Myth # 15: Rope strength can be determined by looking at the manufacturer’s documents.
NO! Rope strength includes the calculation of 3 factors every time a rope is used.
- Knots! A knot will reduce the strength of your rigging by approximately 1/3 of the rope‘s strength.
- Wet/Dry! Nylon rope will lose 15% of its strength when wet. Polyester loses no strength when wet.
- Age! A rope loses about 2% of its strength per year even when it is kept in a like-new condition , even if it has never been used.
These three are allotted for when using the 10 to 1 safety margin. Unless any of the three are excessive, i.e. a 20 year old wet rope with an overhand knot (50-55% efficiency) can push a rope beyond its life support abilities.
( -40% age- 15% wet, -45% knot) = 0 strength.