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Hi You'All,
HMDPE (Spectra and Dyneema) High Molecular Density Polyethylene is a remarkable material. With today’s new high-tech synthetic fibers and advanced rope construction, you can buy rope that’s 10 times stronger than steel with extremely low stretch. Many racers and cruisers have switched from wire to all-rope halyards; others have also opted for high-strength, low-stretch, lightweight ropes for their
running rigging. All-rope halyards have several advantages over wire. Hand-over-hand hoisting is much faster than all-wire; it’s safer, too. Wire is hard on your hands and gear. Rope is easier to splice, it won’t scrape paint or anodizing from your mast, and you don’t have to decide whether or not to rely on a worrisome rope-to-wire splice. Quality rope costs more than wire but is easier to install, lasts longer, and can be recycled in a less demanding capacity. Spectra and Dyneema are simply different brand names for the same thing made by two different companies.
Although Spectra and Dyneemra have significant advantages, here are serious disadvantages:
A frustrating disadvantage of HMDPE-based Spectra & Dyneema is that though the initial stretch of HMDPE is very low, if one leaves a HMDPE rope loaded for a long time it slowly but inexorably stretches, never to return to its original length. While this is not a problem in lines that are adjusted regularly, it is not appropriate in fixed lenght ropes such as shrouds. Therefore, Spectra & Dyneema are not recommended for Shrouds or Forestays.
The exact size and type of rope used is critical to the performance of any rope holding device. While some of the smaller types of cleat work well with laid (twisted) rope, the majority work best with a woven construction. Twisted HMDPE ropes do not incorporate covers to protect them against UV and so they deteriorate too quickly when used on sailboats that are constantly exposed to sunlight. Woven ropes bring their own problems. While the part of the rope in contact with the holding device is the cover, the bit doing the work is the core down the inside. Slippage between the cover and the core can be a limiting factor in holding power if the rope is not well made. Better quality ropes often have an intermediate binder layer incorporated between the core and the cover to help transfer the load; this is certainly a good thing to look for in a rope to be used in a highly stressed situation. The precise way in which the cover is attached to the rope is critical, as it is essential to ensure that the cover pulled down tight to ensure good load transfer back into the original rope. A neat taper into the original rope is also essential if the join is not to snag in blocks. In the case of lines that will have to be made off in more than one position (e.g., halyards for sails that reef) it is important to make sure there is a additional cover at every wear point. Under load, the core will slip through the clutch inside the cover, so the cover has to be sewn to the core in the area in which the clutch is likely to hold it.
Another serious disadvantage is that HMDPE rope is thicker, so it may cause problems for the KSC's boats which have halyard hardware for smaller lines. Replacing the hardware is a possibility though... Also, Spectra or Dyneema braided-core halyards often are stripped of their covers over the length that isn’t handled or exposed constantly to the sun, and the stripped part functions well in a clutch of appropriate size because it’s effectively now a single-braid and the HMDPE fiber is highly resistant to wear.
Also, one must avoid using knots to terminate high-modulus lines. Any knot or bend that comes under load lowers the breaking strength of any line in which it’s tied. In high-tech lines, the reduction can be dramatic. Knots can also slip out of slippery Spectra. All manufacturers and riggers recommend splicing—say, to halyard shackles or sail clews—as being far more reliable. The type of splice used depends on the lay of the line. Most rope manufacturers offer splicing kits and manuals, and the winter-bound sailor might consider periodically practicing with them. Otherwise, it’s best to have splices professionally made which removes one of the advantages of high tensile rope over metal. Splicing services for rope to hardware costs $50 to $100 per splice depending on the kind required. Therefore, use of Spectra or Dyneestra for halyards are a complicated affair and may not be appropriate for the KSC...
Vectran, is the latest in a line of high strength materials and incorporates the latest improvements to Spectra and Dyneestra. Vectran looks like an almost perfect rope maker's material. However, it is as you might expect frighteningly expensive.
However, there are applications where a little bit of give can be a good thing and where the KSC may use Spectra or Dyneestra. Anchor warps are the obvious example but there is a place for some shock absorption in other places too, like trapeze wires.
And that is my GOOGLE search for today...
SALUT! Pierre.
_________________
2015 H16, with spin,
SOLD 1989 Hobie SX18 Sail # 1947 "In Theory..."
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'
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