Is Dymax = Dyneema?

[Captain Kidd]

I'm wanting some 1/8" dyneema. I see this on Amazon. Described as 100% sk65 fibers. Is this a brand name for dyneema? Or a inferior knockoff? Seems I read about some sk78 fibers somewhere.

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[Charles Brennan]

Dale, I'll give it a shot! 
Dyneema was the first of the Ultra-High-Molecular-Weight Poly-Ethylene ropes (UHMWPE) to get manufactured and marketed, so much like Kleenex and Formica, the brand name eventually became a generic description for the product. A good example of brand names getting appropriated by different manufacturers, is Amsteel calling their UHMWPE rope: "Amsteel Dyneema"
Dymax is just another UHMWPE rope made by a different manufacturer, no different than say, another manufacturer making tissue paper, instead of Kleenex.

SK-XX:
Dyneema fibers are produced in several SK ranges, including SK60, SK65, SK75, SK78, and the newer SK99.
Basically, as the number kicks up higher it has an increase in tensile strength, at the cost of a little more stiffness.  Another factor in UHMWPE SK ratings is "creep", a measure of how much the line moves over time.  Since Dyneema type lines are very slippery, you have to tie it with special knots like the Estar knot:

https://www.animatedknots.com/estar-stopper-knot

Fun experiment:
Tie a  reliable jamming knot, like a square knot with two pieces of Dyneema and then pull!   
Yeah, slippery.
For that same reason, eye-splices need to be "locked" with (for example) a locking Brummel splice and further, require much longer "buries" in the eye splice.

Those SK-XX numbers are generally of far more interest to high-performance racing sailors and larger boats, than for boats in our size range.  In practical terms, you could use most any SK number UHMWPE with complete safety on boats in our size.  Another consideration for you to think on, is that ⅛-inch Dyneema type ropes are a little stiff to splice at that diameter, for example, typically requiring a doubled over wire instead of a fid, to do splices.  In your case, the SK65 rating would be much easier to deal with, in ⅛-inch diameters than say, SK78.
In summary, the SK rating is a measure of characteristics, not quality.

Hope this helps,
Charles Brennan

[Brian N.]

Wow - Thanks Charles. You always are so well informed. I am not planning to use Dyneema anytime soon, but the info is interesting. What I do know about UHMWPE (Spectra and Dyneema) comes from archery bow strings, another hobby of mine.

[Doug SC]

CB is spot on as usual. I have done some stuff with Dyneema lines and soft shackles. Not just sailors but ultra lightweight backpackers also use the lines to save weight with their hammock setups. The locking Brummel and long (importantly tapered) buries are needed when splicing. Lots of YouTube videos on how to do this.

[Doug SC]

(Spectra and Dyneema) comes from archery bow strings, another hobby of mine.

I also use the material in the bowstrings I make. I do Flemish strings and add more material to the loops that taper into the string for my longbows and recurves just to spread out the concentrated force on nocks (the bow tip ends).

[Captain Kidd]

Charles,

A good shot! Answered my questions. I have a couple of uses for it on Disciple Ship where I already used some. I have a line that runs under the inwale for tying off things. I have used it for lashings on my boom, etc.

I have used animated knots a good bit in the past and will learn the knot you referenced. I've mostly used a double fisherman's bend to create a loop for the lashings. Nothing has come loose yet: knock-on-wood.

thanks!

downhaul top and bottom

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Under the inwale: this line is not dyneema but I plan to replace it.

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[tjspiel]

I find it very confusing as well so that was helpful. Planning on replacing my lifelines with Dyneema this Spring.

About 15 Years ago, I bought an inflatable catamaran that packed up in two 6 foot bags you could shove into just about any vehicle. It had Dyneema shrouds and I loved how light and easy to pack up they were. No worries about kinking them.

Considering eventually replacing at least part of my standing rigging with Dyneema but have been cautioned that temp changes can cause problems because Dyneema shortens as temps go up and lengthens as temps cool down - which is the opposite of what your mast does. So if your rigging is properly tensioned in July, if you're sailing in very cold temps late in the season, it could be too loose.

For trailer sailers the difference between Dyneema and Aluminum temperature expansion may not make enough difference to worry about. Not sure.

In any case, for trailer sailers, Dyneema rigging would certainly have advantages over stainless in terms of storing and stepping masts.