Rudy writes:
(This is one of the reasons why I (along
with rope manufacturers) suggest that snubbers and rope rodes (any
rope) are
sized not to exceed 15% of the ropes tensile strength.)

Take Rudy's advice with a grain of salt. While not entirely wrong, it
is not entirely right either. Most rope manufacturers specify the
breaking strength of rope as the average of five tests of new rope
pulled to destruction by a test machine. The rope lengths are straight
and the force is slowly and continuously increased until the rope
breaks. You can generally have faith in the manufacturer's
specifications as long as the rope is new and the pull is steady.

Specification of the safe working load is another matter. The
manufacturer takes the breaking strength and divides it by an
arbitrary safety factor ranging from 2 to 10 determined largely by use
and the manufacturer's fear of liability. Most SWL estimates are based
on rope use in the construction industry where rope is often used for
hoisting weights overhead. For these conditions stretch is
undesirable. Also ropes tend to age and weaken in the sunlight and on
exposure to the environment. A safety factor of 5 to 7 is generally
suggested. This is in accord with Rudy's prescription. A nylon rope
with a breaking strength of 1500 lb thus has a SWL of no more than 300
lbs.

But use of nylon rope for anchor rodes and snubbers is another matter.
You want a rode or snubber to stretch under shock absorbing energy and
release the energy relatively slowly. That is the entire purpose of a
snubber. Heavily loaded new nylon rope can stretch up to 40% and
return to its former length without loss of strength. The 40% stretch
point is critical. Stretch it much more than that and it is likely to
part, the ends snapping back with missile like force. The following is
a quote from the Encyclopedia of Boating:
"The three most commonly encountered synthetic materials found on
boats are nylon, Dacron, and polypropylene. Their uses are as
follows:Nylon stretches about 10 to 15 percent under a load equal to
30 percent of its breaking strength; therefore, it is used for anchor
lines, docklines, and towing lines, where its great strength and
stretchability are advantages. Dacron, a polyester fiber, stretches
only about 3.5 to 5 percent under the same conditions, so it is used
for sheets, halyards, and other applications where stretch would be
undesirable. Polypropylene has little elasticity, tends not to retain
knots, and has only about 60 percent the strength of Dacron, but its
light and inexpensive and it floats. Its used for heaving lines,
dinghy painters, and water-skiers tow-lines."

Robert Ogg, inventor of the Danforth anchor, did extensive studies on
the influence of anchor rodes on anchor holding power and came to the
conclusion that nylon line loaded to a maximum of about 30% of its
breaking strength was preferable to either chain or wire rode. The
elasticity prevented lightweight anchors from being jerked out of the
bottom by transient waves or wind gusts. Chain was good too until
stretched tight.

An elastic line transforms a short quick change in velocity into a
long slow change in velocity. Think of the rode as a horizontal bungee
cord. If a bungee jumper lept from a high bridge attached to an
unyielding steel cable (or a chain) the force his body would feel when
he reached the end of the cable would be the same as if he hit the
ground. It would certainly rip his legs off. Attach him to an elastic
bungee cord which stretches as he falls, and the force is reduced to
the point where it is tolerable and even fun. The energy is stored in
the bungee cord and he rebounds almost to the starting height.

The same with a mooring line. The inertial force due to boat motion
from a strong wave impact would be transmitted directly to the anchor
if the rode did not have any elasticity. It is the elasticity of the
line that stretches the force out in time and diminishes its pull on
the anchor. The energy is stored in the line and released by slowly
pulling the boat back into position. A nylon line stores the energy by
stretch. When loaded to 30% of its breaking strength, a nylon line
will stretch almost 15%. That's why Ogg preferred light nylon lines to
get the maximum stretch. In a chain rode, the energy is stored by
gravity, flattening out the catenary. Gravity pulls the boat back into
position as the line resumes its normal curve. The heavier the chain,
the more energy stored and the more elastic the rode.

Bigger trawlers with all chain rodes require a heavier chain to store
enough energy to provide sufficient elasticity. Many trawler owners
confound this with the idea of rode strength. In reality a relatively
small diameter chain has enough breaking strength to anchor all but
the largest trawlers. Wire rope, pound for pound, is much stronger
than chain but is rarely used in trawler anchor rodes because it is
too light to provide much shock attenuation at all.

Of course chain and rope can be used in combination to provide some of
the advantages of each. William van Dorn in "Oceanography and
Seamanship";
Dodd, Mead (1974), presents a graph based on calculations for anchoring
oceanographic vessels in storm conditions. It suggests that the optimum
chain/nylon combination for anchoring vessels < 50ft. in 30 ft. of water
under storm conditions is a 20% chain, 80% nylon rode with an overall
scope
of 6:1. Assuming that the boat's bow chock is 6 feet above the water
and that
the waves are 4 feet (8 feet peak to trough) this works out to a 240
foot
total rode comprised of 48 feet of chain and 192 feet of nylon.
Clearly these
are extreme conditions. In shallower water the rode could be reduced
proportionately. However, the length of chain required approximates
one boat
length and a good working rule for a combined rode is a boat length of
chain
plus whatever nylon is required to give a 6:1 scope. In shallower
water, the
scope should be increased, within swing limitations, to 7:1 to permit
the bow
to lift more easily to the choppy waves near the shore.

Larry Z

This line from a previous post:
"That's why Ogg preferred light nylon lines to
get the maximum stretch."

The tables for elastic elongation provided by manufacturers relate
elongation to either breaking strength or diameter, never length. As
breaking strength (usually)increases with diameter, so does elongation.
Referring to the quoted line, the implication is that a lighter line gives
greater stretch, the exact opposite of fact.
A 'light' line will stretch less than a 'heavy' line of the same length.
Perhaps more important from an anchoring point of view, a heavy line will
absorb and slowly release more energy than a light line.
Richard