The effects of tying a knot in your rope
Knots are convenient and suitable for some applications, but it is very important to be aware of the detrimental effect they have on the strength of your lines.
It is a good idea to avoid tying a knot in any rope because every hitch, bend or knot has a detrimental effect on the strength of the line.
A line with a knot in it will almost always fail or rupture at the point where the line is bent (the knot is formed) or very close to it. If a rope breaks, other than at a knot, it will almost certainly be due to damage or a defect.
Tying a knot involves bending the rope not only around a third-party object, but more critically around itself. When a knot is tightened, the flow of the twisted or laid rope fibres is disrupted causing them to be stressed unevenly. This imbalance leads to a loss of tensile strength.
Technically speaking – Rope Construction is designed for tensile strength in a straight line. Any sharp bend in a rope, will mean that the rope fibres on the outside of the turn will bear most of the load, while those on the inside of the turn will bear only a small proportion or possibly none of the load at all. Even a curve in a tensioned line will cause extra load on the outer fibres and compression on the inner fibres, which prevents them from moving freely to take their share of the strain. A sharper bend, especially when turned on itself, will suffer a correspondingly more serious loss of strength. A Sharp Bend, in this instance, is generally defined as a turn with an internal diameter which less 4 x the size (diameter) of the rope.
The efficiency of individual knots is not an exact science. Variation will depend on several factors: rope fibre content, rope construction, rope size, whether it is wet or dry and individual circumstance.
Approximate values where the straight-line load = 100%
Double Fisherman 65-80% - Bowline 60-75% - Figure of Eight 75-80% - Round turn and two half hitches 65%-70% - Clove Hitch 60-65% - Overhand 60-65% - Reef knot 25%-30% - Sheet bend 40%-45%
It is also worth noting that different rope fibres cope with a knot or sharp bend with varying degrees of efficiency in terms of resultant tensile strength.
LIROS approximate values where the Manufacturer Break Load = 100%
Nylon (Polyamid) 60-65% - Polyester 55-60% - Polypropylene 55-65% - Dyneema 35-50% - Vectran 30-35% - Kevlar 30-40%
A well-constructed splice completed by the Jimmy Green Rigging Team will preserve most of the tensile strength of the rope with an efficiency ranging from 80-100% depending on the type of splice, the rope fibre and the rope construction.