Rope Strength Calculations

Working Limits

Any material has structural limits and we define this for rope in different ways. The minimum breaking strength (MBS) is how much stress it takes to part, or snap, a line. The working load limit (WLL) is how much stress a line is designed to handle in general service.

Natural & Synthetic Breaking Strength

The natural breaking strength of manila is the standard against which other lines are compared. Synthetic lines have been assigned comparison factors which can be plugged into the breaking strength formula as necessary.

Note that for the purposes of testing, the Coast Guard will always give and ask for measurements of natural and synthetic rope by circumference. Once you begin your career, this will not be the case, as rope is practically measured in diameter.

(900 lbs. X circumference^2) x M = Breaking Strength

Where M is the material being measured for breaking strength:
Manila = 1
Nylon = 2.5
Dacron = 2.0
Polypropylene = 1.4

Example: What is the breaking strength of a 4-inch manila line?

900 x 4 x 1 = 14,400 lbs.

Safe Working Load Calculations

To provide a margin of safety a line is calculated to allow a safety factor and unless otherwise specified this is implied to be a factor of five. This means if a line has a MBS of 20,000 lbs., with a safety factor of five, then the WLL will be 4,000 lbs.

To derive the SWL when given the MBS:
Breaking Stress x Safety Factor = SWL

To derive the MBS when given the SWL:
SWL / Safety Factor = MBS

Example: Using a safety factor of 6, determine the safe working load of manila line with a breaking stress of 8 tons?

8 x 6 = 48 tons

Note that practically speaking the breaking strength of a line will reduce over time, whether from rot, UV decay, rust, or other components of weather and age. Also, overloading a line (beyond 75% of its MBS) may permanently damage the line which is more likely to happen during shock loading.