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Discussion Starter #1
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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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

JRP
 

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Mine are 1/2 the diameter of the static (roughly). Seems like I read that somewhere


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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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"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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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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Discussion Starter #8
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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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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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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Discussion Starter #13
Got a helpful pm containing some good info that I thought would be good to share. After a little research and considering my particular application I chose to purchase some 8mm prusiks. Also purchased a petzl tibloc as it looks like a useful tool. Thanks to all the contributions of knowledge. Hope this may come in handy for others when assembling a z drag kit.

Hi,

Everyone seems to be saying 1/2 the size of your main rope. Rather than 5 or 6mm, I use 8mm for prussik strength.
Here's some of the research that I used to decide 8mm over 5mm.

https://app.box.com/s/cc36yll8wkpzqskrq24pq6kgdu239zqz

https://app.box.com/s/145lo5znl8d3kmonsay08c5p65urzb8j

Like was found in the testing/experiment, I have experienced prussiks NOT slipping on a dry main line/rope.
I have only anecdotal evidence/recollection about the prussiks slipping on wet rope. Certainly not consistent or predictable.

Having had smaller prussiks fail under load, I decided that prussik strength was important and have increased the size/diameter of the prussiks I(we) carry in our wrap kits that use an 11mm static main line. YMMV.
You might want to investigate the French Prusik and the Bachmann Hitch. There is some evidence that suggests that those knots/hitches place less stress on their own cord and therefore may break at a higher load instead of the lower breakage point of a traditional Prussik Hitch.
I(we) also have a big river wrap kit (bigger, wider rivers, longer 400' main line) that uses a 13mm static main line and 9mm prussiks. I have also used ascenders and Rescuescenders in place of prussiks in wrap situations, but now try to avoid those in high load scenarios as main line sheath damage seems to always happen. Prussiks and especially double prussiks seem to not damage the main line as often.

Hope all that helps,
Good luck in your search,
 

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.02... Ive done some pretty serious pulling on 7mm prussiks on a wet 1/2"" line. No slip and no fail. Bigger than 7 risks a slip on wet 1/2" static. 5.5 will bite so hard you cant readily move it. I have 7mm in my kit, a bunch of them. I can run doubles if I think the scenario calls for it.My 7mm has a 2500 breaking strength which remains the weak point of the system assuming your anchors and whatever you are pulling are stronger than that. Frame straps and or d rings will likely be the weak point.
 
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