grey which is infinitely less work to maintain than the same piece of wood maintained “bright” by someone who has to constantly oil and varnish that piece of wood. Let’s be honest, it looks pretty but it’s so much work to maintain! Bronze, if left natural, will develop a patina on its surface. It will not corrode and this patina causes no structural damage to the metal, but it is dull and dark looking. By contrast, you could spend the rest of your days polishing all the bronze on your yacht to keep it bright! As you can see, brightwork is a lot of work!

Now to the business side of things. When someone is selling a yacht, the person offering to pay a lot of hard earned money for this yacht is literally handing over a lot of money for “something” in exchange. That “something” is going to be a lot of work (and there is no way around this part, boats are work!) and the more brightwork on the boat, the more that new owner is going to have to work to maintain its level of beauty! An astute owner knows what they are getting into and is prepared for that level of dedication to keep brightwork bright, but an uninformed buyer just knows that “it’s a lot of work” and might get frightened away by the unknown amount of work ahead of them!

Yacht manufacturers have adapted to make selling a new yacht easier. Just as bow and stern thrusters allow yacht brokers to say “it’s so easy, just like parking a car” which then allows them to sell an even bigger boat (that is way more expensive) to someone with no experience at all. By reducing the amount of brightwork on a yacht, they can then reduce the amount of apprehension about maintaining all of that brightwork from potential buyers! This is why all the wood trim has been removed and yacht decks are completely covered in slick gelcoat!

But there is one problem with removing all the brightwork; people like shiny things! If you take all the brightwork away, there won’t be anything shiny on the boat and you won’t catch the eye of potential buyers. So, the metal bits have remained on the boat, but instead of being labor intensive bronze components, they have been replaced by polished or electroplated stainless steel components. Why? Maintenance Free*!

Why the “*”? Because nothing is Maintenance Free on a boat. How often do you see pitted stainless steel components or rust streaks on the boat from a part that was supposed to be shiny? Stainless Steel is “Stain-Less” not “Rust-Proof” and if neglected in a salt water environment, it will corrode! But the important part is it happens slowly and over many years, allowing the owner to blame time and not lack of work for the deterioration of the boat.

So stainless steel replaced bronze on yachts simply because it looks prettier longer and that makes it easier to sell the boat in the first place! So many aspects of yachts have switched from reliability to showmanship because the manufacturers are more concerned with selling the boat instead of the boat performing as a sailboat. Think about what else has changed on a yacht:

• Cockpits got larger to allow more comfort while “hosting a party in the cockpit” while a true seaworthy cockpit is small to keep you safe and minimize the amount of water weight it will hold when the boat is swamped by a boarding wave.

• Salons now have huge “windows” to let it a lot of natural light while a large portlight is a dangerous feature on a bluewater yacht. Imagine if the “window” breaks, now you have a massive hole in your boat where waves can pour through to sink your yacht.

• All the interior joinery is crisp and sharp, like something from an IKEA catalog while in a seaway, such sharp edges pose a serious risk to your health. Imagine the boat being tossed around in a large sea way. As the yacht is picked up and tossed into the next wave, you are picked up and tossed right onto that sharp edge! Yacht interiors need to have rounded edges on all surfaces. Anything sharp or pointy can seriously injure you if you fall on it, while having a blunted edge makes it hurt a lot less (you will still get covered in bruises, it happens). The problem is rounded edges are not the modern design aesthetic in fancy house magazines, and fancy house magazine interiors is what they want in their fancy new yacht.

As always, when the mass market craves something, astute business people will find a way to provide it to them! Since everyone wants stainless steel for every part of their yacht, stainless steel is all you will find for sale everywhere you look. When you know better, you will become very frustrated because finding quality parts in bronze becomes an almost impossible journey to embark on.

Until now! 8 years after I bought my boat and resorting so far as to cast my own bronze components, I finally found a supplier who sells everything you can readily find in stainless steel, but in bronze! They offer components in both Silicon Bronze and Manganese Bronze, and in every size imaginable! The only catch is they are literally on the other side of the world if you live in North America or Europe.

Classic Boat Supplies is located in Australia, but they do ship internationally. This means that you will still have to wait to receive your bronze components. You can’t just drive over to West Marine and pick it up like you can anything made out of Stainless Steel! While shipping and waiting times are an issue, the important part is that you can actually get your bronze components and you don’t have to go on a wild goose chase with no end in sight!