What happens in reality is that the friction is so much higher, that there is virtually no abrasion against the attachment point. All movement is contained in elasticity in the two legs of the loop, that distorts a bit when the pull changes direction slightly. There is less abrasion, because there is less movement over the surface.
it doubles the surface area of the rope contacting the ring,
therefore it halves the pressure.
No & yes : it doesn’t halve because of doubling surface area,
but because of taking the two eye legs around the ring at two
points. (Make that round turn a triple round turn, and you’ll
see that you cannot reduce pressure by this increased surface
contact!) Or, a better way this, it is the area directly opposed
to the force against the ring being doubled that matters; the
other area doubled (in the structure) doesn’t matter.
the friction is so much higher that there is virtually no abrasion …
No, this can’t be : the friction is less or irrelevant --there will be MORE
movement, from availability of material between the eye legs for them
to draw out (when they are not immediately connected to each other
as in the simple case). But as noted above, the pressure will
be less in now two points of opposition vs. the one.
–dl*
As this is the Practical Knots section, I shouldn’t theorise too much. It works, you may try it for yourself and make up just any theory of why it works, but it works. Boats that are moored for months on end chafe their attachment points of the rope without that extra round turn, but with a round turn, there is virtually no chafe.
I made a videoclip to show how it behaves: http://www.youtube.com/watch?v=mEFvEiD8ANA
Great video to demonstrate your assertion Inkanyezi.
alpineer
While I accept the principle that less chafing will happen if the tree is given a round turn, the duration of the felling event is not likely to reflect the sort of chafing exposure seen by a boat moored for several months.
The realities of felling however might lead us in a particular direction when selecting bindings for this job.
Lumber is HEAVY, particularly when it has gained inertial from gravity and especially when this is further enhanced by wind in the branches. The purpose of the guide ropes then is to ensure that these potentially massive forces are met and overcome in order to ensure the tree falls where it is safe.
With this in mind, every knot weakens your rope, so don’t knot the loaded part of the rope. Add to this the possibility/probability that you will not be able to climb the tree to make a tree fixing and you have the basis of an idealised system.
First up, pass a guide cord over some bough high in the tree and use it to haul the working rope up and over the bough and against the trunk.
Bring both ends down to the anchor point - we now have a double line to the tree, potentially halving the load in each line.
Make four or five turns of each rope around the anchor and tie them off to their SP or to a handy bough. If you make the turns of each rope in opposite directions, then the ends can be tied one to the other with a simple square knot. Given enough turns, virtually none of the force from the falling tree will reach the knots in the rope’s ends, so untying will be straight forward, and the risk of overloading the rope and snapping it at an inline knot is eliminated.
A handy trick I have used is to make a Prusik with a short loop of cord and slide this up the guy ropes to roughly the mid point, with a hauling line attached to the Prussic loop. This will allow you to add some serious leverage force to the guy ropes by hauling on them at right angles to the guy lines, and because the guy lines are not knotted to the prusik, the guys are not weakened by any unnecessary inline knots.
One final note is - make sure the anchor is man enough to take what the falling tree throws at it… You do not want the anchor ripping out and following an errant tree on a mission of destruction.
Derek
Hi Derek,
the Prusik at mid-line is a good idea, but so far one that I have not found necessary. If I run short of bull-rope I may try an approach to that need that is at least similar. Perhaps a hitch of some sort that is good for perpendicular pull, for affixing the mid-span attachment of additional line.
But since we’ve delved deeper into the felling subject, I feel compelled to mention that whoever does this task, they should be competent.
The bull-rope should be there to coax the load in the direction of chosen fall. Most ropes or at least the ones around the most homes are nowhere near rated for limiting the falling weight of a sizable tree. Even with going around and around both tree and anchor point(s).
Notching the tree properly is key and most felling is accomplished without rope use.
And yes the mid-line knot will be the location of the most chaffing (as I described in my previous post), but that is why a suitable rope is used. And when that rope is abraded to the suggested (common or uncommon sense inserted here) limit you turn it into something else less demanding.
All that said, I would not use a rope to arrest the leaning/falling of a tree at all if possible.
Scott
Oh Derek, the round turn mentioned was for the towing. Tree felling is a completely different animal that shouldn’t be undertaken without some knowledge of how to fell a tree in the direction you want it to fall. I was fourteen years when I learned to handle a chainsaw and fell trees, and so far, all of them fell exactly where I wanted. You certainly know why.
Usually, when there’s no wind and the tree is straight, it is easy. By removing a wedge-shaped piece (notching) in the direction you want it to fall, and then sawing almost through from the other side, it is usually easy to make it fall where you want it. The difficulty is when the tree is leaning some other way or the wind wants to push it the wrong way; there’s where we need guide ropes and perhaps also have to haul on the tree to make it lean over to the side where you want it to fall. And then of course, if you have a line attached to haul, it makes sense to pull with another rope at right angles to that rope somewhere in the middle, to make the tree fall, so you’re out of harm’s way. I have done that too, and in similar situations, although it wasn’t a house, but electric cables that were endangered.
Okay, this isn’t quite the circumstance I had in mind
–this, to a spar/pile, and I was thinking of an ring,
such as on an anchor; and I was only considering the effects
of tensioning, not shifting angle of incidence.
The behavior shown in the video (where it kinda seems like
the upper rope is given more wiggle, hmmmm! 
)
is something I’ve wondered about affecting knot behavior,
with the cyclical slackening on one leg (maybe only one,
maybe both alternately).
AND, we might conjecture that the movement-from-length
which I pointed to is relatively minor in effect to that of
the shifting of the eye from changes in angle. But do
realize that the video shows angle shift (laterally and also
perhaps vertically --around & along the cylindrical object),
and not pure tension variation.
Around a RING, I don’t see same behavior occurring. And
we do have, in the unwrapped eye, the concentration of
pressure to aggravate if not be the primary cause of rope
deterioration.
Thanks,
–dl*
If it’s a ring it’s the same thing. On cars, this “ring” is often cut out of a plate, leaving edges in the hole. Those edges can cut a rope that is not wrapped with an extra turn, but when the turn is there, there is less movement, and the chafe is less. It works on rings as well as poles, and the difference is greater when the surface is rough.