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From: (no name) (no email)
Date: Thu Jun 28 2007 - 17:36:00 EDT
ANCHORS AND RODES
Lawrence Zeitlin
City University of New York
Part 1
Which is better, the Spade or the Bullwagga, the Danforth or the CQR, the
Bruce, the Rocna, or a rock tied to the end of a rope? There are about as many
philosophies of anchoring as there are boaters. At risk of sounding even more
professorial than I usually do, here is a short tutorial on anchoring rodes
which I liberally cribbed from the class notes for a course I gave to budding
naval architects a long time ago. These guys were planning on making a career
designing supertankers and few had any idea that boats smaller than the Staten
Island ferry had to be anchored too. Soon after the course was given, the tanker
market collapsed and I suspect that many of the students either ended behind
the counter at MacDonalds or started internet dot coms. Most of those are now
at MacDonalds too. The moral is "don't take anything too seriously", even the
advice I give below.
Not too many of us recognize that the nature of the anchor rode is as
important to anchoring success as the shape and weight of the anchor. Many of the
differences in anchor performance published in the boating press can be
attributed to the different rodes used. Just about any appropriately sized modern
anchor can, under the right conditions, provide the horizontal force necessary to
resist dragging under all but the most extreme storm conditions. It depends
primarily on the fluke area and the nature of the bottom. There are differences
between anchors in setting ability and resistance to forces not in line with
the original direction of set. Advantages in one type of bottom may be
disadvantages in another. In comparative anchor tests in a number of boating publi
cations, no single anchor or type emerges as a consistent winner. Boating is a
mature art and the era of modern anchor design started with the development of
the CQR nearly 80 years ago. If a single type of anchor proved its superiority
under all conditions, it would soon dominate the field. Tradition plays an
important role in boating but when it comes to anchors most of us judge by
performance in our typical cruising grounds.
An anchor doesn't hold the boat by itself. It must be attached to the vessel
by the rode, a rope, a chain, or a wire which transfers the forces of the
elements to the anchor. The anchor resists those forces either by sheer mass or by
burying itself in the sea bottom. The load on the rode is determined by the
horizontal forces of wind and current pushing on the boat, the transient forces
of wave action, the depth of the water, and the weight of the anchor rode
itself. To find the required strength and scope of an anchor rode, we must
calculate these forces individually, then sum them to get the total load.
Let me qualify some of the things I've just said. Both chain and nylon rodes,
or any combination of the two, are special cases of a hypothetical general
anchoring system which consists of a weightless, unstretchable line connecting
the anchor and the boat with a single concentrated weight hung somewhere along
its length. The weight serves two purposes. It keeps the anchor shank angle
more nearly horizontal, reducing the chance of the anchor pulling out or
dragging; and, it gives a degree of elasticity to the rode, requiring the weight to
be lifted before becoming taut and jerking on the anchor. The best position for
the weight depends on what you want to do. To keep the anchor shank as flat
as possible, the weight should be attached to the lower end of the rode near
the anchor. For best rode elasticity, the weight should be positioned at one
water depth from the upper end of the rode.
The idea that an anchor chain forms a perfect catenary is another one of
those maritime legends. Typically, in still water with no wind, the chain hangs
down from the bow until is rests on the bottom, then proceeds horizontally to
the anchor. As the wind picks up, enough chain is lifted off the bottom to
balance the wind pressure. This forms a short catenary from the bow to the bottom.
It is only when the wind is quite strong that the chain is entirely lifted off
the bottom forming a catenary from the bow to the anchor. Any increase in
wind strength flattens the shape of the catenary and provides a small amount of
snubbing action, much less than would be provided by an appropriately sized
nylon line. If the wind increases further, the chain becomes bar tight and there
is no snubbing. The main advantage of chain, considered only from the
perspective of anchor dragging, is that the weight tends to keep the anchor shank
parallel to the ground until the entire chain is lifted off the bottom. Further,
as long as a portion of the chain lies on the bottom, the friction of the chain
links lying on the bottom act as a surrogate anchor. No force is exerted on
the anchor until the entire chain lifts off the bottom.
The same effect could be accomplished with a lighter chain or a nylon rode by
using a kellet or weight close to the anchor. Maximum elasticity is achieved
when the rode hangs straight down from the bow to a weight resting on the
bottom, then goes horizontally to the anchor. But such an arrangement really hurts
the bow's ability to lift over the waves. A compromise between the two
requirements is to hang the weight halfway along the rode.
In Part 2 we will look at the actual forces on the anchor and rode.
Larry Z
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