Technical article by Brian Greer
(reprint) Part 2 of 3
Copyright NZ H28
Yacht Owners' Association.
SOME SOLUTIONS
Correction of Pitch
and Surge
The traditional
solution to this problem is to let out
more chain. In the shallow anchorages
(3-10 metres) frequented by H28s,
however, veering more chain does little
to improve the damping of surge motion.
There is not much working stretch while
the chain lies on the bottom, and
virtually none when it lifts clear.
Sudden gusts can bring the vessel to the
limit of its tether with a jerk, perhaps
lifting and breaking out the anchor.
The development of
nylon rope, on the other hand, has
brought entirely new dimensions into
anchoring theory. Nylon gains its
elasticity from stretch rather than from
catenary action, so the load changes on
the anchor are much less violent. In
fact, nylon can recover fully from
having been stretched by as much as 50%
more than its original length.
Naturally, this advantage over chain
becomes more pronounced in shallow
anchorages, where the proportion of
chain that can be suspended in the water
is reduced.
Another interesting
disparity between chain and nylon rodes
is illustrated at Fig. 1. It will
be seen that when using chain, movement
away from the anchor tends to raise the
anchor shank; whereas, with nylon, being
stretchable, the converse occurs!
Fig. 1
A classic example of
ineffectiveness of veering more chain in
shallow water is the experience of W.S.
Kals who, in his 9m sloop "Boheme"
was driven aground and wrecked in the
Bahamas by a fierce squall gust, while
riding to a 35lb anchor and 30m of 9mm
chain in TWO METRES of water. He might
well have avoided this mishap had he
included 20 or 30m of nylon warp in his
anchor rode.
This does not imply,
of course, that nylon is the answer to
anchoring problems. Chain has distinct
advantages in that:
a. it will not chafe
on the bottom;
b. it is essential to
safe anchoring in abrasive areas, such
as coral, or in deep water anchorages,
and
c. its relatively
large weight/length ratio allows for a
great reduction in the scope needed for
a given depth.
Nevertheless, the
conclusion to be reached from all this
is that a mixture of chain and nylon
will gain the advantages of each, and it
can be shown, mathematically, that the
optimum is equal lengths of chain and
nylon. For the H28, anchoring in
favourable conditions, the scope (or
ratio of anchor rode to water depth),
should be about 4:1. Thus, to anchor in
say 5m you should let out about 10m of
chain and 10m of nylon rope. If the
conditions become marginal, (for
example, if the wind forecast for an
exposed anchorage were 60kts, with gusts
to 85kts), then the minimum effective
scope rises to more than 9:1. You would
then require 23m each of chain and rope.
Should you not have that amount of chain
available, the same elasticity can be
achieved by increasing the proportion of
rope; but the length of rope required
rises fast. The man with only 10m of
chain on board would have to add 120m of
nylon to be confident, under such
conditions, that he was not going to
drag his anchor! That may be all very
well if you have plenty of room, But if
you are anchored in company, and 140m
away from your anchor, you are likely to
spend most of the night fending off
other boats as you sail about on your
long warp. Surely there must be a more
subtle solution. There are two:
- Add Another Anchor
Shackle a second anchor onto the
chain of the main anchor, (see
Fig. 2). The distance between
the two anchors should be about 10
metres. In theory, this method
should almost double the holding
power of the first anchor. There are
disadvantages though. You cannot
drop both anchors together, extra
ground tackle is required, maybe
leaving the skipper without an
emergency anchor aboard, and the
gear is difficult to deploy and
retrieve in heavy weather.
- Weight and Anchor
Rode The catenary weight of chain
can be simulated by sliding a weight
down the rode, (see Fig. 3).
In the case described above, 23m of
9mm chain weigh 47kb. If you have
only 10m of chain you are 20kg short
of the required weight. By shackling
that amount (of chain, ballast,
anchors etc.) to the warp, and
letting it out a distance equal to
the average depth of water, you can
achieve the same result without any
of the disadvantages of double
anchoring
Fig 3
