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Old 29-03-2015, 11:40   #16
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Re: Ropework Question

Seaworthy,

I am not sure exactly what you are asking for. But to clear up a little bit.

Spectra and dyneema are trade names for the same chemical compound. That chemical is only made by a company named DSM and is then spun into different products, everything from dyneema cloth to spectra ropes, and sails. The different SK-## have to do with the specific formulation of the chemical which have gone up in number as DSM refines the chemical formula and makes the fiber stronger.

Typically dyneema ropes loose a substantial amount of strength in the first 24 months of use, with a flattening curve over time. It will loose roughly 40% of strength in the first 24 months, and an additional 10-15% of the next 96 months. What is believed to happen is that the outer portion of the fiber gets damaged, but stays in place an acts like a sheath to protect the inner fibers. Based on 8mm sk75

While this may seen like a lot of line strength (and it is) typically dyneema is so oversized for strength that it just doesn't matter. As an example, in this case a piece of 1/64 dyneema line is probably strong enough to hold the boat up, everything passed that is just extra. In this case Dockhead is planning on using 6mm with a MBL of 8,500lbs. Which is so far in excess of the strength needed who cares if uv degradation drops it back to 5,000lbs? It is still hugely overbuilt and will rip the davits off before the line fails anyway.

I keep trying to link to the MDS but fr some reason my iPad is being temperamental and won't let me. It is on page 5 of the DSM Dyneema fact sheet entitled "Ultra High Molecular Weight Polyethaline fiber from DSM Dyneema".

The coating information is from Sampson and is specific to amsteel blue, which is a trade name for SK-75 fiber with the Sampson coating Samthane S. The data on the coating comes from Sampson and is formulated for wear not UV. There may be a company making a braided line with a UV coating, but most people will replace the line for other reasons long before UV becomes an issue.
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Old 29-03-2015, 13:34   #17
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Re: Ropework Question

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Seaworthy,

I am not sure exactly what you are asking for. But to clear up a little bit.

Spectra and dyneema are trade names for the same chemical compound. That chemical is only made by a company named DSM ...
Not that it changes much, but I don't think it is correct that DSM is the source for both. There are a number of UHMWPE pellet manufacturers that supply the fibre makers. Honeywell use them to make Spectra fibres and DSM to make Dyneema. DSM don't supply Honeywell.
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Old 29-03-2015, 15:48   #18
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Re: Ropework Question

Honeywell does the manufacturing of spectra under license from DSM.
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Old 29-03-2015, 19:12   #19
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Re: Ropework Question

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Seaworthy,

I am not sure exactly what you are asking for. But to clear up a little bit.
I have an interest in knots and that leads to a slight interest in line as well. I had always thought that DyneemaŽ and Amsteel Blue were treated to UV stabilise them. I have been trying to look back and see why I was under that misapprehension .

The attachment in my previous post shows the data from DSM on loss of strength with outdoor exposure, but thanks for the extra info. I had previously assumed this was occurring despite UV stabilisation treatment (I just feel like an ass for assuming LOL). Interesting that it is not.

I agree that as Dyneema is massively over specified when it comes to most cruisers' needs, the 60% loss in strength of 8mm line over a decade is not a major consideration (for the same reason if knots can achieve at least 50% of line strength, this is usually satisfactory).

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Old 29-03-2015, 20:48   #20
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Re: Ropework Question

There has been a lot of testing on dyneema knots, and sadly everything comes back saying knots in dyneema seem to fail at around 40-45% of line strength. So reducing the number of knots is critical. The Estar knot has been tested to around 55% of line strength, so it is a real step up from the bowline, but it is still pretty bad.

Load testing
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Old 29-03-2015, 21:12   #21
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Re: Ropework Question

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Originally Posted by Stumble View Post
There has been a lot of testing on dyneema knots, and sadly everything comes back saying knots in dyneema seem to fail at around 40-45% of line strength. So reducing the number of knots is critical. The Estar knot has been tested to around 55% of line strength, so it is a real step up from the bowline, but it is still pretty bad.

Load testing
Evans has done some excellent work.
Yes, it is best to splice Dyneema, but sometimes it needs to be secured quickly and knowing which knots work with this material is then vital. The EStar is a hitch and a hitch may not be appropriate. The Water Bowline with a tuck has similar strength to the EStar and is a good option for a loop. I suspect a Water Bowline with a Yosemite finish would be as well (Evans tested a different extra tuck).

I disagree though that 50% strength is "pretty bad". More would of course be nice , but generally the strength of Dyneema line is way in excess of that required and cruisers are applying nowhere near 50% of the breaking load. A knot holding to 50% is usually way more than adequate.

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