Rope strenghts and knots

[Guy Rossey]

Knots reduce the resistance of lines and ropes. This is quantified in an interesting  test which appeared in the French magazine Voiles & Voiliers n°484 , June 2011.

Interestingly, a 6mm Dyneema shroud resists at a load of 1 ton. But with a bowline in, it will break at 400 kg.

Other results of knots on the strength of lines and ropes (given in % of maximum charge which leads to rupture or slipping of the rope) :

Reef knot :                    PE: 30%   Dyneema: only 8%! , only to be used for ...reefing
Sheet bend:                   PE: 40%   Dyneema: 16%
Fishermans knot:           PE  52%   Dyneema: 25%
Carrick Bend:                 PE: 61%   Dyneema: 23%
Bowline:                        PE: 65% (one of the best)   Dyneema: 40% (still good)
Multiple overhand knot: PE: 57%   Dyneema: 52% , comparable to the bowline but not as easy to undo)
Splice:                           PE 87%    Dyneema: 95% , THE best !

Guy
Catchando Bay ( BR #48)

[Julian Swindell]

Dyneema just doesn't knot. It is too slippery. If you use the open weave braided Dyneema it is easy to splice and that works fine for shrouds and stays. I think other fibres are better for running rigging on the whole. I don't think most Swallow Boats really need low stretch halyards. Not with sprit booms and gunter main masts

[Julian Swindell]

I stand corrected. I don't have low stretch halyards because I can't pull them enough to stretch them in the first place!. I do use a handy billy to get the jib luff tight, so maybe I should use Dyneema there. But nothing is too tight on my boat.

[Jeremy]

There are two primary causes of rope failure at knots; uneven stresses across the diameter of the rope where it makes a tight radius turn under high load and friction searing of any slippage area.

When specifying parachute rigging knots the type of knot used varied with the specification of the cordage.  For example, most people will tell you that a bowline is a good secure knot for making a loop, which is partially true - it works pretty well on natural fibre three strand rope and ropes with low modulus fibres.  Unfortunately it has a tight-radius loaded bend that causes local rope fibre failure when used with high modulus synthetic materials (fibres with low stretch before failure), so reducing the overall safe working load.  For making a strong loop in a high modulus fibre rope a doubled figure of eight is both easier to tie reliably and tends to distribute loads more evenly into the cordage, so is stronger overall.  The snag is that this is a big knot, plus it doesn't tolerate being shaken about a lot (like a reef knot it may loosen if shaken about with no load applied).

Modern synthetic ropes add to the knot problem by having a low surface coefficient of friction, meaning that the constituent parts of the knot tend to slide against each other under load.  To make things worse, modern ropes sometimes have a non-load bearing outer sheath, that doesn't necessarily transfer loads well to the inner core, allowing greater relative movement within the knot.  The failure mode with this type of rope is often friction searing from this movement in the knot when a load is suddenly applied, leading to local softening/weakening of the synthetic fibres.  This is primarily a problem for ropes and cordage exposed to sudden dynamic loads though, so perhaps not a common failure mode on boat rigging.  I've seen cases of parachute rigging line failure where the shock load of opening caused attachment knots to fail - examining them revealed clear indication of melted fibres, you could even feel the roughness of the partially melted surface. 

Personally I'd try and avoid knots in all synthetic ropes if at all possible.  Some braided ropes can be easily and effectively spliced by simply opening up the braid and inserting a long tapered tail, ideally whipping the resulting splice to hold it in place.  Alternatively, a well stitched and whipped loop will be stronger than a knot and will work even on a solid braid rope.  I made all the halyards on my old boat this way, using high modulus braided polyester.  It took an evenings work, sat in front of the TV with a palm, some twine and a sailmakers needle to produce a set of halyards that lasted for years without failure.

[Guy Rossey]

Indeed Claus, here some clarifications on the test :

PE: Braidline, 10mm, 2€/m, rupture: 2080 daN, weight: 74,4g/m, stretch : 7% . Quite flexible
Dyneestar: 8 mm, 4,68 €/m, rupture: 3470 daN, weight : 63,7 g/m, using a Dyneema SK75 core for its light weight, stability to humidity, low stretch ( less than 1 %), 16 strands, two layers: one polyester for protection, the second one to avoid slipping of one on the other.  More rigid.

As for the reasons of failure , SLipping or Breaking:

Reef knot :                    PE: 30%  SL  Dyneema: only 8%! , SL
Sheet bend:                   PE: 40%  SL  Dyneema: 16%  SL
Fishermans knot:           PE  52% BR    Dyneema: 25%  SL
Carrick Bend:                 PE: 61% BR  Dyneema: 23%  SL
Bowline:                        PE: 65% BR  Dyneema: 40% BR
Multiple overhand knot: PE: 57% BR  Dyneema: 52% , BR
Splice:                           PE 87%  BR  Dyneema: 95% , BR

I send you a scanned copy in parallel.