There’s an incredible array of shiny new high-tech rope in every
chandlery. Imaginative manufacturers have matched new fibres with
new construction techniques to try and develop the perfect rope for
every purpose – but how to choose? This is a short guide from
Towergate Insurance, specialist providers of Yacht Insurance helps the
sailor pick their way through the multitude of new rope
products.
Rope seems a rather dull name for the rolls of luminous,
space-age materials that we now see lining the shelves of any
well-stocked chandlery - we’ve come a long way from winding three
strands of hemp yarn together. And it’s not just the materials that
have changed; the modern rope is a two-part construction using an
inner core material and an outer cover. It stretches and kinks much
less than the old three-strand, and works much better in modern
sail handling equipment.
The inner core is where the strength of a modern rope lies, and
there are three ways that it can be constructed. The simplest is
lying the yarns straight, aligned and in parallel. The less twisted
the yarn fibres are from the line of the load - i.e. along the rope
- the stronger the rope will be in a straight pull. So this
parallel construction is the strongest. The disadvantage of the
parallel construction is that it’s weaker when it runs around
bends, as the load is unequally shared amongst the strands. This is
a bigger problem for the low stretch fibres, as they will take up
the load more unevenly.
One solution to this is to braid the core by weaving the yarns
together. This is stronger than the parallel construction around
corners because all the yarn fibres load up equally, but
correspondingly it’s not as good in a straight pull. There will
also be extra abrasion between the braided yarns, so the rope will
wear out more quickly. There is a middle path - constructing the
core with a three-strand weave, just like the old hemp ropes. It’s
not quite as strong in a straight pull as the parallel core, but is
superior around corners. While compared to the braided core, the
three-strand is stronger in a straight pull but weaker round
bends.
The rope covers are always braided, usually with an eight or
16-plait construction: the more plaits there are the more rounded
the rope and the tighter the finish. This affects how it feels and
handles, runs round winches, jams in cleats and so on. The yarn
type is important for the cover. There are two sorts, spunstaple
yarns are made from short lengths, or filaments, of the material,
whereas continuous filament yarns have individual filaments as long
as the length of the rope. The continuous yarns are stronger, for
obvious reasons, and will always be used in the core. But often
spunstaple makes a better cover since it has a matt, almost hairy
feel with plenty of friction, compared to the shiny finish of
continuous yarn covers.
So much for the construction, now let’s get to grips with some
serious jargon – the seven basic rope manufacturing materials. Here
we go:
Polypropylene
Pros: Does not absorb water.
Cons: Low Strength.
Thoughts: Polypropylene has only about 60 per
cent of the strength of nylon and polyester, and so it has little
to recommend it as a working rope. Its big redeeming quality is
that it does not absorb water, and if used in places where the rope
rarely or never loads up, then it can save a lot of weight for
racing sailors.
Nylon
Pros: High-strength.
Cons: High-stretch.
Thoughts: Nylon’s stretchiness make it
completely inappropriate for a halyard, but it’s great as a mooring
rope where it will absorb the energy from the movement of the
boat.
Polyester - AKA: Dacron
Pros: High-strength, excellent weather and
abrasion resistance.
Cons: Moderately low-stretch.
Thoughts: It’s one of the best materials for
the cover of a rope, as well as doing good service in a lot of
applications where weight and stretch are not critical, such as
cruising boat sheets.
Aramid - AKA: Kevlar
Pros: Very low-stretch, high-strength.
Cons: Weak when point loaded, poor UV and
abrasion resistance.
Thoughts: It has the same chemical base as
nylon, but with additional molecules that make the structure more
rigid. This gives the material its great strength, as well as the
ability to withstand temperatures of up to 500 degrees. But the
rigid molecular structure is also why it’s weak when point loaded –
a serious failing for some usages, such as being tied in a knot! In
addition, the poor UV and abrasion resistance means that for most
uses it needs a cover.
High Modulus Polyethylene (HMPE) - AKA: Dyneema and
Spectra
Pros: Very low-stretch, high strength, and high
fatigue life. Doesn’t absorb moisture and floats.
Cons: Creep – under a steady high load the rope
slowly elongates. The material is also slippery, so that it needs
careful design to avoid the core slipping through the cover.
Thoughts: Its characteristics make Spectra a
good material for spinnaker halyards, guys & sheets. It has
better durability than Kevlar.
Vectran
Pros: Very low-stretch, high-strength, low
creep, point loads well and has good abrasion resistance
Cons: Not many...
Thoughts: Although in many key ways it
out-performs HMPE, Vectran doesn’t have quite such a good fatigue
life, or UV resistance, it’s a touch heavier and doesn’t float.
PBO
Pros: Highest-strength and lowest stretch
fibre, with no creep.
Cons: Poor UV and abrasion resistance
Thoughts: This is the rope for the high-tech
race boats, it’s the strongest and lowest stretch, with no creep –
but it’s also the most expensive, and suffers so badly in the sun
from UV degradation that it needs to be kept in a box in the shop,
and should only be put on the boat for regattas!
Final Thoughts...
These seven materials are used either on their own, or combined
to produce a composite rope with the right properties for a
specific job. Just as a hull of exactly the right weight, cost and
strength is built from choosing the right resin system, matching it
with the right fibre reinforcement (like carbon or Kevlar), and
then sandwiching it around the right core material (like balsa or
foam); so the rope manufacturers combine their materials to provide
exactly the right rope for a particular job and cost.
So remember, when you head out to buy a new halyard or sheet,
there’s no such thing as the 'best rope' – just the right one for
the job, at a price you are prepared to pay. Hopefully, with these
notes, you’ll have a much better chance of figuring out which one
that is!
Ask the experts
For risk management and Yacht
insurance advice phone 0844 892 1987.
