Prussik rope sizing for z-drag - Mountain Buzz
 



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Old 03-20-2016   #1
 
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Prussik rope sizing for z-drag

Building a new z drag kit for my round boat and was looking for some input on prussik sizing. So far I have 150' of 11 mm static petzl rope, a couple of 35 kn rated pulleys and quality biners. An 8'x 1"crane strap for an anchor. Even threw in a nice figure eight and a crib sheet to live in the bag. Wondering how much smaller to size the prussik loops. Any input would be appreciated. Thanks!

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Old 03-20-2016   #2
 
Fort Collins, Colorado
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If I recall correctly you want the prussik to be at least a couple of mm smaller than the static rope. I use my yhrow bag as my static rope which has a diameter of 5/16" and mprussiks are all 5 mm and that works fine for me.

I could see an argument for larger prussik cord if you are dealing with heavier loads, but I'm not motivated enough tothink through the math and physics of it to give a quantifiable difference with different sizes at the moment. For a fram of reference, I have only had a need to z drag 14 ft paddle rafts that are open face pinned on mid channel boulders.

Hope this helps

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Old 03-21-2016   #3
 
Jackson, Wyoming
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Mine are 1/2 the diameter of the static (roughly). Seems like I read that somewhere


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Old 03-21-2016   #4
 
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5MM is what is in my kit.
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Old 03-21-2016   #5
 
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I also carry the 11mm Petzl static in my z-kit. I've used 5mm & 5.5mm just fine over the years. I also carry a couple PETZL TIBLOC in my kit and I prefer them. They're lighter and easier to setup/use. Cheers!
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Old 03-21-2016   #6
 
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"The effectiveness of the Prusik hitch relies on the surface area between the hitch and the main line, and the diameter of the cord used. Normally the greater difference between the diameter of the cord used for the hitch and the main line, the greater the ability for the hitch to hold. However, the smaller the diameter of the cord used, the lower its safe working load. In addition, smaller diameter cords often jam too tight when placed under load, and are hard to handle when wearing gloves" (Quote From Wikipedia)

When I was doing some mountaineering training I think I recall a rules of thumb at 1/2 the diameter of the main line.

I also like to use the prusik cord as the failure point in the system, they tend to slip under heavier loads and if it breaks the system will not go flying through the air as it would under a failure of a carbine, pulley, or anchor.
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Old 03-21-2016   #7
 
Hood River, Oregon
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Yes, the 1/2 diameter rule is right on.

Also, Whitewater Spoon's comment about the prusik being the failure point is a great one.
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Old 03-21-2016   #8
 
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Definitely agree on selective coordination of failure. I like the look of the tibloks. I have a petzl mini traxion p07 in the kit and they show the tibloks used in conjunction as a self braking z drag set. Think I will pick up a few and some 5.5 mm cord. Should have a nice kit. Thanks for all the info!
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Old 03-22-2016   #9
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5.5 is small. Don't bother with the static line if you're going to hold it with a tiny prussik. You can grab 11 mil rope with a 7 or 8 mil prussik. With a Z-drag or Pig Rig the Prussik holds the weight. 5.5 mil won't cut it when pulling hard.
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Old 03-22-2016   #10
 
River City, Oregon
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There are a wide variety of souces out there. Most that I have found say the prussick should be anywhere between 50% to 70% of the diameter of the main line.
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