|Posted on Monday, May 12, 2003 - 09:26 am: ||
Aha! If the line has the appearance of being melted, then the fibers may have become very hot during the rope-breaking process. We're talking nylon line here, or some other material that melts easily.
Think of all the tension on the line as potential energy stored in the elongation of the line. As the line breaks a portion of that energy gets concentrated at the break site. While the line is pulling apart, but before it completely parts, a lot of energy is available in a very short time. Enough to melt the line, perhaps. The rest of the energy whips the ends of the rope around.
|Posted on Friday, May 09, 2003 - 12:41 pm: ||
My experience has been that when a line breaks, it IS at the bend and it has the appearance of having been melted. There is apparently a lot of heat generated by the deformation of the molecules during breaking.
The above assumes line in good condition and NOT breaking at a weakened part of the line.
|Posted on Thursday, May 08, 2003 - 11:08 pm: ||
Your diagnosis of knot failure is commonsensical and I'm not sure that anyone has empirical evidence to the contrary. However, the fact is that the line does not break IN the knot, but just before the knot, which doesn't negate your conjecture, it just gives it an interesting twist. We have a rope manufacturer in OC and I visited them. The GM was not unfriendly, but not real happy with the visit. He handed me a photocopy of the standard list of knots with their designated strength in answer to my questions. The machine that tests ropes wraps a few turns around two opposing large drums so that no knot is part of the test. In answer to the question of testing knots, he said that they have done it occasionally, and then only to see what would happen. And he confirmed that the rope breaks before the knot, not in the knot. Frustrating. I sure would love to have a few hours with that machine. Of course we are ignoring the factors of age, fraying, salt exposure, sun exposure, etc. Tie a knot in an old line with dried salt crystals inside, what do you get?
|Posted on Thursday, May 08, 2003 - 09:56 pm: ||
Wow, that was informative and thought provoking. I must find a copy of Ashley, of whom I was ignorant of the existence of, previously.
For my intuitive two cents, I suspect that the thing that weakens the line in a knot or wrapped around a shackle is the degree to which the line is bent in relation to its diameter. The sharper the bend, the weaker it makes the line. Which is why thimbles work.
I would further suppose that this is because some fibers, outer or inner in relation to the bend, are stressed more than the others and they fail sooner, weakening the rope and leading to ultimate failure. I also suspect that rope fibers must slide past each other slightly as the line stretches to evenly distribute the stress. If they are captured in a knot or other sharp bend, they concentrate The stress in a portion of the cross-section of the line rather than distribute the stress throughout the line.
The fibers on the inside of a bend are probably in compression, so they leave fewer fibers on the outside of the bend to hold the tension. Something like that anyway.
I agree with Ed that the only thing that makes a knot completely shakeproof is a completely jammed knot, which probably really weakens the rope following the previous logic or seizing the line with thread, wire, or such.
|Posted on Monday, May 05, 2003 - 11:30 am: ||
The following is forwarded from Ed Hooper:
Everyone who works with knots should read the first chapter of Ashley,
especially all the authors who have come after him. We all know the game
where you sit in a circle and whisper a story to the person next to you, and
then he/she whispers it to the next person, and so forth, until it returns to
you, and it's not the same story. That's what has occurred with Ashley.
Everyone has a part myth, part truth, part concoction of what was actually
written. There are TWO parts to knot strength: knot security - does the knot
fail, and knot strength - does the knot break. Most authors simply state a
knot's strength with one percentage number, such as, for intance, that a
bowline retains 82% of a rope's strength. This has been copied from one
author to another (of course, they all change it slightly) in a line somehow
stretching back to the original tests done by Ashley. Ashley counted
bounces: how many bounces does it take to unravel a knot. He also tested
line with a knot in it until it broke, noting that the break always occurred
just before the knot, never in the knot. And made no other comment. My
issue is that most people now learning knots are mislead into thinking that
they can ask the question: how strong is that knot?
A knot degrades rope strength. Do different knots impact ropes
I do not know. Intuitively, I want to say a bigger knot is badder, but I
suspect that the simple fact of a knot, any and all knots, are equally
degrading to rope strength. The old time sailors used common sense and
experience, and I think we are stuck with them, because the knot authors do
not help us here. Most of the ropes used in sailing have a strength is eight
or nine times maximum tension. Under moderate load a knot by my guesstimate
knocks off, say, 10%. That's one knot, first usage, removed after usage. If
the knot is not removed and stored, then the fibers will be moulded into the
shape of the knot and become weaker if asked to move in a different
direction. Take off another10%. If the load is more than moderate, the rope
is then stretched, including the part in the knot. We all know this one: the
rest of the rope elastically returns to its original shape, the knot now has
a brittle portion with the load stored in it. If this rope is returned to
load, it's elasticity is now downgraded by the position of that brittle knot,
and that knot is now a weak point, maybe terminally so.
Knot security can be determined by this one fact: if it is easy to take
apart, it will come apart easily. A knot can be made more secure by adding
loops and hitches: if it is more difficult to take apart, it will come apart
with more difficulty. If it cannot be taken apart, if it jams, it is 100%
secure. You could probably make a good case that a jammed knot is
inherently weaker than a non-jammed, but it will not come undone.
My pet peeve is the bowline on the jib clew. I've had many tell me that
the bowline is THE knot that maintains 100% of rope strength. Why, they say,
I've had bowlines tied in my jib sheets for years and never had a problem,
never seen one of them break. I like a bowline in a jib sheet, but once it
is stored in the line, it morphs into a splice, a bad splice. It's moulded,
it's jammed and it weakens the sheet. It's just that the load never comes
close to the rope strength, so the knot never fails.
The right hand, left hand, same, oppie for the Sheet Bend does come from
the days of heavy usage of twisted lines, some with a right hand twist, some
with a left. Ashley tested "the principle of the knot"(not his principle),
which states that "no two parts which move in the same direction, if the rope
were to slip, should lie alongside of and touching each other." The Sheet
Bend with the ends on the same side violates this principle and the oppie
complies. Ashley's test, however, demonstrates that the oppie "has a poor
nip and is unreliable." A fact lost in the shades of time.
The name Larkshead is disparaged by Ashley on page 11 as being a name for
the Ring Hitch or Bale Hitch taken from a discredited British "Book of
Knots," printed in 1866, containing many errors and copying knot names from
the French, such as this one, tête de alouette. Imagine British Tars using
|Posted on Thursday, April 24, 2003 - 10:10 pm: ||
By Anonymous on Monday, May 05, 2003 - 10:15 am: