Throw Bags, Accessories and WorkShops "Gear For Life"

Testing For Prusick Slip

Testing For Prusick Slippage With Throw Bag Rope Paddlers Normally Carry
-- Jim Simmons
Several of us were interested to conduct testing to learn slippage points with prusick cord when placed on a haul rope. Both the cord and the rope were common sizes and types that whitewater/river paddlers carry. After deciding the various combinations we wanted to test, Mike Gardner, ACA Canoe Instructor in Claremore, OK, used a pneumatic ram to conduct the actual testing. We collected donations from our group to pay for materials and Tim Jones, dealer with Sterling Rope Company, secured the materials (a few we already had on hand). Instructors involved were: Mike Gardner, Tom Jenkins (retired), Jim Jones, Tim Jones, Don Harwood, and Jim Simmons.
The testing was done with both nylon and dyneema cord; and with grabline and dyneema rope of different sizes. Given here are the breaking strengths (lbs) of the prusick cord and rope:
3/8" Grabline (Maxgrip) Rope --- 3282
3/8" Ultraline (Dyneema) Rope - 5261
5/16" Grabline (Maxgrip) Rope --1600
1/4" Ultraline (Dyneema) Rope - 2608
*Dyneema is ultra-high molecular density polyethylene and similar to spectra.
5 mm Nylon Cord -- 1169
6 mm Nylon Cord -- 1978
6 mm Powercord -- 4789
*Powercord is dyneema and is stiffer/harder than regular nylon or polyester cords.
Table 1. Figures in table one are the initial slip points (lbs).
Prusick Knot 3/8" Grabline 3/8" Ultraline 5/16" Grabline 1/4" Ultraline

5 mm Nylon 3-Wrap

750 500 575 300

5 mm Nylon 3-Wrap

(Tandem Prusicks)

1000 750 700 400
5 mm Nylon 4-Wrap 975 825 750 550

5 mm Nylon 4-Wrap

(Tandem Prusicks)

1150 950 1150 500
6 mm Nylon 3-Wrap 750 675 475 --

6 mm Nylon 3-Wrap

(Tandem Prusicks)

1050 825 -- --
6 mm Nylon 4-Wrap 1050 975 725

6 mm Nylon 4-Wrap

(Tandem Prusicks)

1800 1125 -- --

6 mm Powercord 3-Wrap

(Technora)

725 800 -- --

6 mm Powercord 3-Wrap

Tandem (Technora)

1450 1300 -- --

6 mm Powercord 4-Wrap

(Technora)

1650 1400 -- --

6 mm Powercord 4-Wrap

Tandem (Technora)

2000 2100 -- --
Discussion:
We caution the reader that these tests results will obviously vary when setting haul systems in the field and should be considered only guidelines. We suggest you practice setting haul systems to learn how your gear, ropes, prusick cord, and webbing perform for you. For the tests, we used grabline and ultraline from Sterling Rope Company because it was readily available to us as Tim Jones is a dealer for Sterling. We also use spectra rope in throw bags as well, but it was not available to us in bulk quantity (spectra and ultraline are virtually the same w/ ultraline having a slightly higher strength rating).
Testing: Our group wanted to learn about slippage points with the prusick cord and types of rope that we currently are using. Mike Gardner did the actual testing with a pneumatic ram and the tests were Slow Pull Tests. While Mike applied mechanical principles there could be factors that might cause errors in the test results. Such factors might be: age and condition of the cord and rope; and differences between sheath of the grabline and ultraline and between the nylon cord and ultraline powercord. On test repeatability Mike reported that he did several trials of each test and obtained the same results.
Mike reported these testing characteristics--
1. When the prusick began to slip on the haul rope it did so slowly, grabbed, and then began to slip again. With the dyneema line, once the prusick began to slip it continued until it burned away the sheath. The force required to make a prusick slip is difficult to determine; it is affected by several factors: brand/make of both the rope and cord (tightness of weave, pic count in sheath, fibre used); whether it is wet or dry, clean or dirty and how well the prusick is tied, dressed and set on the haul line. Mike was meticulous with these factors.
2. With the powercord prusick material he used a standard double-fisherman's knot instead of a triple fisherman.
3. Nylon prusick cord grabbed better on grabline rope than on the dyneema line.
4. You can see from the table that 5mm nylon prusicks grabbed well on the smaller, 5/16 inch grabline rope.
When a haul system is loaded, as more force is applied more energy (heat) is generated and the resulting heat can melt cord and rope causing failure. When the prusick begins to slip it is time to stop and re-evaluate what you need to do. Rescue books and other sources have previously reported that prusicks will begin to slip somewhere between 900-1200 lbs. This testing recorded the initial slippage point and varied according to materials used.
Summarizing Our Suggestions.
1) Keep in mind there are risks to paddlers/rescuers when using a mechanical advantage haul system for unpinning boats. Be sure you have practiced the basics and are confident in what you are doing.
2) Be familiar with the prusicks materials you are using and they are compatible with the throw bag that will serve as your haul rope.
3) We recommend carrying a 75 ft. throw bag rope that will serve two main functions--a) for general rescue use, and b) for setting a haul system, if needed. Be well practiced with the throw bag that you are carrying.
4) Considering cost, availability and good performance you will not go wrong choosing nylon/nylon, or polyester/nylon prusick cordage and one inch nylon tubular webbing.
5) Grabline rope is a good choice at a much lower cost, but its main drawback is the amount of stretch when setting haul systems. However, both 5/16 inch grabline (1600 lbs) and regular 3/8 inch poly (2000) have successfully been used in haul systems for recovering boats and rafts. We have also noted that the grabline tends to become heavier with much use (may be that the double-braided sheath collects sand and grit). If you choose the higher strength rope (spectra, dyneema) and cordage (dyneema and other powercord) expect to pay more.
ADDITIONAL INFORMATION (for interested readers):
Previous Testing of Rope, Cordage, and Webbing: Technical testing of the various kinds of rope, cordage and webbing have been done using kevlar, spectera, vectran, and now dyneema, along with other high tech materials. Using such materials was pioneered by the sailing industry to gain an advantage of higher strength, low stretch, and low moisture absorption of the cord and rope. Spectra, grabline, ultraline (dyneema) and polypro line have been used in whitewater paddling and swiftwater rescue applications for many years. You'll also recognize spectra from its use in making plastic grocery bags and plastic jugs.
Characteristics and Cost of Rope/Cordage: Grabline (also called maxgrip) has a double-braided sheath and is a sophisticated version of poly. It has a lot of stretch, a main disadvantage in setting haul systems. Both dyneema and spectra (which we also use) have 'creeping' characteristics, which is slow elongation under a load. Over time both become stiffer from use. Grabline retails for about 50/cents/foot; dyneema and spectra for $1/foot or more. High strength cord such as the Sterling powercord sells for 85-90 cents/foot, and nylon and polyester cord for between 40-45 cents/foot.
Factors to Consider When Choosing Rope and Cordage: A general principle for selecting prusick material is to choose a type that is different than the composition of the rope on which it will be placed. Example: cord made of nylon sheath/nylon or polyester sheath/nylon used on grabline rope. A softer lay (sheath) will grip better but will be more difficult to loosen and move on the rope, plus it will wear out faster. Harder/stiffer lay cord is easier to loosen and move along the rope but will not grip as well. Refer to results in Table One and you'll notice that the test results confirm this difference in materials.
It is important that the prusick cord type and size be compatible with the haul rope. Generally, the cord size should be no more than 60-70% of the size of the haul rope. For river paddlers commonly used 3/8 inch size rope (comparable to 10 mm size) would accept a 6 mm cord--roughly 60% of rope size. If the prusick cord is the same, or nearly the same, size as the haul line it won't work. When forming the prusick loop a double fisherman knot is most often used to join the ends. With the stiffer powercord, however, a triple fisherman's is commonly used; or else use a trace '8' knot to join the ends.
A prusick hitch is the most common cinching knot used in haul systems; it is actually multiple girth hitches around the rope and can be loaded in either direction. A triple wrap has been traditionally used in river rescue but more wraps (as indicated in the testing) increase the prusick's gripping ability. Refer to the table for contrasting results between the number of wraps with nylon cord and the powercord.
References:
-Swiftwater Rescue Handbook, by Slim Ray (1997).
-Whitewater Rescue, by Sundmacher and Walbridge (1995).
-Technical Paper on Comparative Testing of High Strength Cord, by Tom Moyer, Paul Tusting and Chris Marmston, 2000.
-Rope Rescue Tech Sheets, 2005, British Columbia, Canada.
JS and MG; August, 2010
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