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!
