Synthetic Standing Rigging is a wonderful system to work with. It weighs next to nothing compared to steel, yet holds your mast up with even more strength. The term "Synthetic Standing Rigging" is a generic term for standing rigging that does not use steel cables to hold the mast up. This broad category tends to include PBO, Vectran, Spectra, and Dyneema .
PBO is prohibitively expensive and tends to only be seen on race boats. It will not creep and is incredibly strong. It's major disadvantage is that it will quickly degrade when exposed to UV light. This is fine for sponsored race yachts where money is no object, but not very practical for the average cruiser.
Vectran is a wonderful product for standing rigging. It is in the Kevlar family and exhibits incredible strength with virtually no creep. While both of these features would make it wonderful for standing rigging, it is very susceptible to UV damage. The use of covers will help prolong its lifespan, but at the cost of higher windage.
Spectra is the same as Dyneema, but is produced by DuPont. They have transitioned over to government armor contracts and no longer produce rope for civilians.
Dyneema is produced by DSM and is the same as Spectra. Dyneema is made of Ultra High Molecular Weight Polyethylene (UHMWPE) and is my favorite material for standing rigging on a cruising sailboat. It is incredibly strong with minimal creep, but with very good resistance to UV damage. This allows the fibers to be left exposed to the sun, minimizing the bulk of each stay to minimize windage.
So you decided that Dyneema is the material of choice for your synthetic standing rigging. Wonderful! Then you go to purchase the line needed for your standing rigging and they ask "Which one do you want?" Instantly you are flooded with numbers and strange words like SK78 or Heat Set. Lets take a look at the different types of Dyneema.
Dyneema comes in many flavors, each with its own properties:
SK75 was the be the best form of dyneema available for a while, but we have come a long way since these fibers. Some still tout SK75 as the best because of its incredible strength and resistance to UV damage, but it creeps a lot. Creep plagued SK75, and spurred the development of the newer fibers that do not creep as much. Most applications for SK75 have been phased out and replaced by SK78.
SK78 is the improved version of SK75. It offers the same high strength of SK75 but with greatly reduced creep. SK78 has pretty much replaced SK75 for all uses on the boat and many manufacturers have stopped offering SK75 since the introduction of SK78.
SK90 is the improved version of SK78 and offers significant increases in strength but no improvement in creep properties. SK90 offers a 10 to 15% increase in strength which made this fiber the best thing since sliced bread when it was launched in 2009. It reigned supreme until 2013 when an improved version replaced it.
SK99 is the improved version of SK90. It was launched in 2013 and offers 20% more strength as compared to SK78 but still no improvement over creep. SK99 may sound like a miracle fiber, but its price is equally set. For this reason, SK99 is not seen as often as SK78 which offers incredible strength with a more reasonable price.
DM20 is a different class of dyneema which has nearly zero creep but less strength as compared to SK78. The lack of creep would make it seem like the ideal fiber for standing rigging, yet it doesn't seem to be the popular choice. This is because there are treatments that SK78 can receive to improve its properties.
Heat Treatment is part of the manufacturing process whereby the Dyneema is subjected to heat and tension which causes molecular changes in the fibers themselves. Dyneema is made of polyethylene chains. The heating process under tension causes the fibers to stretch out further and creates more crystalline structures in the fiber which give the rope more strength. It also creates longer chains of polyethylene which are able to bear more load than the shorter chains found in untreated fibers, which results in less creep. The end result of heat treatment is a much stronger rope with less creep.
This may sound like a magic bullet, but sadly, there are drawbacks. The rope becomes much less resistant to bending and abrasion. This is not a problem for standing rigging because they are straight and do not move; just be sure that any bends are distributed over an appropriate radius to avoid damaging the rope.
Lets take a moment to digest all of this information.
SK75 is strong but creeps.
SK78 is strong and has low creep.
SK99 is awesome but too expensive.
DM20 does not creep but is weak.
Heat treating SK78 results in low creep and incredible strength.
The downsides of Heat Treated SK78 can be easily overcome. Providing properly sized thimbles produces the proper radius turns needed and protective chafe coverings guards the line from abrasion.
For these reasons, Heat Treated SK78 is the best rope for synthetic standing rigging. The price is not too much of a jump from regular SK78, and its strength and creep properties are vastly improved.
I personally prefer New England Ropes STS-HSR which is Heat Treated SK78 and use this rope for my own synthetic standing rigging.
I'm sure as new technologies come to the market, these views will change. For now, this is the best product for the money and it will have a long service life as standing rigging if properly cared for.