With regard to the weight of ropes, it may be said that ropes of all kinds are usually measured by their circumference. The weight of clean, dry, hemp rope in pounds per fathom is one-fourth of the square of the circumference in inches; for example, a 3-in. hemp rope (about 1 in. in diameter) weighs ¼ × 32^2 = 2¼ lb. per fathom (6 ft.). A flat hemp rope, with a width of about four times the thickness, weighs in pounds per fathom about twice the square of the circumference in inches; for example, a 3-in. by ¾-in. flat hemp rope will weigh about 2 × 7 = 14 lb. per fathom.
Round wire ropes weigh in pounds per fathom seven-eighths of the square of the circumference in inches; for example, a 3-in. wire rope weighs about ⅞ × 3^2 = 7⅞ lb. per fathom. A flat wire rope weighs in pounds per fathom ten times the sectional area in square inches; for example, a flat wire rope, 3 in. by ¾ in. = say 2 sq. in. area, will weigh about 10 × 2 = 20 lb. per fathom.
The maximum safe load on a rope depends on many circumstances, such as quality, age and dryness of rope, nature of load, mode of lifting, etc. Approximately, the safe load on a new hemp rope in hundredweights with direct lift is three times the weight in pounds per fathom. On a sound old rope fall one-half the square of the circumference is sufficient load. A Bessemer steel wire rope will safely carry in hundredweights three times the square of its circumference in inches, and a crucible steel wire rope four times the square of its circumference. For hemp ropes the minimum diameter of sheave should be circumference of rope + 2, and for wire ropes the diameter of sheave in inches should be equal to circumference of rope in sixteenths.
The principle of rope making is very readily shown by holding the ends of a piece of twine or whipcord, about a foot long, in the hands and twisting it so as to increase the lay. If the twine be now slackened by bringing the hands nearer to one another, a loop will first form in the middle of the twine, and it will continue to twist itself up into a compact cord which will not unlay, as the tension to which the strands have been subjected causes friction between them, which holds them together. In other words, the tendency of each part singly to unlay, acting in opposite directions, is the means of keeping them together when joined.
Some very interesting experiments were made by Réaumur, the purposes of which were to ascertain the loss of strength occasioned by laying up the fibres of various substances, one or two of which are given.
1. A thread, consisting of 832 fibres of silk, each of which carried 1 dram and 18 grains, broke with a weight of 5 lbs., though the sum of the absolute strength of the fibres is 104 drams, or upwards of 8 lbs. 2 oz.
2. Three threads were twisted together, their mean strength being nearly 8 lbs. They broke with 17½ lbs., whereas they should have carried 24 lbs.
These experiments prove that though convenience and portability are gained by twisting the fibres, there is a great loss in the strength of the resultant rope.
In speaking of the size of a rope, the circumference and not the diameter is alluded to. Thus, a three-inch rope would be slightly less than an inch in diameter.
In practising knotting it is as well to use a tolerably firm material, such as whipcord, for small common knots, or, still better, line used for sea fishing. Either can be tied up and undone over and over again without injuring it, which is not the case with twine; it is also more easy to see which way the parts of a knot lie in the harder material, and then to find out whether the turns are properly made or not. For more complicated knots, particularly those where the strands of the rope have to be unlaid to form the knot, such as a wall knot (p. 66) or a Matthew Walker (p. 70), it is advisable to use three strands of fishing line, each about a foot long. If a “seizing” (a seizing is shown in Fig. 57, p. 51) be put round them in the centre, so as to hold them firmly together, a good representation of a rope with the strands unlaid ready for working is obtained. A knot can be made and unmade as often as required in this way, without detriment to the strands; but the strands of a rope, owing to their loose nature, will seldom bear knotting more than once or twice. If desired, the knots can be made as above described and kept for future reference. In string also it is better to use hard laid stuff at first, but when these matters are thoroughly understood knots can be made on any sort of cordage without difficulty.