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 Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Glenn Ashmore wrote:
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.


I'm extremely skeptical of this calculator for several reasons.
First, I plugged in a few numbers to see if it thought I needed more
than 50 feet of chain. The result was that if I held the chain
constant and increased nylon (with no kellet), the horizontal tension
decreased, implying that it would hit its limit sooner if you fed out
more scope. This is completely counter to all experience.

Secondly, the basic premise is that you hit some sort of limit when
the chain get lifted off the bottom so that the angle to the anchor is
no longer zero. In fact, anchors are designed to keep digging in even
with a small up angle of about 8 degrees. This means that a properly
set anchor will not try to pull out if the scope is 7:1 or greater,
even if the rode is straight.

And finally, although I didn't have enough time to look closely at the
spreadsheet, it seemed to have some problems, and nowhere does it
actually describe the math behind its claims, or even the meaning of
its "answer." Also, the sheet doesn't seem to correspond to the
examples in any way.

It only take a few seconds of auditing to see how the spreadsheet works. It
is fairly simple. It is limited to 200' horizontal distance however so it
works fine up to about 220- 250' of rode depending on the depth. Beyond
that limit it starts dropping off chain and the result is invalid.

While it is not some prime limit if you discount the point where the chain
lifts you really discount the need for chain or kellets.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Jeff" wrote in message
...
Glenn Ashmore wrote:
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.


I'm extremely skeptical of this calculator for several reasons.
First, I plugged in a few numbers to see if it thought I needed more
than 50 feet of chain. The result was that if I held the chain
constant and increased nylon (with no kellet), the horizontal tension
decreased, implying that it would hit its limit sooner if you fed out
more scope. This is completely counter to all experience.

Secondly, the basic premise is that you hit some sort of limit when
the chain get lifted off the bottom so that the angle to the anchor is
no longer zero. In fact, anchors are designed to keep digging in even
with a small up angle of about 8 degrees. This means that a properly
set anchor will not try to pull out if the scope is 7:1 or greater,
even if the rode is straight.

And finally, although I didn't have enough time to look closely at the
spreadsheet, it seemed to have some problems, and nowhere does it
actually describe the math behind its claims, or even the meaning of
its "answer." Also, the sheet doesn't seem to correspond to the
examples in any way.

Glenn Ashmore wrote:
It only take a few seconds of auditing to see how the spreadsheet works. It
is fairly simple. It is limited to 200' horizontal distance however so it
works fine up to about 220- 250' of rode depending on the depth. Beyond
that limit it starts dropping off chain and the result is invalid.

Yes, it appears that it functions better if the total rode is kept
under 200 feet. However, since the depth is initialized at 44 feet,
attempting to determine the trade off between scope and chain will
lead to wrong conclusions. The sheet is explicit in saying it works
up to 250 feet, this is clearly bogus.


While it is not some prime limit if you discount the point where the chain
lifts you really discount the need for chain or kellets.

Yes, I do discount it. And so do experts like Van Dorn and Hinz. My
issue is that the critical point is not when the angle goes above
zero, it is when the angle exceeds about 8 degrees. Modern anchors
are designed to keep digging in even when there's a small up angle.
As long as the scope is over 7 to 1, that angle is not exceeded.

This spreadsheet gives a biased view of the situation, implying that a
lot of nylon must be added to equal the holding power of a small
amount of chain. The example I used was: in 20 feet of water, 40' of
chain and 100' of nylon yields a horizontal tension of 254# to lift
the catenary, which is actually the load of a 35 to 40 foot boat in 15
knots of wind. The truth is the scope is already 7 to 1, and a well
set anchor will hold and probably continue to dig in and in fact
handle a much higher load.

Adding to this scenario, 20' of extra chain raises the tension to
400#; to do this with nylon takes over 50 extra feet. However, at
this point, the scope is almost 10 to 1, beyond what is normally
needed. Another approach is to replace the 100' of nylon with chain:
the all chain 7:1 rode yields a tension of 643#, implying that it has
150% better holding than the rope/chain rode, which is not true.

The prime benefit of the kellet comes when you need to anchor in short
scope, under 7 to 1, and weight is needed to keep the angle low.
However, by the sheet, even all chain at 5:1 is fully lifted at 228#,
and is even pulling at 12 degrees, which could unset the anchor. Even
adding a 20# kellet does not significantly raise the tension point,
although there is benefit in light loads.

Thus, the sheet can lead us to two faulty conclusions: First, that all
chain is vastly superior to a mix, and second that a kellet or heavier
chain allow you to use very short scope.

I'm not denying that the kellet can help in some situations, or that a
reasonable amount of chain is essential for secure anchoring. I'm
only claiming that its more important to understand what's really
going on.

"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

News f2s wrote:
....
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!)

Actually, I think you overstate the amount of "spring" a chain rode
provides. When there is still chain on the bottom, there is spring in
the system from lifting that chain, but when fully lifted into the
classic "catenary" shape, there is very little left. While the
deflection in the rode may look reassuring, it is the extension that
actually provides shock protection. For a typical situation
(according to Hinz) a 200 foot 3/8" chain will have only 2 feet of
extension left in winds as low as 15 knots. At 30 knots, it about a
few inches. In other words, if the boat tries to move aft, the chain
allows only 2-3 inches - the remaining movement will come from
dragging the anchor or tearing the deck fittings. Thus in a lull, or
between wave surges, the system may relax, but in the next gust or
wave the boat will be thrown back an then stopped short with virtually
no shock absorption.

Your system of 100 feet of nylon is prudent. I use 50 feet of chain
spliced to nylon, so in a blow I typically have 100 feet of stretchy
line out.

"Jeff" wrote in message
...

Actually, I think you overstate the amount of "spring" a chain
rode provides. When there is still chain on the bottom, there
is spring in the system from lifting that chain, but when fully
lifted into the classic "catenary" shape, there is very little
left.

I completely agree, Jeff.

The model I offered was, as you probably realised, the 'lift off
the ground' model. I presented the best possible case for the
catenary, to show that, even then, nylon still wins as a shock
absorber.

The catenary myth is an article of faith among many charter
sailors (and those who brief them) in the Mediterranean. The
common mooring there is bows or stern to, when the rode is usually
stretched off the ground already. I've had many a quayside
discussion (complete with bits of string and jugs of beer)
explaining why the surge from a ferry caused an anchor to snatch
out. Logic doesn't always win when you challenge an article of
faith though, even after the bribe of a beer or two, unless it is
a blindingly clear demonstration!

JimB

On Sun, 18 Dec 2005 18:03:41 -0000, "News f2s"
wrote:

The
common mooring there is bows or stern to, when the rode is usually
stretched off the ground already. I've had many a quayside
discussion (complete with bits of string and jugs of beer)
explaining why the surge from a ferry caused an anchor to snatch
out.

When you are bow (or stern) to the quay you really don't want a lot of
stretch in the system however. Better to use more scope if needed to
get the required holding power, or a better anchor.

"Wayne.B" wrote in message
...
On Sun, 18 Dec 2005 18:03:41 -0000, "News f2s"
wrote:

The
common mooring there is bows or stern to, when the rode is
usually
stretched off the ground already. I've had many a quayside
discussion (complete with bits of string and jugs of beer)
explaining why the surge from a ferry caused an anchor to snatch
out.

When you are bow (or stern) to the quay you really don't want a
lot of
stretch in the system however. Better to use more scope if
needed to
get the required holding power, or a better anchor.

I assumed that adequate scope and anchor have been used. The
stretch you want will depend on the surge you face. Without
stretch to match the surge you're (as you rightly pointed out
earlier) either risking your fittings or going to snatch out the
anchor. So far I've seen more anchor snatches. But also fittings -
torque failure on a winch drum shaft, tilted anchor winch.

Suitable stretch depends on the cruising area and the harbours. I
don't know your cruising area, but fast ferries entering some
Greek harbours cause a surge giving up to 3 metre fore and aft
movement relative to the quay with about an 8 to 12 second period.
Poros west quay, outside berths in Paros harbour and Ermoupolis
town quay in Syros immediately come to mind. Other quays facing
east to the fast ferry routes in Ithaca are also affected. And
quite a few south Spain marinas suffer similar surges in winter
storms.

It means, of course, that you have to tie off at least 2m from the
quay (3m is my preferred distance) pulling the boat in against the
stretch for that brief period when you dance ashore (or back
afloat) along a bouncing passarelle. Not my wife's favourite trip!
Or use the dinghy.

JimB