lashing distance for high mod lifelines

[seawolf]

Hello all,
Is there a standard distance between the lashing thimble on the end of the line and the lashing point that is good to use when lashing amsteel, or other hi mod line to ensure enough room for take up and creep, and not to much for a lashing to be to long and unattractive.
It would make final line measurement easier if there is a proven absolute or formula depending on length and diameter of the line.
Thanks and fair winds.
Bob

[jfranta]

Hi Bob, we have done some extensive research and testing on dyneema lines and it's sister products like Spectra so I feel qualified to answer this.

Like most questions should be answered, it depends...

Creep is a time weighted parameter, that is, you need load for a period of time to get it. For sailboat applications we mostly use pretension in designing for creep. For offshore boats that can see high dynamic loads for extended periods we factor in those loads also when sizing the line. We generally size line for approx .1" of creep per year (1" in 10 years). Before I get too long winded, if you use 5 mm or larger dyneema you would need around 500 lbs of preload on your lifelines to see any significant creep. Your lifelines will obviously not see anywhere near those static loads so you will not see any creep.

If you use non heat stretched line like amsteel you will get some constructional stretch, which is what alot of people have called Creep in the past and this is dependent on total line length. Constructional stretch will stop however, unlike creep but you still have to allow for it in the lashing length.

We generally use about 12 inches for lashing line distance in lifelines, really just for a good lashing but we mostly use 5 mm Dynex Dux (heat stretched) line as we don't have any constructional stretch and only have to deal with the resetting of the splices for length measurement. The allowance for constructional stretch really depends on the line diameter.

One other thing I would like to say about lifelines is to be careful about the stanchion holes and any sharp edges for chafe on the line. Take advantage of the high break strength of Dyneema and use the largest diameter that you can put through the holes in your stanchions, this will give you the largest safety factor on your lifelines. You will see any chafe issues well before you run into any strength limitations. Dux is great stuff for lifelines as it is the most chafe resistant line you can buy which is why we recommend it, especially for lifelines, which are aptly named "life" lines. 5mm dux breaks at over 11,000 lbs so you can cut halfway thru it and still be stronger than 3/16 1x19 wire, lots of room to feel safe there even with chafe/UV issues.

By the way, we use ceramic knifes when we splice Dux and we have to sharpen them after about 10 splices. These are knifes that the manufacturers say never need to be sharpened. I haven't kept track of how much amsteel or normal SK-75 line we can cut but I know it would be alot more than 10 splices. We literally have to saw thru Dux with a very sharp knife so the difference in chafe resistance (from non heat stretched sk-75) is remarkable.

Sorry, I did get a little long winded here.

John Franta, Colligo Marine

[seawolf]

John,
Thank you for your reply.
The information was very helpful, and absolutely not long winded. Just very informative.
Fair Winds.
Bob