Rope Breaking Strength vs. Safe Working Load
-- Jim Simmons
There is a difference between the tensile strength of a rope and its safe working load. Tensile strength is the force required to break it, which can be determined by testing. However, in the real world of rivers and the outdoors there are factors that impact or reduce the strength of any rope. These include; tying a knot in the rope, its age, environmental deterioration (hot and cold), abrasion and rubbing on objects, harmful substances, water and sand/grit, and the rope's previous use and care. If a person is on the end of a line we obviously don't want the rope to break. To factor in the above variables we use an arbitrary number to reduce the actual load that is placed on the rope. This number is called the safe working load. The ratio between the working load and the rope's tensile strength is termed the safety margin. If a rope is rated at 5000 lbs tensile and we wanted to maintain a 10:1 safety margin, the maximum working load would be 500 lbs.
Throughout various professional associations there remains controversy about what standard should be adopted for safe working load of rope. Generally, cavers, mountaineers, and other outdoor activity groups accept a safety factor of 10:1, but the National Fire Protection Association (NFPA) recommends a 15:1 safety factor, as does the National Association of Tower Erectors. That would mean that on the 5000 lb rope the maximum working load would be about 333 lbs. If a rope held a 200 lb rescuer, plus a 150 lb person being rescued, the total of 350 lbs would exceed the safe working load of 333 lbs by 17 lbs. While the rope may not break in this instance...well, you get the concept.
When recovering boats, rafts or kayaks from a pinned situation paddlers and rescuers may not be much concerned with safe working load because they want to retrieve a boat. Even then, it is still important to practice 'safety first' so no one is injured from the backlash if a rope should break while hauling on a pinned craft. That is a main reason that we recommend paddlers consider carrying substantially stronger throw bag rope than just a minimum strength polypro line. It is also good to practice with your gear and ropes so you're familiar with how it all performs in live haul systems.
General Rope Properties:
Recreational whitewater paddlers and rescuers; along with Search and Rescue personnel will likely be using rope made of spectra, dyneema, polypropylene, polyethylene, or perhaps even nylon. Nylon was used a lot for rescue line in throw bags in the early years of the modern era of whitewater paddling and swiftwater rescue. Polypro line usually chosen is either regular polypro or grabline/maxgrip, a sophisticated version of polypro and much stronger. Spectra and dyneema are much stronger ultra high molecular weight polyethylene materials.
Nylon has excellent shock load absorption ability, resists the the UV's in sunlight well and has moderate elongation (extension under a load)but doesn't float well. Polypro floats well, has very high elongation (which can be a disadvantage), excellent resistance to UV's, doesn't absorb water and usually has an inexpensive price tag. Polyester has low elongation characteristics, excellent resistance to UV's, rates well for low absorption of water, but doesn't float well. Spectra and dyneema float well, don't absorb water, are excellent in resisting UV's but both are much more expensive than other rope.
1) Sterling Rope Company Booklet--Guide to Rope Engineering, Design, and Use (Vol. 1); Sterling Rope Company, Inc, 31 Washington Avenue, Scarborough, ME 04074.
2) Swiftwater Rescue Manual by Fred Ray, 1997.
3) Unpublished Booklet on Knots, Ropes and Rope Work, by Vernon Seaman.
4) General information from a variety of rope manufacturers.
Jim Simmons, 7/2010