Marlinespike - 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.
(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 = 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.
(comparison factor X 900 lbs. X circumference2 = breaking strength)
Following is a comparison factor chart for synthetic lines.
Comparison Factor (greater than manila)
Using the example above, lets 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 = 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 slice. However, a splice is stronger than a knot.
Just being able to calculate breaking strength doesnt 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 dont know if that particular piece of line was above average or below average. Next we will discuss safe working load.