Author Topic: Cordage(incl.tape) -- Structure & Materials  (Read 3300 times)

Dan_Lehman

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Cordage(incl.tape) -- Structure & Materials
« on: July 26, 2009, 05:51:05 PM »
It seems that in many discussions of knots, especially regarding that one datum
that seems (illusively, I assert) to be objective --viz., knot strength-- , there is a
big omission of an important factor:  material -- meaning both the type of
fibre(s) and the type of construction (lay or braid) of the cordage.  And here I
want "cordage" to include webbing/tape; i.e., I intend to discuss all knottable
media (what else is there to string/cord/rope/tape ?).

I'm also getting the impression that much of knotting literature that comes to
us gift-wrapped in nautical cover has drawn from the behavior of medium-lay
natural-fibre ropes; that the diversity of cordage in general usage some half-century
ago was vastly less than is the case today.

Among common cordage today are:
 springy/slick/flexible laid PP ropes;
   "  <ditto>  "  hollow-braided PP ropes;
 slick/flexible polyamide (nylon) solid braid cords;
 moderately firm/stiffish/smooth-slick nylon kernmantle ropes;
 bending-resistant/slick/firm monofilament nylon fish line;
 soft/flexible/slick nylon/polyester braided yacht lines, dock lines;
 soft/flexible/slick soft-laid nylon dock lines & general-purpose rope;
 1" tubular nylon webbing;
 half-inch solid/soft/lubricated(!) polyester cable-hauling tape;
 soft/flexible/slick/12-strand hi-mod (HMPE, Technora, Vectran) ropes;
 firm/stiff/slickish kernmantle "static" (low-elongation) ropes;
 moderately flexible/slick/12- & 16-strand arborist ropes;
 firm/slick/braided nylon or polyester boot/shoe laces.

And ... ?

But there is some continued naive belief that the knot-strength attribute
attaches to the general knot qua schematic structure (to stumble for
appropriate terms -- need a Nomenclature thread in Knot Theory!).

On this forum, we had Paul Kruse report performing breaking-strength tests
on a moderate-sized (3/4" - 1" or larger?) soft-laid nylon rope and seeing
the Butterfly eyeknot  break at lower forces than did a bowline (and both
about 50% or less); whereas Dave Richards's testing in 3 kernmantle ropes
saw the Butterfly do quite well (70% or better), and significantly better
than a bowline.  Now, there is an issue as to the formation AND the loading
of the asymmetric Butterfly, but, still, those are pretty dramatic differences,
both in relative order, and in stated break strength (% of tensile).
And that, we should note, is for but one type of strength -- that of new rope
on a slow-pull, steady extension testing device.  This isn't all such a good
model of how knots are stressed in practice, where repetitive and sudden
loading is more likely the force acting on the knot, and also where some
abrasion might affect the knot after months of usage.

I have been a scavenger of "play ropes" for some decades, and now have
quite a collection; it impresses me how little of all of that haphazardly gotten
material is duplicative.  To be sure, there are repeated finds of like PP laid
ropes, and some hollow braids, and solid braided nylon.  But e.g. beside
me now is a very hard-laid PP rope, 20yrs? old, no signs of UV damage
(most of my ownership has seen it in dark, admittedly), and no concession
to being spliced -- "just try opening my lay, Mr. Marlinespiker Man !" (well,
someone did, but for just two tucks of an eye splice)  Then there are two'
CoEx 5/8" soft-laid ropes, one with nice soft-feeling fibres and the other
with rather coarse, manila-like fibres (said to soften with usage, but this
rope is pretty new), and another that I think is CoEx flat fibre & not
the thinner-fibre, fibrilated/split-film PP (which ropes can seem almost
weightlessly light!), that is of a slicker, waxier feel.

Further:  I have poly-combo ropes of interior monofilament (increasingly
more often non-round fibres) PP wrapped by multifilament polyester;
but also a square-ish cross-sectioned one with black PP (also another
of blue-green CoEx) mono.fibres running S (left-laid) and white multi.fibres
going Z (right-laid); and a like-looking B&W one where these same two
fibre types are set adjacent and is an 8-strand diamond (over-under-o-u...)
braid (hollow but rather firm, considering).  And there are fairly flexible
low-elongation (caving) kernmantle ropes vs. some intractably stiff ones,
of nylon -- where the stiff ones refuse in manual force to bend around
one diameter, resisting even at a 2-dia curvature!

Thin (8-6mm) "dental floss" climbing slings of HMPE fibres (with some nylon
mixed in, I hear -- details aren't so easy to get) have a much more square
cross section than solid or tubular 1" tape that used to be common in
the rockclimbing field; these new tapes are so far available only in sewn
slings, though, so knotting amounts to tying Clove Hitches or Overhand
eyeknots or "Girth hitching" sling-2-sling (so far!).  But the Overhand,
"Water knot" looks much different in this relatively thick HMPE tape
than in the quite flat 1" or even 9/16" tubular tape.  So, shall we
make unqualified assertions about that knot?  (No, but it's done.)

---------------------

Part of the research in determining what knotting is done by the various
user groups (commercial fishing (and groups within this), kermantle-rope
users (SAR, caving, rockclimbing, canyoneering), medical, angling, ... )
is getting a good understanding of the materials in which they work.
(Also, one needs to understand working conditions & demands.)

In the sailing world, where usage dates back centuries through various
materials, there seems to be not much concern for the Bowline needing
some additional precaution against coming untied or slipping; but in the
kernmantle-ropes environment, users are well advised to not use an
unprotected Bowline!  Hmmm, in the commercial-fishing world of which
I have some insight, many/most knots are given some end-securing
addition, though I have also shown unguarded bowlines (sometimes
capsized).  No doubt, similar distinctions are needed for other knots.

--dl*
====


Sweeney

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Re: Cordage(incl.tape) -- Structure & Materials
« Reply #1 on: July 26, 2009, 06:34:54 PM »
Dan

Can I add one to your list, please? Tubular polyethylene with usually a PP core (was sisal years ago) diameter about 2 - 4 mm used (in the UK at least) for washing lines both post to post and the rotary kind - also available with a steel core consisting of a number of mild steel (not stainless) strands laid in parallel. This is probably the most widely used cordage there is in terms of yards consumed and number of users as just about every garden has a piece yet I think most knot tyers would look at it with disdain as not being "real" cordage. It gives rise to that well known knot, the 37 round turns and 14 half hitches seen regularly around posts hereabouts.

Barry