Lines (ropes) are commonly made from two (2) types of material: natural fiber such as manila, sisal, cotton, and hemp while man-made fiber rope is typically made of Nylon, Dacron, and Kevlar. Natural fiber rope is subject to rot and is easily abraded. On the other hand, man made line is much stronger than natural fiber cordage and wears better. Regarding man made fiber rope, Nylon is stronger than Dacron. It also stretches which is advantageous when it is used for anchor or dock-lines. Dacron and Kevlar are low stretch they are often used for halyards, since the line will not have to be adjusted with increasing wind loads. Lines may be braided or a three strand (laid) line. Nylon laid (stranded) lines has the maximum stretch. Small stuff is generally line with a diameter of less than 1”.
Twisted rope should be put into round coils. right-laid rope, as most twisted rope is, should be wound clockwise, while left-laid rope should be wound counter-clockwise. Preserving the lay of the rope in this way will make for line that coils easily and plays out smoothly. Braided rope has no preferred direction and often loops into figure eights naturally. This will also run out smoothly.
Natural fiber line must be thoroughly rinsed and dried before stowing since rot fungi will rapidly grow in the organic materials which the rope is made from. A good method to detect rot within a line is to open the lay of the rope to see if mold exists. Line should be either faked or flemished (cheesed) in order to promote drying. To fake (not to be confused with flaking a line) a rope, line, or hawser by winding alternately in opposite directions, in layers usually of zigzag or figure of eight form, to prevent twisting when running out. Flemishing a line (from the Dutch speaking mariners of northern Belgium) is to coil the excess line into a spiral pattern by taking the free end and twisting it until you have a flat coil of line.
The breaking strain is the force needed to break a line (rope). The working strain is the greatest amount of force that can be placed on a line without damaging the line. Nylon has a working strain of about 10% of its breaking strain. Dacron has a working strain of about 20% its breaking strain. Lines also come in a three strand weave or braided. The braided weaves are approximately 20% stronger than stranded line. A knot decreases the breaking and working strain by 50%. A splice decrease the breaking and working strain by 10%.
Type of Line
Stretch at a Load of 50% Breaking Strength
Stranded Nylon 30%
Braided Nylon 15%
Stranded Dacron 10%
Braided Dacron 10%
Kevlar & Spectra 5%
Rope Breaking Strength
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.
(Circumference2 X 900) = Breaking Strength
When you purchase line you will buy it by its diameter. However, for purposes of the USCG license exams, all lines measurements are given in circumference not diameter. In the real world, to convert diameter into circumference use the following formula.
Circumference = π(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 = 2,218 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.
(900 lbs. X circumference2 ) X comparison factor = 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.
(1.572 X 900) X 2.5 = 5,546 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 splice. 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.
The safe working load (SWL) of a line is computed using safety factors. Usually, a Safety Factor (SF) of 10 is used when the work directly involves human life. A SF of 6, when working a safe distance from a load. For example, what would be the SWL with a SF of 10 of a line with a BS of 8,100 pounds? SWL would equal the BS divided by the SF, or SWL = 8,100 ÷ 10 or 810 pounds. Hint: a short ton is 2,000 pounds while a long ton is 2,250 pounds.
Breaking Stress and Safe Working Load Study Problems
1. What is the breaking stress of a 1 ¾” circumference manila line?
2. What is the breaking stress of a 1 ¾” circumference nylon line?
3. Using a safety factor of 10, determine the safe working load of a Dacron line with a breaking stress of 15 tons?
4. What is the breaking stress of a 2 5/8” circumference polypropylene line?
5. Determine the size of a manila line with a breaking stress of 12,000 pounds.
6. Using a safety factor of 6, determine the safe working load of a 1” diameter nylon line.
7. Using a safety factor of 6, determine the safe working load of a nylon line with a breaking stress of 72,000 pounds.
Answers: 1. 2,760 2. 6,900 3. 1.5 tons 4. 8,700 5. 3-5/8” 6. 3,400 7. 12, 000
Knots, Bends, and Hitches
A good knot must be easy to tie, hold without slipping, and be easy to untie. The choice of the best knot, bend, or hitch to use depends largely on the job it has to do. Always follow this rule: never tie a knot on which you are not willing to stake your life.Each of the three terms--knot, bend, and hitch--has a specific definition. In a knot, a line is usually bent or tied to itself, forming an eye or a knob or securing a cord or line around an object, such as a package. In its noun form, a bend ordinarily is that used to join the ends of two lines together. In its verb form, bend means the act of joining; bent is the past tense of bend. A hitch differs from a knot and a bend in that it ordinarily is tied to a ring, around a spar or stanchion, or around another line. In other words, it is not merely tied back on itself to form an eye or to bend two lines together.Tying a knot, bend, or hitch in a line weakens it because the fibers are bent sharply, causing the line to lose varying degrees of its efficiency or strength. A general rule to follow is to use a knot, bend, or hitch for temporary work and use a splice for permanent work because it retains more of the line's strength. For example, the following indicates the retained strength of the line:
Square Knot – 40 – 50%
Sheet Bend – 50 – 60%
Carrick Bend – 50 – 60%
Bowline – 65 – 75%
Anchor Bend: 5/8” radius – 55 – 65%
Anchor Bend: 4” radius – 80 -90%
Elements of the Knot, Bend, and Hitch
The overhand knot is the basis for all knots. It is the simplest of all and the most commonly used. It may be used to prevent the end of a line from untwisting, to form a knot at the end of a line, or to be part of another knot. When tied to the end of a line, this knot will prevent it from running through a block, hole, or other knot.
Figure Eight Knot:
The figure eight knot is used to form a larger knot at the end of a line than would be formed by an overhand knot. It is used to prevent the end of the line from running through a block. It is an easy knot to tie.To tie this knot, form an overhand loop in the line and pass the running end under the standing part, up the other side, and through the loop. Tighten the knot by pulling on the running end and the standing part.
Use the square knot to tie two lines of equal size together so that they will not slip. For a square knot, the end and standing part of one line come out on the same side of the bight formed by the other line. This knot will not hold if the lines are wet or are of unequal sizes. It tightens under strain but can be untied by grasping the ends of the two bights and pulling the knot apart. Its strength is 45 percent.To avoid a "granny" or a "fool's knot" which will slip, follow this procedure. Take the end in your right hand and say "over and under." Pass it over and under the part in your left hand as shown in With your right hand, take the end that was in your left hand. This time say to yourself "under and over." Pass it under and over the part in your left hand.
Sheet or Becket Bend:
Use a single sheet or becket bend to tie two lines of unequal size together and to tie a line to an eye. Always use a double sheet or becket bend to tie the gantline to a boatswain's chair. The single sheet or becket bend will draw tight, but will loosen when the line is slackened. The single sheet or becket bend is stronger than the square knot, with a strength of 55 percent, and is more easily untied than the square knot.To tie a single sheet or becket bend, take a bight in the larger of the two lines. Using the smaller of the two lines, put its end up through the bight. Then put it around the standing part of the larger line first because it will have the strain on it and then around the end of the larger line. Next put the end of the smaller line under its standing part. The strain on the standing part will hold the end. Notice in the double sheet or becket bend that the end of the smaller line goes under its standing part both times.
Use the bowline to tie a temporary eye in the end of a line. A bowline neither slips nor jams and unties easily. An example of a temporary use is that of tying a heaving line or messenger to a hawser and throwing it to a pier where line handlers can pull the hawser to the pier, using the heaving line or messenger.To tie a bowline, hold the standing part with your left hand and the running end with your right. Flip an overhand loop in the standing part, and hold the standing part and loop with the thumb and fingers of your left hand. Using your right hand, pass the running end up through the loop, under the standing part, and down through the loop. Its strength is 60 percent.
A bowline on a bight gives two loops instead of one, neither of which slips. It can be used for the same purpose as a boatswain's chair. It does not leave both hands free, but its twin, non-slipping loops form a comfortable seat. Use the bowline on a bight when:
Strength (greater than a single bowline) is necessary.
A loop is needed at some point in a line other than at the end.
The end of a line is not accessible.
There are at least seven (7) types of bowlines. The pinnacle of which is the “flying bowline”.
The bowline is easily untied and can be tied at the end of a line by doubling the line for a short section.To tie a bowline on a bight double the line, form an overhand loop, and put the end of the bight through the loop. Put your hand through the bight, take hold of the bight under the loop, and pull it through the first bight to tighten the knot.
Use a French bowline as a sling for lifting an injured person. For this purpose, one loop is used as a seat and the other loop is put around the body under the arms, then the knot is drawn tight at the chest. Even an unconscious person can ride up safely in a properly secured French bowline, because his weight keeps the two loops tight so that he will not fall out. It follows, though, that it is necessary to take care not to allow the loop under his arms to catch on any projections. Also use the French bowline where a person is working alone and needs both hands free. The two loops of the knot can be adjusted to the required size.
Double Carrick Bend:
A double carrick bend with its ends seized is recommended for tying together two hawsers. Even after a heavy strain, it is easy to untie because it never draws up. Its strength is 56 percent. However, a double carrick will draw up if the ends are not seized.
Use the half hitch to back up other knots, but tie with the short end of the line. Never tie two half hitches by themselves. Instead, take two round turns so that the strain will be on the line, not the hitches, and then tie the hitches.
The best knot for tying a line to a ring, a spar, or anything that is round is a clove hitch. It will not jam or pull out. Its strength is 55 to 60 percent.
Wire rope may be made by either of two methods. If the strands or wires are shaped to conform to the curvature of the finished wire rope before laying up, the wire rope is termed preformed. If they are not shaped before fabrication, the wire rope is termed non-preformed. When cut, preformed wire rope tends not to unlay, and it is more flexible than non-preformed wire rope. With non-preformed wire rope, twisting produces a stress in the wires; and, when it is cut or broken, the stress causes the strands to unlay. In non-preformed wire rope, unlaying is rapid and almost instantaneous, which could cause serious injury to someone not familiar with it. The main types of wire rope used have 6, 7, 12, 19, 24, or 37 wires in each strand. Usually, the wire rope has six strands laid around a fiber or steel center. Two common types of wire rope, 6 by 19 and 6 by 37 wire rope, are shown in figure 1-4. The 6 by 19 type of wire rope, having six strands with 19 wires in each strand, is commonly used for rough hoisting and skidding work where abrasion is likely to occur. The 6 by 37 wire rope, having six strands with 37 wires in each strand, is the most flexible of the standard six-strand wire ropes. For that reason, it is particularly suitable when you are going to use small sheaves and drums, such as are used on cranes and similar machinery.
Wire Rope Defects
Tension BreaksTension wire breaks typically have one end of broken wire coned or cupped. Necking down of the broken ends is typical of this type of break. Where tension breaks are found, the rope has been subjected to overloading, either for its original strength (new rope) or for its remaining strength in the case of a used rope. Tension breaks frequently are caused by the sudden application of a load to a slack rope, thereby setting up calculable impact stresses. The necking down of wire rope indicates excessive tension.
Abrasion BreaksAbrasion breaks typically has broken ends worn to knife-edge thinness. Abrasive wear obviously is concentrated at points where the rope contacts an abrasive medium, such as the grooves of sheaves and drums, or other objects with which the rope comes into contact. Unwarranted abrasive wear indicates improperly grooved sheaves and drums, incorrect fleet angle, or other localized abrasive conditions.
Fatigue BreaksWire breaks are usually transverse or square showing granular structure. Often these breaks will develop a shattered or jagged fracture, depending on the type of operation. Where fatigue breaks occur, the rope has repeatedly been bent around too small a radius. Whipping, vibration, slapping and tensional stresses will also cause fatigue. Fatigue breaks are accelerated by abrasion and nicking.
Corrosion Breakseasily noted by the wire's pitted surface, wire breaks usually show evidence of tension, abrasion and/or fatigue. Corrosion usually indicates improper lubrication. The extent of the damage to the interior of the rope is extremely difficult to determine; consequently, corrosion is one of the most dangerous causes of rope deterioration.
Cut or ShearWire will be pinched down and cut at broken ends, or will show evidence of a shear-like cut. This condition is evidence of mechanical abuse caused by agents outside the installation, or by something abnormal on the installation itself.
Wire Rope Measurements
Rope diameters are determined by measuring the circle that just touches the extreme outer limits of the strands — that is, the greatest dimension that can be measured with a pair of parallel-jawed calipers or machinist’s caliper square. A mistake could be made by measuring the smaller dimension.