On Mar 27, 2010, at 8:09 PM, Bill Spencer wrote:
> >
> > If anyone else has tried Amsteel, it would be interesting to hear your thoughts about it's applications.
Amsteel is Samson's name for 12 strand Dyneema SK75. You can buy virtually the same line from a dozen different vendors, although Samson's marketing department are the only ones evoking "steel" with the naming and the color.
Dyneema and Spectra are both Ultra High Molecular Weight cross linked polyethylene plastics. They make a line that is very slippery, floats in water, is abrasion and UV resistant, and extremely strong and stiff. Its two negative points are that it has a low melting point (some racers do not like it for spinnaker sheets as it is possible to melt it with a quick "burn" on a winch) and it suffers from "creep", or stretch very slowly and permanently if kept constantly loaded. For this reason many racers will not use it on white sail halyards. There are low stretch and low creep variants, such as Dyneema SK78 and Dynex Dux which are more suitable for halyards. Dyneema is more tolerant of small sheaves and being spliced around shackles and the like - partly because of its creep characteristics - compared to other high tech lines like Kevlar or Vectran.
I use Dyneema for nearly all the running rigging on my boat except for sheets (which do not need the strength or stretch resistance). I am using it for the halyards as well, but I am having trouble with stretch (not creep) in the main halyard, which is loaded to about 4000 lbs.
Now, why use it in lifelines? I am using 8 mm Endura 12, a New England Ropes product very similar to Amsteel, except in a nice yachty white color! This has an average breaking strength of 13,300 lbs when new. It will stretch approximately 3% at the breaking point. If I remember my engineering correctly, a 30 foot length of this line will absorb around 6000 ft lbs of energy as it breaks, or a 200 lb man falling 30 ft.
Ordinary stainless steel lifeline installed on recreational sailboats is 7x7 construction 3/16 diameter vinyl coated 304 alloy, breaking strength quoted by the Loos Company is 3700 lbs. You could use 1/4 diameter, with a quoted breaking strength of 6100 lbs. Loos gives the stretch at breaking to be 1.2% for both. For the 3/16 wire, a 30 ft length will absorb about 670 ft lbs, or a 200 lb man falling a little over 3 ft. The 1/4 wire absorbs 1100 ft lbs, or a 200 lb man falling 5.5 ft. Just to remind everyone, the beam of a Nonsuch 30 is over 11 feet.
The lower figures for wire are due partly to the lower strength, but also to the lower stretch - energy absorption being proportional to the square of the stretch distance times the spring constant. A shorter length will absorb less energy because though it is stiffer, it will stretch less at breaking.
Dyneema lines also do not corrode, do not stain your towels you hung on them, feel good in the hand, and can be replaced easily with spare cordage on board. It does however, cut more easily with a knife than SS wire.
Also of note is that the ISAF offshore special regulations now specifically allow (2010-2011 rule) Dyneema lifelines:
3.14.6
TABLE 8
Lifeline Minimum Diameters, Required Materials, Specifications
a) Lifelines shall be of :
- stranded stainless steel wire or
- single-braided Dyneema® rope
Jon Fitch
'Anomaly'
