What is the issue?Knots weaken the rope in which they are made. When knotted rope is strained to its breaking point, it almost always fails at the knot or close to it, unless it is defective or damaged elsewhere. The bending, crushing, and chafing forces that hold a knot in place also unevenly stress rope fibres and ultimately lead to a reduction in strength.
Why address this?All knots reduce the breaking strength of rope, and any service which that line provides is in jeopardy when it has a knot. The most extreme example is an anchor rode in heavy weather, if it has a knot its service load is significantly reduced placing the vessel in jeopardy.
How to address this?''Every knot diminishes the strength of rope to some degree. The reason for this is that in any sharp bend of a rope (less than four times the diameter of the rope), the rope fibres on the outside of the bend carry the majority of the load on the rope. The fibres on the inside of the bend will carry very little of the load or none at all.''
Vines and Hudson, High Angle Rescue Techniques, 54
A straight segment of rope is the strongest because it distributes the load evenly on each of the fibers and stresses them uniformly. A curved segment weakens the rope because it distributes the load unevenly, which strains some fibres and compresses others. This causes it to fail, normally just outside of the knot itself.
Photo: Leonard J Matthews via CC BY SA 2.0
Whenever possible use a tensionless hitch around a cylindrical object that is at least a 4x or more the diameter of the line. Observing this keeps the line safely on the right side of the 4-to-1 Rule. The 4-to-1 rule states that if a rope is bent around an object at least four times its diameter, there will be no loss of efficiency due to bending. So the “tightness of the bends” also determines the efficiency ratings of knots. Any bend tighter than 4x reduces the strength of rope because they stress the rope in flexure, the fibres’ weakest direction. Going wide, on the other hand, reduces and entirely eliminates the problem. To maintain maximum line efficiency one should also avoid acute bends. The sharper a bend a line is put through, the greater the efficiency loss.
If this is not possible, then it is a matter of selecting a knot with the best relative knot strength, also called 'knot efficiency'. This is the breaking strength of a knotted rope in proportion to the breaking strength of the rope without the knot. If a straight line with no knot has 100% of line strength, these are the efficiencies of commonly used sailing knots:
- • Double Fisherman’s 65-79% of the rope's original strength.
- • 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%
These are all mean numbers. Determining a precise value for a particular knot is difficult because many factors can affect a knot efficiency test: the type of fibre, the style of rope, the size of rope, whether it is wet or dry, how the knot is dressed before loading, how rapidly it is loaded, whether the knot is repeatedly loaded, and so on.
Rope splices, however, can maintain nearly all the rope's full strength with a well-made eye splice reducing the breaking strength of the line by less than 5%. Unfortunately, in most situations forming loops and bends with conventional knots is far more practical. The working load limit of a rope is generally specified with a significant safety factor, up to 15:1 for critical applications.
With thanks to:Michael Harpur, Yacht Obsession.
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