T&T: stern line/anchor rode elasticity

[Bob Austin]

I paused after I had written the "anchor jerked out" phrase--considered, and
left it at that.  The elasticity is not for comfort--it is to keep the boat
anchored.  The ideas are compimentary--because they are on both ends of the
boat.  However I can describe one of the most uncomfortable nights I have
spent at anchor was due to elasticity of the anchor rode.  I was behind a
reef, and during the night a swell came up with 8 to 10 foot rollers.  The
boat would fetch back, until the nylon line was almost singing--the wave would
break under the counter, then the boat would surge foreward on the stern nylon
rode--and the opposite would occur.  If the anchor had been jerked out--I
would have lost the boat on the beach--but instead I just had an uncomfortable
night!

Sorry to disagree--the chain cantary does provide shock absorption up to a
point--at that point the chain is bar tight--and there is no further
elasticity, or shock absorption.  For this reason a snubber is used.  This is
usually 20 to 30 feet of relitatively light nylon three strand line.

Some people do use braid on braid or solid braid for anchor rodes--and it is
not felt to be quite as good an anchor rode.  It is slightly more abraison
resistant, and is much easier on the hands.  Nylon braid has different
elasticity depending on the nature of the braid--but it is still relitatively
elastic in comparison to Spectra or even Dacron.

New England Ropes had a "mega braid II"  which in the smaller sizes (1/2" to
5/8" is recommended to be used as dock lines and anchor lines.  They also have
a MegaBraid, (Size 5/8" to 1 1/2 ") has a suggested use for dock lines, but
not anchor rodes.

The characteristics of an anchoring system has to do with a number of factors,
which do include the anchor--but also the rode and its characteristics.

In the Boat US article on hurricane anchoring the following points are made:

"
Five Critical Factors That Predict How Rope Will Fare in a Storm

1. Breaking strength is determined by wrapping new rope around two large
diameter capstans and slowly tensioning the line until it breaks.

All things being equal, a braid-on braid line will have the most breaking
strength followed by plait and then three strand. You might think that
breaking strength (tensile strength) would be the ultimate criteria for
selecting an anchor line. After all, the line that's the strongest would have
the best chances of surviving the tremendous forces in a hurricane. Alas, the
forces on a rope in a hurricane are not applied slowly on large diameter
drums; it takes more than breaking strength for a rope to survive something as
violent as a hurricane. Breaking strength is only one of six factors.

2. Stretch. While breaking strength is clearly important, stretch is also
essential to act as a shock absorber to absorb the tremendous amounts of
energy. There are two ways that a rope absorbs energy: 1. The material itself
stretches and 2. The weave of the material can expand and contract
mechanically. Before the fiber itself stretches, the lay of the rope untwists
and absorbs energy.

Nylon stretches more than polyester and, all things being equal, a
three-strand twist line will stretch more (and absorb more energy) than a
braid-on-braid line. A plaited line (similar to three strand but softer and
with a square profile) will stretch more than either three-strand or
braid-on-braid.

Without something that stretches to act as a shock absorber, the intense gusts
and surging waves are much more likely to yank an anchor out of the bottom. A
line that stretches also helps to prevent a boat from "sailing" back and forth
on the rode, which puts additional strain on the anchor and rode. The greater
its ability to absorb the sudden and violent stresses, the less likely an
anchor line will fail."

The other features are chafe, age and line quality.



>From Don Casey's article on anchoring:  "Unlike oversizing the anchor,
oversizing the line is not recommended because that reduces its beneficial
elasticity."

I don't know what Doug Gould's anchoring experience is--but mine is well over
3000 nights--and I want that elasticity to keep the anchor stuck in the
bottom!


Bob Austin