Ropes are used on nearly every Scout outing. Knot tying is practiced frequently. Nearly every experienced Scout can tie a bowline with his eyes closed! Give a First Class Scout a length of rope and he can probably line a canoe down a rapids, recover a raft marooned on a mid-stream rock, haul a food pack up where no bear can find it, or make a simple tarp into a shelter that will withstand a gale of wind. But the rope comes first.
Unfortunately, it is surprisingly hard to find information on the types of rope that you can choose from. Your corner hardware store will carry manila, cotton, nylon, polyester (Dacron) and polypropylene ropes in diameters from 1/8" to 1/2" or larger. Outdoor specialty stores carry ropes advertised for climbing, rappelling and perhaps even rescue use. Prices may vary from a few cents to a dollar or more per running foot. To make it even more confusing, each of these ropes might be available in several different 'braids' and 'twists'! Which rope is right for your use? We hope these notes on what we’ve learned will help you in selecting the right rope for your job!
Strength Specifications
You’ll often find terms like "ultimate load", "tensile strength" and "working load". Ultimate load and tensile strength are really the same thing. They are basically the breaking point of a new sample of rope determined under laboratory conditions. The manufacturer's recommended working load is determined by taking the breaking strength (ultimate load or tensile strength) and dividing it by a safety factor of up to 15:1. The working load rating is intended to more accurately reflects the maximum load that should be applied to a given rope to assure a comfortable safety margin and longevity of the line. The safety factor varies with industry practice, the type of fiber, the rope construction, and with the intended use. As a rule of thumb, the higher the risk to human life, the higher the safety factor.
In the U.S. we are accustomed to the "English" system of weights and measures. Though not technically precise, we commonly use "pounds"(lbs) when referring to loads and strength ratings. Ropes and climbing/rappelling accessories are often rated with metric system specifications using kilonewtons (kn or Knt) or kilograms (kg). You can use these conversion factors to help get a better feel for the different ratings you’ll see:
1lb = 4.45nt \ 0.225lb = 1nt
10,000nt = 10Knt = 2,248lbs
(example: a pulley with an ultimate load spec of 18kn has an equivalent rating of 4,000lbs. Be sure to apply an appropriate safety factor to these ratings!)
Safety Factors
The working load for most kinds of rope is between 15% and 25% of the tensile strength providing safety factors of 7:1 to 4:1. These are for new, undamaged laboratory tested samples. Ropes intended for use in climbing, rappelling or any type of rescue work should have higher safety margins. In the U.S., the National Fire Protection Association (NFPA) specifies that the working load of rescue ropes be specified at 15:1 of the breaking strength. Many rope manufacturers use the NFPA guideline for climbing and rappelling ropes as well. Climbing ropes should also be certified by the UIAA (Union Internationale des Associations díAlpinisme) to have assurance of other important performance characteristics, though we consider strength to be the #1 issue!
Impact loading is also an important load to consider when selecting and using your ropes. The following table illustrates the energy created by a sudden drop. Long fast descents with quick breaking action can create these types of stress on a rope. Multiply a load’s weight by the distance it falls to estimate the energy released when a rope is required to stop the fall quickly. For example:
|
Static Weight |
Distance |
Energy Released |
|
100 pounds |
dropped 5 feet |
500 foot-pounds |
|
100 pounds |
dropped 25 feet |
2500 foot-pounds |
|
200 pounds |
dropped 5 feet |
1000 foot-pounds |
|
200 pounds |
dropped 25 feet |
5000 foot-pounds |
Converting foot-pounds of energy to a strength rating in lbs requires a bit of calculation involving the above conditions plus the rope’s elongation characteristics – a bit too complicated for us! Suffice it to say that even short drops with quick stops can overload an otherwise suitable rope! Dynamic ropes help minimize this effect and are recommended for all climbing uses and other applications where impact loading is an issue. NOTE: dynamic ropes are not recommended for rappelling or rescue work as the inherent stretch in a dynamic rope can cause dangerous snap-back!
Knot Breaking Strength
Any time you tie a knot in a rope you dramatically reduce the rope’s ultimate strength. This is primarily due to the reduction in the number of rope fibers sharing the load due to sharp bends in the rope made when tying knots. So...the less the angle of bend from the knot allows more fibers to share the load thus increasing the rope's strength to nearly its original strength. Certain kinds of knots damage the line less than others. A 50% loss of ultimate strength is a good general rule to live by. The Figure-8 knot reduces the ultimate strength by approximately 35% instead of 50% for other common knots. This is why the family of "8's" are used almost exclusively for rescue work. As a rule of thumb, knots that are easy to untie after being loaded, such as the bowline, have less reduction in rope strength and cause less long term damage to the rope and are thus the preferred type of knot.
Different knots have varying impacts on rope strength. The following tests were done by Mr. Jerry Smith of the California Mountain Company using Wellington’s 1/2 inch Rhino Rescue Rope pulled end to end. Results will be different on other sizes and brands.
|
Knot |
Strength in Lb. |
Percent Lost |
|
Control Rope |
10,705 |
-- |
|
Bends |
|
|
|
Figure Eight Bend |
8,640 |
19% |
|
Double Fisherman’s |
8,440 |
21% |
|
Loops |
|
|
|
Bowline |
7,180 |
33% |
|
Inline Figure 8 |
6,280 |
41% |
|
Double Figure 8 |
8,820 |
18% |
Dressing and setting a knot is necessary to minimize the knot’s affect on the rope’s strength. Dressing a knot involves the orientation of all the knot parts so that they are all properly aligned, straightened or bundled, and so the rope parts of the knot look like the picture books. Neglecting this activity may result in an additional 50% reduction in knot strength.
Setting a knot involves tightening all the parts of the knot so that all of the rope parts touch, grab, and cause friction upon other parts of the knot so as to render it operational. A loosely tied knot can easily deform under strain and change in character. For instance, an effectively set bowline requires tension in three directions at the same time.
Rope Construction
There are two major categories of ropes that you should be familiar with; dynamic and static ropes.
All UIAA certified ropes used for lead climbing are dynamic - they are designed to stretch when fallen on, absorbing and dissipating at least some of the energy generated by the fall (like a rubber band). Dynamic ropes are often multicolored and may stretch as much as 10% or more (a 100 ft rope could stretch to 110+ feet).
Static ropes are designed for minimal stretch and are used for rappelling, caving and rescue work. Normally static ropes are a solid color or one color with a dark strip. Static rope will stretch about 1-2%.
Aside from extensive special training and practice, serious rescue work requires high tensile strength static line. Climbers use high strength dynamic line that stretches when shock loaded i.e. when they fall. Rescue rope should always be static line that is designed to have very little stretch.
Diamond or hollow core braided polypropylene rope is a commonly available dynamic rope. Its low cost, light weight, ease of splicing, and positive buoyancy make this a very popular rope for water sports (think water ski rope!).
Most of the laid rope you'll find in hardware and boating stores is laid rope. Laid rope is built up from strands twisted together. Each strand, in turn, is made from yarns, produced by twisting individual fibers into a unit. A common laid rope is constructed from three strands with a right hand (or ‘Z’ twist) lay. It's relatively inexpensive, easy to splice, and perfectly adequate for most purposes. Laid or twisted rope does have a tendency to untwist and stretch as weight is placed on it (it’s a dynamic rope type!). A common 7/16 3 strand twisted nylon rope can have an ultimate strength in the range of 4500-5500 pounds. We use 3/8" twisted poly rope for our every day throwbags and canoe painters. This type of rope has more limited strength ratings (generally about 1000 lbs) and has a lower resistance to UV. Be sure to check your outdoor poly lines frequently! As an aside, this type of rope is often used by utility companies as underground ‘pull rope’. You can often get the used rope for free – but don’t trust it for life line uses!
Serious climbers and professional rescue personnel have found that laid nylon and poly ropes are ‘slippery when wet’. Though fairly common up to the late 60’s (with brand names like ‘Goldline’), you won’t find professional grade climbing or rescue ropes using laid construction any longer. Also, rescue grade throw bags have discontinued the use of laid ropes. The preferred rope construction for these high demand applications in the kernmantle style as it offers more strength and a better grip.
In braided ropes the yarns are plaited and braided around an inner core, forming an outerwoven jacket or sheath. This rope is called "Kernmantle" rope and consists of a core or "kern" of many small laid cords. Each cord is made up of twisted filaments. Every filament runs the length full length of the rope. The cords are held together by a tightly plaited sheath called a "mantle" which protects the rope from cuts and abrasion. 11 mm Bluewater II with nylon filaments has a strength rating of 6,000 lbs. Braided ropes are available in both dynamic and static designs. Solid color lines tend to be static, but check the manufacturer’s specs to be sure.
The only kinds of synthetic fibers that float are polypropylene and polyethylene. Nylon and Dacron (polyester) sink but are substantially stronger. Up until just recently, nearly all commercially available throw bags were made of solid braided polypropylene, typically 3/8" diameter with tensile strengths of 900 to 1200lbs. The newer kernmantle-polypropylene lines with linear solid strands have ultimate strengths between 1600 and 2000lbs. The most recent innovation is "spectra core" which has a polypropylene kernmantle and a polyethylene core. While much more costly, spectra core lines have ultimate strengths of 2200lbs for 1/4" line and 4500lbs for 3/8" line.
Proprietary braided poly and spectra lines are available with tensile strengths of up to 1940 and 6300 pounds respectively for 3/8" line. Due to their cost, you wouldn’t want your entire troop to go out and buy spectra core throw bags, but having one or two in the general trip inventory is not a bad idea. Standard throw lines are perfectly acceptable for what they are designed to do and that is primarily to retrieve swimmers or assist boats in getting to shore. While working loads for these lines may only be a few hundred pounds, its doubtful that a rescuer(s) or victim(s) could actually hang on to loads over 200-300lbs.
Basically it boils down to this; use the right rope for the job. While it has been done, don't always expect to unwrap a seriously pinned raft by using a throw line, or worse, applying mechanical advantage to a throw line. The recoil forces can and have caused serious injuries when the line or some other portion of the system (like D-rings) have failed. Likewise, its not healthy to use throw lines for logging, mountain climbing or towing cars. Besides, it can overload and thus ruin an otherwise good throwbag! For canoe and raft outings, we carry a ‘recovery bag’ with higher strength ropes and hardware. To keep our costs under control, we carry an 11 mm 200 foot static line along with several 24kN (quick, how many pounds can this handle?) carabiners, pulleys and lengths of tubular webbing in our ‘recovery bag’. We use 7 mm (1/4 inch) for our prusik lines. Do not use natural fiber ropes (manila, hemp or sisal) for climbing, rappelling or any type of line.
In general, don’t use ropes less than 6mm (1/4") diameter; they are weaker, can cut your hands too easily and are very difficult to hang onto. Prusik lines are an obvious exception.
How much rope do you need? Our canoe painters are 20 foot lengths of 3/8" poly (it floats, is fairly strong, and is very affordable). Our troop made throw bags use 50 foot lengths of the same rope, for the same reasons. Though a shorter recovery line usually works (I wouldn’t recommend anything less than 75 feet), we carry a 200 foot 11mm static kernmantle rope for recovery use. Our prusik lines and webbing are in 6 foot to 20 foot lengths.
Rope wear, knots and mishandling can seriously reduce the load figures discussed here. Be sure to always inspect your ropes before coming out. Replace life and rescue lines regularly.
A word of caution, it's surprisingly easy to twist a loop of rope around a foot or arm! When tracking or lining, therefore, always keep a sharp knife handy. Many rescue personnel recommend one with a fixed blade and a blunt sheep's-foot point, well-protected in a metal-lined sheath. BSA requirements prohibit fixed blade knives so to be in compliance we use the new thumb actuated lockblade knives that can be easily opened with one hand. Be sure to get a knife with at least a partially serrated blade to quickly cut through rope!
Care Of Your Ropes
Under actual field conditions, rope strength is related to a number of variables, not the least of which is the way a rope has been used and stored. Climbing, rappelling and rescue ropes should be treated as if you life depended on them! Follow these simple rules to get the most out of your life lines:
Some Simple Rules for Rope Care
- Use the correct type of rope for your application
- Use rope from reputable commercial manufacturers (Bluewater, PMI etc.).Keep rope clean. Wash in cold water using a mild, non detergent soap. Air Dry ONLY.
- Store away from heat, sunlight and chemicals. Use a rope bag for protection (a canvas bag makes a good container for a painter).
- Do not use for any other purpose (i.e. towing a car)
- NEVER step on the rope (dirt gets between the fibers and cuts them when squeezed together. In addition dirt acts like sandpaper on decenders)
- Rinse dirty ropes in clean, cool water, drying them thoroughly afterward.
- Inspect your rope before and after each use. Document status in a rope log. A little surface fuzz on the strands is OK, but that's all.
- Fuse ends to keep it from unraveling.
- When in doubt, retire it. It's not a bad idea to retire tracking and rescue lines after three seasons of use, even if they show no wear of any sort.
- NEVER buy used equipment. You have no guarantee of how it was used. NEVER buy surplus military equipment. It is designed for different functions.
How do you know if your rescue line and throw ropes are still in good condition? It certainly helps to know their history. Has the line been subjected to sustained loads, shock loads or loads over three times the recommended working load? Is the line unusually stiff in certain areas? This symptom suggests the line has been overloaded or subject to excessive heat. Are there visibly worn or cut strands in the outer construction? Has the line been exposed to prolonged periods of direct sunlight or used often in mud, sand and dirt without being adequately washed off? If any or these symptoms are present, downgrade the line to a less demanding application or replace it.
Some Notes on Webbing
Webbing (sometimes called tape) is essentially flat rope. It most common usage is for slings, harnesses (Swiss seats) and as an anchor for rope rigging. The size most often used is 1 inch and has a breaking strength of 4,500 lbs.
Webbing is made in two forms: flat and tubular. Both appear flat, but tubular is hollow. Tubular is usually stronger and more flexible than flat. Tubular is constructed in two very different ways: spiral structure (shuttle loom construction) which is very strong and chain structure (needle loom construction), which is weaker and has a lengthwise seam.
Melt ends to prevent unraveling.
Mark ends with identification and length.
Store the same way as rope.
Create a chain to store easily.
Check stitching for looseness.
Home sewn webbing is only 10% of the strength of commercially sewn web.
--------------------
Thanks to Troop 940, Bluewater Ropes, the riverpeople web site and the GEC's own HAT Team for help with this article.
This page revised September 5, 2007