Repairing Dyneema Rigging

One of the shining features of Dyneema over steel is the fact that Dyneema can be repaired.

Chafe is the biggest problem facing synthetic rigging, and one that is easily avoided by routing the running rigging in a way that it won’t contact, and then chafe, the standing rigging. Should chafe occur and become serious, Dyneema can be easily repaired with mending splices to insert a new portion of Dyneema replacing the damaged portion.

After 6 years of sailing, our synthetic standing rigging has suffered no major chafing or damage, but our synthetic lifelines have! Everyone always worries about the stays chafing, but the workhorse lifelines are neglected to a life of chafe and abuse! Fenders are tied to them, gear is laid upon them, sheets and sails rub on them! Miles and years go buy where these poor little lines are subjected to abuse right next to the prized standing rigging that is shielded from all extraneous duty.

After all those years, while in Gibraltar, I decided it was time to carry out repairs on the two areas which had chafed. One was due to the windvane control lines tugging harshly on it, which I repaired the proper way; the other was subjected to a sharp metal bicycle pedal rubbing on it the entire way across the Atlantic!

The second repair was carried out in a more “creative” and less “professional” manner. The reason for the variation in repairs is simple: I wanted to see how well both repairs would hold up to similar abuse? Both repairs were done on the same day, so their subsequent wear would be similar. The proper repair is the control, as this is how repairs should be carried out, while the “creative” repair is the variable being measured against the control.

The correct repair is carried out using an end to end splice which mends the chafed area by replacing it with new Dyneema. This method is relatively simple to do, creating two locked splices and requiring access to at least one free end. There are four tapered tails that need to be created and buried and the whole process is rather lengthy, but yields an impeccable repair which is as strong as the original line.

The creative repair is able to be carried out in a situation where you do not have access to either end. A mending piece of Dyneema is inserted into the chafed line using two long bury splices. Additional locking stitches are needed to properly secure the splice as it is not locked on its own and could easily be pulled out if someone wanted to (or if gear snagged on it just the right way). This method only requires one splice and two tapered tails, making it quicker and easier to perform.

After crossing the Atlantic again with these repairs and carrying lots of gear on the deck which chafes on the lifelines, I can say with confidence that both repairs are holding up the same. The end to end splice does look a lot nicer while the creative repair looks a little shabby.

I personally prefer the end to end splice as I prefer locking splices over splices which require locking stitches to secure them, but if you have a situation where there simply isn’t the required free end to fix it, know that this other method will work well to repair your lifelines!

Not All Dyneema is Created Equal

Dyneema is a wonderful modern fiber that is incredibly light and incredibly strong. It can be used for all sorts of different purposes, ranging from anchoring giant oil rigs, mooring huge cargo ships, mining, logging, and even standing rigging on a sailboat!

The only issue with using Dyneema for your specific application is you need to be informed about “which” Dyneema you want to use. It has been many years since the first generation of Dyneema came to market, and since then it has gone through many evolutions as particular needs were found, and then met.

Saying “I’ll rig my yacht with Dyneema” is the same as saying “I’ll have a dog guard my house”. Are you talking about a Chihuahua, a Pug, or a German Sheppard? Some dogs are better for specific jobs and the same holds true with Dyneema.

As Dyneema products improve, their prices fluctuate. What used to be the latest and greatest comes down in price as the new “best product” takes the high price on the market. This makes it tempting to buy “cheap Dyneema” for your rigging, but it is important to look at what kind you are getting.

An early form of Dyneema is called SK-75. This rope is incredibly strong and lightweight, but it doesn’t do well under a constant load as it will begin to creep (rather heavily).

SK-75 was then replaced by SK-78 which had the same properties as its predecessor but with improved creep resistance. SK-78 was then improved upon even further with an entirely new generation of material called DM-20.

Naturally, SK-75 is very inexpensive, SK-78 is a little more expensive, and DM-20 fetches a higher price. All three fibers are single braid 12-strand rope, so why can’t you use them interchangeably?

This will all become apparent when the three different types of Dyneema are subjected to the same load at the same temperature.

The three fibers were subjected to the force of 300 MPa at 30*C and the creep was measured.

• SK-75 creeped 0.02% per day

• SK-78 creeped 0.006% per day

• DM-20 creeped 0.00007% per day

The percentages seem small, but on a boat, the “per day” is indefinite! Rigging is always under a load and every year is 365 days. In one year SK-75 would creep 7.5%. If your stay is 20m long, you are talking about it creeping an additional 1.5m (4.9 feet)!

Steel standing rigging has an expected lifespan of 10 years, that’s 3650 days. That would be 73% creep! This sounds pretty extreme but the stay would have failed from a creep rupture long before ever reaching that point, meaning it would fail long before the 10 year mark. SK-75 tends to fail at around 50% creep, which means you could expect it to fail in about 6 years.

SK-78 creeps significantly less than SK-75, about 3 times less actually. This means that it will hold your rigging and avoid creeping out of control before your eyes! Over 10 years of that grueling experiment, you can expect the a stay to creep 21.9%. This is significantly less than with the older generation of SK-75 fibers and really good news to someone who uses this as their standing rigging. SK-78 tends to rupture from creep at around 30% creep elongation, meaning that SK-78 would outlast steel rigging (if a yacht were the torture chamber that was this experiment). If your rigging lived in that torture chamber, it would be expected to fail at around day 5000, or 13.7 years into the experiment!

Naturally, one can expect that DM-20 raises the stakes for what is considered ideal The creep resistance improvement from SK-78 to DM-20 is 85.7x better (from 0.006% per day to 0.00007% per day)! After 10 years in the torture chamber, the stay would have creeped a mere 0.255%. That is practically nothing! In our imaginary 20m stay in this torture chamber, the stay would only creep 0.05m (1.96 inches) over 10 years! Compare that the the 1.5m of creep from SK-75 in the first year!

While SK-75 tends to rupture from creep at around 50% in torture tests and SK-78 tends to rupture around 30% from creep elongation, none of the tests that I have seen has managed to cause DM-20 to fail from creep. 30*C is wonderful for accelerating the flaws of creep, yet tests at 70*C (which manage to rupture SK-75 and SK-78 in a few days) failed to cause DM-20 to fail. After 6 months, the test was ended without a creep failure in DM-20.

While you can find “cheap” Dyneema to rig your yacht, it is worth the extra expense to buy the right kind of Dyneema for your yacht. Creep will not be a concern and you will be able to rest and relax knowing that your rigging is incredibly strong and secure, even against creep!

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.

Dyneema and its Coefficient of Thermal Expansion

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.

Dog-bone Dyneema Soft Shackle

Want to replace your steel standing rigging with synthetic standing rigging? Easy!

Simply swap out your steel stays for Dyneema stays.

Now all you have to do is swap out all of your bronze hanks that will chafe on the synthetic stay with soft hanks. These with dog bones are very secure and incredibly easy to operate!