GroundquestMSA wrote:See
Myth#4 "A High Strength Tie-off (Frictionless Hitch) needs 3 wraps around the anchor."
I won't argue the claim that two wraps is
often enough, but my goodness, doesn't the
Incorrect!!! caption for a perfectly fine rig seem a bit overdramatic? And the "terrible waste of resources"... Boy. That extra 2 seconds and four feet of rope used. What a tragedy.
I would argue that two wraps is not neccessarrily enough, and there exists too many variables to phrase myth (#4) the way they did. I am glad that this "myth" was pointed out. The great part about most things we do within rope craft especially within rope rescue involves physics...and equations...and makes it easy for us to call BS. Anytime we wrap rope around any object (like a tree for an anchor, a descent device, or a carabiner) we create an equation in which we can determine the friction gained. As for the High-Friction Tie-Off anchor in myth #4, the success and physics behind this, is the capstan equation.
As a rope bends over a small segment of a drum (Tree), the tension in a rope will increase from T to T+dT in an angle dθ. The normal force is the differential dN, since it acts on a differential of area. The frictional force is μdN, and acts to oppose slippage.
Equilibrium in the x direction requires the sum of forces in the x direction to equal to be zero.
It is the capstan equation and principles that become relevant when discussing the High Friction Tie Off, clove hitch and the static and kinetic coefficient of friction properties of the munter hitch. The factors influencing the success or desired result in these techniques are; 1. Tension on rope, 2. Coefficient of friction, and 3. the total angle of contact.
So two wraps may work, but it depends on the weight of those on the rope and the estimated amount of force (tension) they place on the rope which will pull on the wraps. Next is the coefficient of friction between the rope and the object the rope is wrapped around (tree or whatever object is your anchor). This is almost impossible to precisely figure out. Wet rope, new rope, old rope, muddy rope, bark on tree, etc all play a role in the coefficient of friction. Lastly is the contact angle. This is the total "degrees" of angle produced by your various amount of wraps.
Heavy rappellers = more wraps. Wet ropes or wet anchor = more wraps. Smaller diameter tree = more wraps
Preaching 2 wraps = not true
A great way of thinking about the criticality of contact angle is the munter hitch. When the munter is in the break position (ropes are parallel)you or your load are in hanging in a static coefficient of friction. When you begin to move your break strand out by decreasing the contact angle of the rope and carabiner your load moves (kinetic coefficient of friction).