GENERALITY
The introduction of new technology to produce fibres
for textile use (man made fibres) and the parallel development
of naval industries, has consented the introduction
for the common employment of products realaized in syntetic
fibres.. Particulary are used fibres derived by macromolecules
of polymers. For certain specific application , e.g.
school ship Vespucci e Palinuro, are again used natural
fibres like Manilla or hemp.
All the more polymers used are:
Polypropylene –
Polyester - Polythylene-
Poyiamide
TYPES OF CORDAGE FOR NAVAL USE
Before to analyze some uses in naval application, it
is important to keep back some definictions :
Cordage : product indipendent of size,in
wich twisting, plaiting or braiding of any fibrous material
produces a finished article, sensibly round in cross
section. The collettive therm for rope, line and cord.
Twist : the number of turns applied
per unit length
Yarn : twisted entity composed of filament,
staple fibres, or fibrillated film held together by
twist
To lay : to twist two or more components
about each other to form a helix around the axis of
the resulting laid item , which may be a strand or a
rope. With this operation we can produce a strand or
a rope or a braid.
Strand : product obtained by twisting
together two or more yarns prior laying into a rope
Hawser laid rope : rope of three strands
where the strands are twisted to form helices around
the central axis.
Braided cord: cord of braided construction
Braided rope : rope formed by braiding
or plaiting the strands together as opposed to twisting
them. NOTE the terms “ braiding” and “plaiting”
are considered to be synonymous.
Eight strands plaited rope :rope normally
composed of four pairs of strands plaited in a double
4 strands coreless round sennit.
Pitch (or plait pitch): longitudinal
distance along the axis of the rope from a point on
tha crown of one strand (or plait) to the corresponding
point one strand (or plait) along.
The most important component in a rope
is the yarn or base yarn (yarn obtained from the manufacturer,
consisting in a wound package of parallel filaments
on which subsequent twisting operations are performed).Its
values of identification are linear density or linear
mass (the mass for unit length of linear material) and
tenacity (expression of breaking stress of cordage,
related to its fineness, rather than more usual relationship
to cross-sectional area. It is determinated by dividing
the breaking strength of the material by its linear
density to produce a value for tha tenacity of the material
in Newton/tex)
For naval use , it is possibile to find
three principal types of cordage:
- Hawser laid
- Plaited rope
- Braided rope
HAWSER LAID
A certain number of yarns are twisted together
to form a strand. The number and size of yarn to make
each strand depend on the size of the rope it is intended
to make. This stage il know as “ twisting the
strands” and again the twist can be left handed
or right handed. Three or four stands are noe made in
up into a left handed or a right handed rope. This process
is called “ laying” or “closing”
and is always carried out in the direction opposite
to that used in the previous stage of twisting the strands;
it is, moreover, distinct from the simple spin or twist
and is two fold in that:
- the strands are twisted up together to form the rope
and at the same time
- the strands are rotated individually in the direction
of the original twist.
When this not done, laying the strands up together would
tend to untwist the yarns in each strand. As the rope
is laid up, its length contracts like a coiled spring,
giving it a certain elasticity. The harder the twist
given to the strands in laying. The shorter will be
resultant rope and thus a rope is said to be hard laid,
ordinary laid or soft laid. In pratice, three strands
of 275 mt lay up into a rope of about 220 mt in length.
Three strands so laid up constitute a hawser laid rope.
The strands tend to unlay unless the end of the rope
is whipped (i.e. firmly bound) with twine. The rope
will stretch under load and will a not completely recover
when the load is removed. The rope acquires a permanent
and irreversible set; the higher the load in relation
to the breaking strength, the greater the set. The set
may be observed by the extension in length and reduction
in diameter when the rope is slack and will eventually
render the rope unifit for service. The older and more
worn rope, the less elasticity it will possess and the
weaker it will become. Rope under load will tend to
twist in the opposite direction to that of its lay and
thereby tend to unlay itself, but it should regain its
normal form when salck. When wet, will usually shrink
in length in proportion to the amount by which it swells
in diameter, but it will recover its original length
when dry and after use. Rope which is continually subjected
to heat and damp – when in the tropics, for example-
will lose its elasticity and strength sooner than rope
used under normal conditions of temperature and humidity.
PLAITED ROPE
The rope is constructed of eight strands arranged in
four pairs, two pairs of left hand lay and two pairs
of right hand lay. This arrangement is know commercially
as “squareline” but in naval use is commonly
referred to as multiplait. Its properties are very similar
to hawser laid except that it is softer rope and does
not kink.
BRAIDED ROPE
This rope , know commercially as core/cover rope, is
constructed by crossing and recrossing the yarns or
starnds in “maypole” fashion such as that
each yarn or strand passes alternatively over and under
one or more of the others to form a circular tubular
sheath, which may contain a core. All braided ropes
fall into one of the following categories:
- braided sheath around a braided core having a heart
of parallel strands
- braided sheath around a hollow braided core
- braided sheath around a core af either parallel strands,
or a three strand rope, or a mulriplicy of three strand
rope core members
- braided sheath with no core (hollow centred rope)
The use of braided rope is limited to certain specific
applications. Braided construction gives the following
advantages over hawser laid ropes:
- good flexibility and easy handling when wet or dry,
new or worn;
- non rotating and will not kink.
- more grip on capstains or warping drums because of
the greater contact area
.
|
