Determining Rope Strength

Tensile Strength vs Working Load

Tensile strength is the average strength of a new rope under laboratory conditions. This is determined by wrapping the rope around two large diameter capstans and slowly tensioning the line until it breaks. Some manufacturers refer to this as Breaking strength.

Working load is determined by taking the tensile strength and dividing it by a factor that more accurately reflects the maximum load that should be applied to a given rope to assure a comfortable safety margin before it reaches the breaking point. That factor varies with the type of fiber and the weaving construction. Adhering to the working load also prolongs the life of the line. These are only guidelines, factors such as the age of the rope and degradation through use must also be taken into consideration when judging the working load and breaking point of the rope.

Some manufacturers refer to Working strength as working load and define it as the weight in pounds that is recommended for safe working conditions. They apply this term to new rope in good condition with appropriate splices and only under normal service conditions. Where dynamic loading may occur, the recommended working load should be adjusted accordingly.

Elastic Stiffness, Why it Matters to Sailors

When sailors are selecting line for their running rigging, the most cited specification, breaking strength, is often not the most critical. For nearly all applications on a recreational sailboat, the strength of modern lines far exceeds both the load that it will experience, and the strength of the hardware connected to it. Other characteristics, such as size, hand, weight, coefficient of friction, and flexibility are more important criteria for selection. Of these, elastic stiffness, or stretchiness of the line is most critical.

Elastic stiffness is the resistance of a line to stretch under load. This is determined by the size, material, and construction of the line; and this is what sailors care about, because it impacts the way a rope handles. Before we can define the elastic stiffness of a rope, we must define one more thing: Strain, which is the percentage change in length of an object when under load. In other words, the higher the elastic stiffness, the lower the strain.

What does this mean, and how do we select the optimum stiffness of a line? Lines that control sail camber, such as halyards, outhauls, cunninghams, or jib sheets, benefit most from stiffness. If, when beating to windward in a fresh breeze with your sail in the ideal trim, you are hit by a puff, the pressure on the sail would increase and thus the load on all lines controlling that sail shape would increase, causing them to elongate. No one wants that. This elongation increases the depth of the sail camber, increasing both lift and drag on a sail.

There are cases where choosing a line with some stretch (lower elastic stiffness) is beneficial. A boom vang that is able to stretch when a puff hits allows the mainsail to twist and dump power from the head of the sail, thus preventing the boat from being overpowered. Similarly, using a lower stiffness line for a boom preventer is advisable since it lowers the shock load on hardware and deck fittings.

Beware How a Knot Affects a Line’s Breaking Strength

It may surprise you to find out that the working load for most kinds of rope is between 15% and 25% of the tensile strength. Now consider the fact that any time you tie a knot in a rope you effectively cut the tensile strength in half. With enough tension, a knot can cut the line. While certain kinds of knots damage the line less than others, the 50% loss of tensile strength is a good general rule to live by. Research has shown that the figure 8 knot reduces the tensile strength by approximately 35% instead of 50% for other common knots tested.