Rope is constructed in two basic ways, laid and braided, although there are variations on the theme. The first is "3 strand" line. The direction of twist is called the lay of the rope. Three strand twisted line can be "laid" right or left, and should always be coiled with the lay of the line. This rope is described as S-laid (left-laid) or Z-laid (right-laid) according to whether the twist follows the line of the center part of the letter S or Z. Most three strand rope is Z-laid (right-laid). If you hold a length of 3 strand, right-hand laid twisted line at arm's length and eyeball it, you will see the wrap of the line twisting to the right.
The construction of stranded line, whether natural or synthetic, is much the same. Individual fibers are twisted into yarns, the yarns are twisted into strands and the stands are twisted into line. Right-laid is twisted clockwise and left-laid is twisted counterclockwise. The fibers are twisted in the same direction as the strands, however the yarns are twisted in the opposite direction. This right, left, right for right-laid line helps give strength, keep the line from kinking and hold its shape.
The other marine anchor construction type is braided line. This type of line does not stretch to the degree that twisted line does, and is more difficult to splice. However, it goes through a pulley or block very well because of its rounded shape, and is stronger than its equivalent-sized twisted line. Braided line also tends to snag when used as docking line if the pilings are rough.
There are many materials used today to make rope; synthetic fiber, natural fiber and wire. The most popular is nylon, a synthetic. It is strong, holds up well to the weather and stress, and coils without kinking. Line is also made from natural fibers like cotton and hemp (manila), and other synthetic fibers such as dacron, kevlar, and polypropylene.
Polypropylene line is the least expensive of the synthetic lines, however, it deteriorates quickly from ultra-violet rays and wear. It is not a good line for dock line because its hard surface tends to slip from cleats and can cause cuts if it runs free through your hands. It floats, so it is good for rescue lines. It is also appropriate for ski lines, dinghy painters, short mooring pendants or other applications where you want to be able to see the line on top of the water. Not for use as dock, anchor or towing lines.
Nylon rope has a lot of stretch (up to 40%) and is very strong for its size, allowing it to absorb shock loads well. However, when it is wet it can lose up to 25% of its strength. It wears well, resists mildew and rot, and does not float. Nylon three-strand is the preferred line for dock lines since it stretches sufficiently to dampen the shock of wave action and wind against your cleats. Just make sure it does not stretch too much for the situation in which you use it.
Polyester rope wears better than polypropylene, is almost as strong as Nylon, and retains its strength when wet. It does not stretch as much as Nylon and does not float. Polyester (such as Dacron) is used for sailboat running rigging, anchor rode, towing lines and other applications where you don't want line stretch to interfere. It will, however, chafe easily so check it often and protect as necessary.
When cutting synthetic rope, prevent the ends from fraying with a temporary binding or whipping. Synthetic rope ends can be sealed by melting, either with a special heat tool for the purpose of cutting and sealing (as shown in photo), or by melting over a flame to fuse the fibers. Adhesive tape wound around the ends can be a temporary binding. Small line ends can be dipped into acetate glue or a commercial "liquid whipping" material. Plastic heat-shrink tubing is also available.
Synthetic lines are lighter and stronger and more rot-resistant, generally, than natural fiber ropes. Synthetic lines are slipperier than natural fiber ropes so be sure to check your knots to make sure they are secure. Synthetic lines should be cleaned with fresh water and detergent, kept out of sunlight, inspected frequently for chafe, and stored dry.
Natural fibers such as manila, sisal, hemp and cotton will shrink when they get wet and also tend to rot or become brittle. Manila is still used today on large ships and is the best natural fiber for mooring lines, anchor lines and as running rigging. Manila has a minimum of stretch and is very strong. However, it has only about one-half the strength of a comparable-sized synthetic line.
Natural fiber line should be uncoiled from the inside of a new coil in order to prevent kinks. Always whip or tape the ends of natural fibers to keep them from unraveling. When natural fiber lines have been in salt water you should rinse them in fresh water and allow to dry thoroughly. They should then be properly coiled and stored on grates above deck in a dry, well-ventilated place to help prevent mildew and rot.
Each type of line, natural fiber, synthetic and wire rope, have different breaking strengths and safe working loads. Natural breaking strength of manila line is the standard against which other lines are compared. Synthetic lines have been assigned "comparison factors" against which they are compared to manila line. The basic breaking strength factor for manila line is found by multiplying the square of the circumference of the line by 900 lbs.
(900 lbs. X circumference2 = breaking strength)
When you purchase line you will buy it by its diameter. However, for purposes of the USCG license exams, all lines must be measured by circumference. To convert use the following formula.
Circumference = p PI (3.14) X diameter
As an example, if you had a piece of ½" manila line and wanted to find the breaking strength, you would first calculate the circumference. (.5 X 3.14 = 1.57) Then using the formula above:
1.572 X 900 = 1,414 pounds of breaking strength
To calculate the breaking strength of synthetic lines you need to add one more factor. As mentioned above, a comparison factor has been developed to compare the breaking strength of synthetics over manila. Since synthetics are stronger than manila an additional multiplication step is added to the formula above.
(comparison factor X 900 lbs. X circumference2 = breaking strength)
Following is a comparison factor chart for synthetic lines.
Line Material Comparison Factor (greater than manila)
Using the example above, let’s find the breaking strength of a piece of ½" nylon line. First convert the diameter to circumference as we did above and then write the formula including the extra comparison factor step.
2.5 X 1.572 X 900 = 3, 537 pounds of breaking strength
Knots and splices will reduce the breaking strength of a line by as much as 50 to 60 percent. The weakest point in the line is the knot or slice. However, a splice is stronger than a knot.
Just being able to calculate breaking strength doesn’t give one a safety margin. The breaking strength formula was developed on the average breaking strength of new line under laboratory conditions. Without straining the line until it parts, you don’t know if that particular piece of line was above average or below average. Next we will discuss safe working load.
Knowing the maximum safe working load for line can help prevent accidents and tragedies. You should never stress a line anywhere near its breaking strength. As line is spliced, stretched, wears, is subjected to sustained loads, shock loads, loads of many times the recommended working load, subjected to great heat or ultraviolet light for long periods of time it will continually loose some of its strength. Each line should be inspected prior to using it in extreme load conditions and if chafe, excess dirt, cut or worn strands, stiffness or hardness are found the line should not be used.
Important: Do not allow anyone to stand in line with, or within 45 degrees on either side, of a line under tension. Should the line part, the recoil force may cause serious injury.
Safe working load is generally thought of as no more than 1/5th of a line’s breaking strength. Said another way, the breaking strength should be five times the weight of the object the line is going to hold. You are not expected to memorize the tables below but you should remember this 5 to 1 safety rule. You should always choose a line with its intended safe working load in mind. The American Boat and Yacht Council has published charts of safe working loads for various types of line and are outlined below.
American Boat and Yacht Council Safe Working Load of3 strand twisted nylon line
|Diameter||Circumference||Safe Working Load|
|3/8"||1-1/8"||407 lb / 185 kg|
|1/2"||1-1/2"||704 lb / 320 kg|
|5/8"||2"||1100 lb / 500 kg|
|3/4"||2-1/4"||1375 lb / 625 kg|
|7/8"||2-3/4"||1980 lb / 900 kg|
|1"||3"||2420 lb / 1100 kg|
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