"Glenn Ashmore" wrote in message
news:qvznf.11456$fz5.4020@dukeread04...
If anyone wants to explore the effect of rope, chain and kellets
on rode centenary and maximum horizontal tension
http://www.johnsboatstuff.com/Articles/anchor.htm has a good
spreadsheet for calculating it. Scroll to the bottom of the
page.

Glenn, I'm a bit late to this one, but the graphs here represent a
largely static view of anchoring stresses. It's easy to do
elaborate math calculations to high levels of accuracy - but they
miss the point.

The reality is that whatever chain to rope ratio you use (kellets
too) you will occasionally end up with a straight line to the
anchor for brief periods. This is because in wave conditions,
and/or when the yacht is veering from side to side, very high
inertia forces are created.

My conclusion has been that (if there's any wave or wind threat)
it is necessary to put out enough rode so that the anchor will
always still be digging in, even if the rode from boat to anchor
is straight. The critical pull angle varies from type to type of
anchor, but is between 8 and 12 degrees. As Jeff has pointed out,
that implies depth to length ratios (at the waterline!) from 1/5
to 1/8 - depending on your anchor type. Any less rode, and the
anchor can snatch out. OK, it'll re-set on the next surge, but
you'll have given a bit of ground, and some times you've picked up
a chunk of weed or whatever, and it won't re-set.

Forgive me if from now on I'm teaching grandma to suck eggs - but
here I go with my opinions . . .

Of course, the best way to reduce inertia stresses is to provide
lots of 'spring'.

On the matter of percentage of rope to chain in the rode, the
traditional story is that the catenary of a chain gives you
'spring'. It does, but only half what's available from a length of
nylon in most conditions.

The crude way to see this is to play with a bit of string. Put a 6
inch length of string on the table, pull its centre 1 inch of off
the straight line between the two ends, and measure the amount of
'spring' (assuming of course that chain alone will not stretch!).
Or do the maths. The answer is 0.3 inch spring. Give or take a
bit for curvature. That's 5%. Your nylon gives you 10% spring
without deformation, and depending on the weave, a further 10%
leaving you permanently stretched line (which you'd better scrap!)

So, why use chain? Bottom chafe resistance, and also good damping
as you yaw from side to side. If these aren't factors (as in a
Mediterranean moor - bows or stern to a quay) you might as well
use all nylon - it's easier to pull up! But in open water
anchoring when there's a bit of a weather threat, I always secure
my chain with 30 metres (90ft) of nylon spring - 10 metres of
which run from the bow back to the jib winch.

In other words, I add as much nylon as is practical to to the
chain. Crude, but simple.

JimB