[Public-List] Hemp vs polyester

[Gordon Laco]

One thing my student’s experiment confirmed was the academic description of how synthetic rope breaks.  Apparently a great amount of the energy being put into the rope is translated into heat when the rope is stressed to near it’s breaking strength.  Examining the broken ends of the rope back in the lecture hall that day, sure enough, we could see that the ends of the fibres were melted into little knobs.

We had occasion to rediscover the same phenomena aboard SURPRISE one time.  I merrily sailed her up onto a sand bar while heeled over… we were well and truly stuck.  Along came a friendly motorboat who offered to tow us off, so I passed them a line… 1/2” double braided nylon, actually two pieces joined with a reef knot.  I lengthened the reach so the motorboat would be in deep water.

The first thing that happened was the motorboat operator’s companion spread her legs and brace herself to pull… luckily the operator saw that and didn’t push his throttle.  Once the line was made down to the boat’s waterski towing point, they tried again.  

They pulled us off, but when we recovered the tow line we noticed two odd things.  One was that the reef knot was welded in place.  The rope was very hard and there was clearly no way to undo the knot.   The other was that the braided length of rope (the other was three strand) had a hard ‘crick’ in it ever couple of feet or so, which it retains to this day.  My guess is that the rope was near breaking and had started melting its fibres.

The odd thing is that the three strand half of the line was ancient polyester, it showed no signs of distress at all… but the nylon part, which was new double braid, was clearly about to break.

I cut out the welded knot to use as an exhibit when I do rigging talks… the kinked nylon got coiled up and has hung on a nail in the garage for the past fifteen years… I really shouldn’t trust it…

Gordon Laco
www.gordonlaco.com




> On Mar 12, 2020, at 11:51 PM, Michael Grosh via Public-List <public-list at lists.alberg30.org> wrote:
> 
> To add to Gord's great breaking strength story:  knot or splice...
> A boat I worked on years ago was having issues with lines parting
> prematurely, and the manufacturer wanted sample lengths to test to
> destruction. We made up about 4 6' lengths and off they went. 3 of those
> lengths were 2 tapered eye splices each end (a real pain in the neck to put
> together) and one was the way we always spliced them-3 tucks. I have never
> seen 3 tucks pull out, btw. Yes, I know the Coast Guard wants 4.
> Anyway, bottom line is, there was no difference in the parting strength of
> the samples, and all up to spec, the line parting issue was elsewhere.
> I have always wondered what that test machine looked like, this was 9" 3
> strand polyester (dacron).
> I would say about 75% of the time, assuming no cuts or chafe, lines wanted
> to part within 6" or so of the end of the splice. I always figured that had
> something to do with the strain of opening up the lay for splicing-which
> would be the arguement for tapering, but, testing, inconclusive.
> Michael Grosh
> #220
> 
> On Thu, Mar 12, 2020, 21:09 Gordon Laco via Public-List <
> public-list at lists.alberg30.org> wrote:
> 
>> Good morning Stephen -
>> 
>> The figure you quote is for flax, not hemp… and added to the disparity is
>> that braided rope is generally stronger by diameter than twisted.
>> 
>> Gordon Laco
>> www.gordonlaco.com
>> 
>> 
>> 
>> 
>>> On Mar 12, 2020, at 1:44 AM, Stephen Gwyn via Public-List <
>> public-list at lists.alberg30.org> wrote:
>>> 
>>> A quick bit of Googling suggests that 10mm hemp rope has a breaking
>>> strength of 760kg vs 2600 kg for 10mm Marlowbraid polyester.
>>> 
>>> SG
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