'Tippet knots...how weak?

'Tippet knots...how weak?

Tracy&James | Thursday, 26 October 2017

My Instron is up and running again now, apparently during the routine electrical safety testing there had been a mix up between circuits and the wrong fuses had been replaced. This meant that my machine instantly blew the fuse it was on, whereas some other equipment wouldn’t have had the protection it needed – so the safety testing actually introduced a hazard, I’m sure there’s a lesson in there somewhere.

Anyway, I thought I’d do some knot testing, specifically tippet knots i.e. joining two identical pieces of nylon together. My set up for these tests involved two metal bobbins around which the nylon could be wound, without knotting. These were held in the Instron mechanical tester where force and extension were measured as the bobbins were driven apart. I’ve mentioned in my previous mechanical testing FPs that I’d prefer to ‘shock’ the leader material in order to replicate the kind of failures usually seen in a fishing scenario, however I’m bound by the maximum speed of the machine. As such, each test took a couple of seconds to break the nylon.

For these tests I found a jumbo spool of nylon that I was willing to sacrifice, this was marked up as 8lb ultimate tensile strength (UTS).  Firstly I performed some ‘control’ tests i.e. lengths of material containing no knots, the results from these are shown below.


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There are a couple of things to note or clarify about these results.  Firstly you’ll notice that the applied force does not quite start at zero, this is simply the mass of the lower bobbin that is suspended on the test sample and, as such, is real and accountable.  You’ll also see that there is some horizontal shift in the data, this is because the lower bobbin was attached to a pivot – this needed to be pulled tight vertically before any additional force (above that of the mass of the bobbin) could be applied.  This arrangement allowed me to take up the inevitable slack, thus preventing the nylon from unwinding itself from the bobbins.  The horizontal inconsistency is therefore due to differences in the amount of slack during set-up and doesn’t affect the results in any way.

 

The data from the control tests was very consistent with UTS values of 39.6, 40.6, 41.4 and 40.7 Newtons recorded, thus averaging 40.6N equivalent to 4.1kg or 9.1lbs (indicating the UTS was slightly understated on the spool).  The breaking strain, measured from the point where the lower pivot joint pulled taught (and the load increases rapidly), was typically just over 0.5 (50% stretch before failure) i.e. a ~80mm extension from a ~150mm original length.  (If you want to call the UTS figure ‘breaking strain’ then that’s up to you – I’ve written about the common misuse of this term previously).  Above approximately 30N it is clear that the gradient of the curves change significantly, this is typical of polymeric materials where plastic deformation, i.e. a permanent stretching, begins to dominate over the previously (mainly) elastic response. 

 So on to the tippet knot testing.  The testing arrangement was identical to the control samples described above apart from the insertion of the knot under test, mid-way between the two bobbins.  Three knots were initially trialled; a loop to loop connection using two perfection loops, the 3 turn water knot (the one I currently use when fishing) and the double grinner (or uni) knot.  In each case 4 individual tyings were pulled and in all tests the failure was observed to occur in the knot.  It was also noteworthy that all breakages occurred whilst the nylon was still in the elastic response region of the force/extension curve.

The results from these tests are tabulated and charted below.  I must admit to being a little disappointed in the performance of these knots, especially the one I actually use.  The loop to loop knot exhibited the best average but still lost 35% of the nylons strength.  The other two knots both lost 40% on average.  The most inconsistent of my knots was the 3 turn water knot (the one I’m most used to tying!) which recorded both the highest and the lowest individual knot strength, although the mean was in-line with the double grinner.  Seeing these rather non-impressive results started me thinking about the strength of a leader containing a ‘wind-knots’ so I added a single overhand knot into the experiment.  This was formed in a single piece of nylon rather than two pieces joined.  This again indicated a 40% reduction in leader strength.

 

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I intend to carry on with this testing next week with the figure of eight knot, the double blood and any other knot that is suggested.  I’m hoping that perhaps one will shine above the others, but perhaps I’m being too optimistic?  In the meantime perhaps the best option would be to avoid tippet knots altogether so if any tapered leader manufacturer wants to start making bonefish leaders with a built in level tip of 6ft or so (to make the total leader up to ~15ft) then I’d be your first customer.

Let me know if you have any suggestions for a knot to test and I’ll include it in next week’s results.

James.