Choosing the Right Rope
When it comes to most activities that require the making of knots, whether it is in sailing or decorative arts or another area, it is helpful to understand how the properties of the rope you choose to use affect your application. Some ropes have a very specific application, while others are general-purpose. Not all ropes can be used correctly and safely for any activity. When you choose your rope, consider its properties and how they apply to your particular needs.
Strength under Strain
You need to know the strain that an application will place on the rope and whether this strain will be steady or variable. A steady load is one where the tension will remain constant, such as when binding something that will not shift or suspending something that will not move. Here the strength needed can be as easy to determine as just looking at the weight that is being lifted. However, if the load will not be steady, the strain on the rope will be higher and more difficult to determine.
A shifting load can increase the strain on a rope and can lead to shock loading. You can try to prevent shock loading by securing the load to prevent it from shifting, but sometimes shock loading cannot be avoided and must be accounted for in determining the strength of the rope needed. The less stretch a rope has, the stronger it has to be to resist shock loading; a rope with more stretch can absorb more of the energy of the shock, and therefore doesn't need to be as strong.
ALERT!
One way to deal with shock loading is to use a rope strong enough to provide for a margin of safety. When rope is too weak for the sudden load it receives, a number of problems may occur. The rope may break, it may be weakened due to strain, and any knots that were tied in it may overheat or even melt as a result of the friction caused by being rapidly tightened. Also, if a fitting the rope is secured to is not strong enough, it may break free and fly through the air like a missile.
Rope is best used at or below its “working load” level. Using a rope near its breaking strength will cause what is called “fatigue,” and the rope will have lower strength as a result. The working load rating is many times less than its breaking strength and is often labeled on rope packaged for sale. This is the amount of load that can be used on the rope repeatedly without causing fatigue.
As important as strength is to your choice of rope, it may actually be the last criteria that you use. That is because most ropes come in many sizes. You may choose a rope based on other factors such as whether or not it floats, its durability, tendency toward stretch, and so on. After narrowing your choice down to a single type or rope, just choose the size that corresponds to the strength needed.
QUESTION?
How do I determine the strength of a rope?
The rope's strength is determined by the area or cross section of the rope. An otherwise identical rope with twice the diameter will have four times the strength, and vice versa. Moreover, the bigger rope will have less stretch when put under the same amount of strain as the smaller one.
Stretch under Strain
Ropes vary in how much they stretch under strain. Some applications require very little stretch, while some benefit from it. Nylon is an example of a cordage material that stretches, and polyester is a common material used in low-stretch applications. A low-stretch rope can help you avoid shock loading in an object by firmly holding it in place.
Some applications can only use ropes of low stretch. If the rope holding up a sail on a sailboat were able to stretch when the wind picked up, it would allow the sail to move and thus change shape. A hammock should be made of, or slung from, low-stretch materials in order to keep it from sagging to the ground when occupied. Some people want a little stretch in their anchor line to help prevent sudden jerks in the line, causing the anchor to break free. Some fishing lines are made to stretch, which helps keep them from breaking under sudden tension, and some have very little stretch for extra sensitivity. When using ropes and twine that stretch, some of the stretch must be taken out if it is to be tightened. This is easily done with a Trucker's Hitch or another adjustable tie. Some applications require very little stretch and very little “creep,” the tendency of a rope to stretch slowly when under continuous tension. Steel cable is often used for this.
Additional Rope Properties
Some properties have to do with the type of abuse the rope will take. Abrasion resistance is sometimes needed, or resistance to ultraviolet rays, insects, mildew, heat, high voltage, rough handling, and repeated knotting. If appearance is important, then you would choose rope based on structure, material, color, surface texture, or size. Other factors may include cost, availability, ease of splicing, quality, tradition, or uniformity with similar uses. Rope can be biodegradable, edible, flammable, glow in the dark, or electrically lighted. If you can't find what you want, you can even make your own!
Sometimes you will have to make do with the rope you have on hand or any substitute you can find. Keeping the properties of rope in mind will help you to make the best use of it. There are knots that can collect the ends of many strands, or that are useful for tying together ropes of very different materials, sizes, and structures. There are also knots that can be tied in bungee cords, straps, sheets, or torn clothing.
The properties of knots do not stand alone, but depend on the substance they are tied in. Keeping this in mind as you learn from this book will make you less frustrated, and a much more competent and safe knot tyer.
