Jere Lull wrote:
On 2007-09-08 14:00:14 -0400, jeff said:

For instance, invoking the very concept the "catenary" is pedantic and
not relevant to practical anchoring. The problem is that once a chain
has completely assumed the catenary shape, it is virtually fully
extended and about to be broken out. The purpose of an all chain rode
or a kellet is to minimize the catenary and keep as much of the rode
as possible sitting on the bottom.

Oops! You got catenary backwards:

Nope, I have it right.

Catenary: "a curve formed by a wire,
rope, or chain hanging freely from two points that are on the same
horizontal level." (okay, the two points on an anchor aren't on the same
horizontal level, but you get the idea.)

Actually, the curve of the anchor chain is a "half catenary;" you work
out the math assuming twice as much chain.


When the chain *loses* it's catenary, it's fully extended and the stress
on the anchor is substantial.

Nope. Its essentially physically impossible to lose the catenary that
way. The fully extended point is when the full chain is in the catenary
shape. Anything short of this and there will be chain sitting on the
bottom, and a shorter catenary to the the boat. In practice, most of
the the chain will be sitting on the bottom, and the only catenary will
really be near vertical. In this mode, any force on the vessel will be
absorbed in lifting the chain off the bottom - this is where the virtue
of an all chain rode (or a kellet) lies.

Getting back to the "almost fully extended" point. It is impossible to
make the chain literally "rod-straight" so the catenary can never quite
be lost, only reduced. Sometime people claim that in the process of
"reducing the catenary" there is significant shock absorbing, but this
is a fallacy. The problem is that there is little extension left at
this point as there is only a few inches left before the anchor is
pulled out.

My point is that the shock absorbing is not in the catenary, it is in
the chain lying on the bottom that will get lifted as the force
increases. Consideration of the catenary does nothing for the
understanding of this phenomenon - you might just as well think of the
chain as a straight line form the point where it departs from the bottom
to where it gets to the bow.


Also, I find it most useful to have an anchor of a different type than
the primary for short-distance cruising.

I certainly agree with that - I'm always amazed when I see two CQR's on
the bow of a cruising boat. FWIW, I use a 35# Delta with 50 feet of
chain as a primary, and a Fortress FX-23 with 12 feet of chain as a
secondary. A Danforth and spare rode is in a locker; if I cruise
extensively I carry a large Fortress broken down.