Cordage 101
By Michael Argyelan
Strings, lines, ropes, running rigging, cordage, whatever
you want to call it, understanding the basics of cordage is an important aspect
of being a good sailor. If you don’t know the breaking strength of your
halyards, you might be putting yourself and your crew at risk. Do you know what
stretch and creep are? How about core dependant versus cover-core dependent?
No? Well you will after reading this article. First, let’s explore the fibers
and materials used in the covers and cores of lines.
Polyester: A general grade line for applications not too
concerned with stretch. Most covers of lines are constructed of polyester.
There are different grades available to the line manufacturer, which affect
color retention, sheen, wear and UV degradation. Polyester stretches less than
nylon, and is durable and economical.
Spectra: Very strong and durable. It has super low stretch,
and is light enough to float. Spectra has the best fatigue life of most any
fiber. One issue with Spectra is “creep”; under high static loads (think main
halyard), Spectra will elongate very slowly. After a period of time, you might
notice that your main halyard now seems an inch looser. Using one size larger
will greatly reduce creep, but it may still be noticeable. Spectra is the
ultimate material for high fatigue and lightweight spinnaker halyards, guys and
sheets. Spectra does not absorb moisture, which keeps it lighter on the boat.
Dyneema: Dyneema is like a fraternal twin to Spectra.
Dyneema has a slightly different molecular structure than Spectra and higher
breaking strength in the larger diameters. It displays slightly better wear
characteristics in heavy use areas, but has more elasticity than Spectra. Like
Spectra, Dyneema has a relatively low melting point making it susceptible to
friction damage.
Vectran: A liquid crystal polymer fiber, it is another fiber
that is very popular for low stretch applications. This fiber has very low
stretch and high breaking strength, low creep, good in tight bends, and has
good chafe resistance. Vectran is very popular for control lines and halyards
requiring extremely low stretch. It has an excellent track record and is used
on many boats. Vectran is an alternative to Dyneema as it has less stretch and
creep, although it does not have quite the fatigue life or UV resistance of
Dyneema. It is also slightly heavier and does not float.
Technora: Technora is an Aramid fiber, similar to Kevlar. It
has very high strength and low creep, but is more susceptible to fatigue,
especially around small diameter sheaves and tight bends. It is also very
susceptible to UV degradation and so must be protected with a cover or a
coating on the core. It is more suitable for halyards and other lines that
remain static when loaded. Cruiser/day sailors with furling jibs that stay up
all year would benefit greatly using a line with a Technora core.
Creep vs. Stretch: Creep is non-recoverable elongation,
whereas stretch is elasticity or recoverable elongation. A “stretchy” line (ie.
Polyester) will return to approximately its original length when unloaded.
Lines that are prone to creep or elongate (especially Spectra or Dyneema) will
gradually stretch under load but will remain at the stretched length after the
load is removed. They will eventually stretch and then creep to failure. All
line materials are susceptible to stretch and creep. Now let’s explore the
difference between a core dependant line and a cover-core dependant line.
This
image shows a polyester cover and a Spectra core.
The typical day sailing boat with run-of-the-mill cordage is
likely to have cover-core dependant lines on their boat such as New England
Ropes Sta Set (polyester cover and core). If you’ve ever looked at your lines
closely, you’ll notice that there’s a cover (white or colored with some sort of
flecked color or a solid color without a fleck) and inside is either a white or
colored core. The strength of a cover-core line is completely dependant on both
the cover and the core being in optimal condition.
Let’s say you have a main halyard that is starting to chafe
in the area near the shackle where the line goes around the sheave when at full
hoist. Any chafe on the cover of your halyard essentially means that you now
own a compromised line holding up your main sail. The strength of the halyard
is dependant on the cover and the core working together.
Lines can also be compromised by UV degradation. Have an old
halyard that is left on the boat year round with the mast up? It’s likely that
your cover-core dependant line has UV degradation and its strength has been
compromised. Nothing like a halyard snapping during an enjoyable day sail to
ruin that freshly poured rum drink!
Core dependant line is just that, the strength
characteristics are dependant on the core of the line. Many, if not most,
common core dependant lines have polyester covers to help protect against UV
degradation, provide better chafe resistance, and make the line easier to
handle. However, the difference between a cover-core dependant line and a core dependant
line is, if the cover starts to chafe or break down due to high UV degradation,
your core is likely still operating at a high level and uncompromised like a
cover-core dependant line.
Typical core materials are likely to be the ones we covered
earlier in this article: Spectra/Dyneema, Technora/Aramid, and Vectran. Many
race boats will help save weight on the boat by stripping back the cover of the
line except where the line meets the clutch or winch and leave just the core of
the line exposed. Doing this of course limits the life of the line as the core
is now exposed to UV and chafe but has not changed the strength of the line.
This image is of a Spectra core.
Some lines are sold without a cover at all. Of these, some
have a special coating to help with UV issues, and others do not. In fact, many
boats have started to use Spectra/Dyneema lines in lieu of wire for lifelines
as the cordage is lighter and in most cases, stronger than the wire they
replaced.
This image is of Yale PhD cordage. This is
the example of where a “cover” is simply spun over the Spectra “core”.
Not all lines have a “core” per se. Some lines like Maffioli
Swiftcord and Yale PhD have the “cover” spun around the “core” and therefore,
look like a core only line. Most often the spun cover is there to protect the
core of the line from chafe, UV, and to provide a good “hand” or grip when
using the line. The polyester (likely) spun cover also provides a good grip on
winches, cam cleats, and the like. Some spun materials will even have an added
“special sauce” to it so grip is enhanced. Yale PhD uses what they call iGrip
to improve the feel and grip. A huge bonus of lines like these is that they are
very light, feel great in the hand, and have almost unbeatable strength to
weight ratios.
Of course there are also other lines made from other
materials. Some lines are made of mixed materials like poly/aramid covers, etc.
In the world of running rigging materials, it’s sort of like Burger King, have
it your way.
Choosing line diameter: To determine what line size
(diameter) is appropriate, you will need to know what size line your winches
(self tailing), clutches, blocks, and sheaves can handle. You may want a big,
fat, grippy 1/2" line for your jib sheets, but if your self tailing winch can
only handle up to 7/16” line, then the max line you can use is 7/16”.
The same criterion applies to halyards. If you want a larger
diameter line because it’s easier for your crew to hoist and easier on the
hands, you will need to know if the halyard sheave at the top of the mast can
handle the line size you want. You will also need to know the max diameter that
your clutch and cabin top self tailing winch can handle. If you have a mast
base block and deck organizer (likely) then you will have a max diameter that
you can use in those too.
Someone with little to no knowledge of cordage would think
choosing line is pretty easy. Pick a color, pick the size that feels good, and
go. This is obviously not the case. Or at least it’s hopefully clear to you
after reading this article! Always pick your running rigging based on the
application, the performance and characteristics of the line, the size of your
sheaves, winches, clutches, and then and only then, your budget and favorite
color.
A quick note on splicing: We highly recommend that you
splice halyards to your shackle. A proper splice will retain almost 100% of a
line’s given breaking strength. When you tie a bowline knot, you will lose
almost 20% of the load carrying characteristics. As you may have noted above,
some high tech lines don’t perform well in tight bends. This is even more
reason to have your halyards spliced to a shackle.
If you have any questions on cordage, want a running rigging
inspection of your vessel, or advice on the right lines for your particular
boat, please do not hesitate to contact me at
mma@crowleys.com
or
773.364.1344.
I hope this finds you well and warm.
