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Old 06-04-2014, 01:45   #91
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Re: Bigger is better, part 2

I would go for 16mm on a boat that size*, meself, particularly for those waters.

* ie Kiwiroa
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Old 06-04-2014, 01:56   #92
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Re: Bigger is better, part 2

And getting back to chain

Andrew,

From what you are saying G3 chain has a high, advantageous, ability to be ductile. Relatively it has a low yield, but after it reaches its yield point it might stretch and stretch. G7 is not like this, once it reaches its yield point it soon thereafter snaps.

Why is this an issue?

Both chains have the same ultimate failure point, they break under the same certain load - under laboratory conditions. The G7 is a smaller link - but same strength. The G3 chain starts to stretch much earlier (and thus has a greater ability, before it fails to absorb stress) but at the end of the day both snap (to paraphrase Craig)

Is it true that all G3 chains have this lower elastic limit and G7 chains a higher elastic limit - so all G3 chain is the same or similar (its an immutable law) and all G7 chains are more elastic and then have no ductility (another immutable law). Or the same immutable law - at either ends of the scale.


Is it not possible to have a high tensile stress, low elasticty and high(er) ductility?

I am simply showing and admitting my lack of metallurgical knowledge

Jonathan
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Old 06-04-2014, 02:00   #93
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Re: Bigger is better, part 2

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I would go for 16mm on a boat that size*, meself, particularly for those waters.

* ie Kiwiroa
Presumably G3?

I'm not being picky, just trying to differentiate between a desire for catenary and a desire to have the right balance of elasticity and ductility (if one can differentiate).

Jonathan
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Old 06-04-2014, 02:27   #94
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Re: Bigger is better, part 2

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Presumably G3?

I'm not being picky, just trying to differentiate between a desire for catenary and a desire to have the right balance of elasticity and ductility (if one can differentiate).

Jonathan
Yup, G3 would be peachy, to my way of thinking.

I don't think in terms of "elasticity", but of the elastic limit. Elasticity could mean a variety of things, but engineers have different names for all of them. The slope of the stress strain curve, in that initial straight-line phase, is called the "Elastic Modulus", and the stress at the top of that straight line is the elastic limit. The former is virtually the same for all steels; so the latter is really the only thing we need to characterise the elastic behaviour

Engineers have more precise measures (notably the "0.2% proof stress") because it can be hard to pinpoint just where the elastic limit is for many alloy steels, but that's an unnecessary distinction when we're talking about the concepts of rupture vs yield, so I'll just talk about the elastic limit.

If a chain is sized for catenary, then it is not necessary (or beneficial) to push for a higher elastic limit than G3/Grade L.

I'm a fan for a mixed rode with about 70m of heavy chain, (preferably in a chain locker back at the mast) so that the rope is only deployed in deep anchorages or awful conditions, when it automatically becomes a snubber.

If the chain locker is designed appropriately, the chain can be spread out in the bilge when off soundings and restrained with lashings, (or in the top of a box keel) as a substitute for internal ballast. A great advantage over conventional ballast is the ease with which it can be jetisonned for later retrieval.

But I can readily sense your eyes glazing over, Jonjo, as these concepts hold no attraction for a multihuller.
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Old 06-04-2014, 02:40   #95
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Re: Bigger is better, part 2

Yup, I have ballast over the keels, same same, mine is wine, lashed down with quick release lashings in case of need.

But do not let the idea that my fascination with all things light has a solely multi focus,

In the grand scheme of things as real yachts disappear all we will be left with is AWB lightweight construction vessels with minimalist chain lockers and not even storage for wine over the keel. It might be said that such yachts (are not yachts) and will not venture outside sight of land - but that's a different story. Its what fills our marinas and moorings its what fills the second hand listings and are being increasingly used for interesting voyages.

So if they can increase their rode length, reduce hobby horseing etc (and do so safely) I think it a topic that merits exploring.


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Old 06-04-2014, 02:44   #96
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Re: Bigger is better, part 2

Say it ain't so, Jonjo!

Say you wouldn't

(gulp)

Jettison WINE ?

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Old 06-04-2014, 03:17   #97
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Re: Bigger is better, part 2

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Say it ain't so, Jonjo!

Say you wouldn't

(gulp)

Jettison WINE ?

Of course we do, but we always drink, but never gulp, it first
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Old 06-04-2014, 03:24   #98
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Re: Bigger is better, part 2

JonJo

Patience, grasshopper ! You’re asking questions which my promised explanation will render unnecessary (at least, I hope so).

Having said that, your latest questions lead neatly into how I proposed to begin.
We’ll get to the crux in due course, I promise, it’s just a concept that requires a bit of foundation building.

The fundamental problem that bedevils your questions is that they presuppose (probably without your realising it) that an anchor chain will fail under steady-state load.
In fact, what you say would be exactly true in such a world.

A steady-state (or very slowly changing) load is how the figures of elastic limit, UTS, elongation etc (all the figures, in fact, other than impact strength) are arrived at in the test lab.

A moment’s thought will reveal that your presupposition is almost impossible, because it implies a steady-state load which distances the atoms further than they have ever been distanced in the past.

But peak loads will ALWAYS be higher than steady-state loads, and furthermore, the more suddenly they are applied, the more the distancing of the atoms will ‘overshoot’ the crucial maximum distance their interatomic bonding strength can sustain.
(This is simplified for the purposes of this discussion, but I do not consider the simplifications to be misleading)

The next post (provided you don’t interject some more ‘pesky’ questions [just joking] will deal with how engineers measure the potential to do damage, arising from a *suddenly applied* load.

This bears little resemblance to the orderly world suggested by steady state calculations.
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Old 06-04-2014, 03:45   #99
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Re: Bigger is better, part 2

Keep going,

I'm a very confirmed advocate of snatch loads, I'm with you so far.

J
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Old 06-04-2014, 04:46   #100
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pirate Re: Bigger is better, part 2

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Yup, I have ballast over the keels, same same, mine is wine, lashed down with quick release lashings in case of need ...
Jonathan
Ho ho ho.

Gentlemen, this has been a great read. Can't guess what this info would cost if one had to pay. Yes, my non-techy eyes have glazed over once or twice but I am hanging in.
Thank you.
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Old 07-04-2014, 02:18   #101
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Re: Bigger is better, part 2

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Andrew,

...If G3 fails at 20% stretch, then 10% might be elastic and 10% (so 20% total) might be permenant deformation, G7 might be 15% and 5% (and G100 18% and 2%). I'm assuming same strength, but smaller and smaller chain.

If this uneducated 'summary' is correct then why is not this high elasticity good on the further assumption that WLLs are similar and are not exceeded. ...
When I responded to this in my post #81 I emboldened the words "are not exceeded", because that's when things get interesting and different. But I forgot to address that point.

More specifically, the problem is, what happens if the elastic limits are exceeded.

You might say “Oh, but I won’t ever do that if I go to G70 chain”

I’m not actually sure you can guarantee this, especially in the envelope-pushing load regime implied by your challenge to me (which I paraphrase): “if you can’t show evidence of G70 chains breaking, they must be strong enough”.

This (in terms of the implied factor of safety tending towards 1:1) is not very far from the Formula One mindset of “if none are breaking, they’re too strong”

I’m only doing some mopping up for posterity in raising this, because I think you (JonJo) probably realise by now it’s actually beside the point, and here’s why:

I think I’ve persuaded you that chains don’t break at constant load, which is what “elastic limit” and “WLL” are all about.

Whether or not anchor chains break is all about absorbing or recycling or dissipating excess energy.

Energy is not the same as load:

it's load x speed x speed,
or load (ie force) x deceleration x distance,
or some other equivalent.

To take the high WLL of chain intended for restraining cargo or lifting heavy loads as being indicative of high performance as an anchor chain is a bit like looking only at the muscles and sinews and top speed of a cheetah chasing an antelope, and deciding it would be a good animal for carrying messages .

If we tried that, cardiovascular constraints would make it overheat within a few tens of seconds at that high performance level: it’s a problem with excess energy, which the cheetah can neither absorb and recycle (like a kangaroo’s tendons) or dissipate (like a racehorse’s heart and lungs)
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Old 07-04-2014, 02:29   #102
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Re: Bigger is better, part 2

The important question is this: what maximum energy absorption will the material of your chain will be required to provide? This is impossible to quantify, but what we CAN do is get an idea of the difference in energy absorption inherent to steels supplied at different tensile strengths.

We can compare the energy absorption capabilities of (say) 10mm G70 vs 10mm G30 chain, at least to a rough approximation. Then we can use past experience (eg from generations of proven matching of G30/Grade L chains to boat displacements) as a rough guide to what should be a safe rule of thumb when using stronger steel.

Unfortunately it’s a lot more complicated when we try to compare (say) 12mm G30 with 8mm G70, because there is currently a pervasive misunderstanding in the “leisure-boat anchoring” community. (LBAC !)

This misunderstanding, to do with catenary, is similar to the one which I’ve hopefully dispelled above to do with WLL and elastic limit.

The arguments both fail because they deal with steady-state, static loads.
Testing-lab loads, in other words.

But the “real-world” explanation for catenary, under the suddenly applied loads which break chain, is likely to be beyond the concentration span of anyone from the LBAC.
At least, I’ll use that as my excuse for not attempting to lay it out on this forum.

I don’t think anyone would disagree that catenary, *if the chain is heavy enough in relation to the boat*, adds significantly to energy absorption of suddenly applied loads.
The disagreement is in whether a REALISTICALLY heavy chain can have a useful influence (fuelled by a misleading static analysis by Fraisse)

There is also disagreement as to what constitutes realistic. (eg, is a 16mm chain realistic for a 52’, 25 tonne vessel) This is not the sort of question which has *an* answer; everyone has to decide for themselves.

In my next post I’ll hopefully lay out how we can characterise energy absorption within the microstructure of the steel.
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Old 07-04-2014, 02:40   #103
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Re: Bigger is better, part 2

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Ho ho ho.

Gentlemen, this has been a great read. Can't guess what this info would cost if one had to pay. Yes, my non-techy eyes have glazed over once or twice but I am hanging in. Thank you.
The information is free which is great. The cost of implimenting the information well that is likely to see your local chandler telephone a posh restaurant and make a booking on a fraction of the profits he is about to make.

Jonjo, I think you need one of those strain gauges the Scots block makes and see if your real world senarios justify the costs.

If my 8mm G3 snaps at 3 tonnes well that will probably save the front deck from being torn out, or perhaps the deck might give way first Do I want to swop to 6mm G7, no thanks I will stick with what I have.

Pete
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Old 07-04-2014, 03:23   #104
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Re: Bigger is better, part 2

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The information is free which is great. The cost of implimenting the information well that is likely to see your local chandler telephone a posh restaurant and make a booking on a fraction of the profits he is about to make.

Jonjo, I think you need one of those strain gauges the Scots block makes and see if your real world senarios justify the costs.

If my 8mm G3 snaps at 3 tonnes well that will probably save the front deck from being torn out, or perhaps the deck might give way first Do I want to swop to 6mm G7, no thanks I will stick with what I have.

Pete
Well not quite true, G7 is expensive but because you use the smaller size not as expensive as you think.

I have a load cell, the results are all published. The big problem is no-one wants to use a load cell above, say 35 knots - you usually have concerns other than furthering 'knowledge'. It was from the load cell work that I fully agree with Andrew - the loads are snatch loads, not gently developing loads as used in chain (or anchor) testing. We did some snatch load testing of anchors - scary!

Forget dropping from 8mm to 6mm - Noelex dropped from 12mm or 13mm to 10mm - same, or similar, discussion. In any event it turns out 6mm x G7 is not available - but it might be. That does not negate the discussion, of going from 10mm G3 to 8mm G7, or following Noelex (or not - I wait to see how Andrew's thesis develops - I'm ready to be converted, hopefully I'm still of an age to be open minded).

Jonathan
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Old 07-04-2014, 03:36   #105
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Re: Bigger is better, part 2

I am not sure were should be overly reassured buy the historical low numbers of reported of chain failures.
Anchors have improved and holding powers are higher. The advent of powerful, reliable anchor winches has seen anchors get larger, further increasing holding. Modern anchors are much better at staying put that their forefathers (where some gradual creep or movement in the stronger gusts is not unusual).

At the same time chain sizes are shrinking as the importance of chain weight is is no longer thought by many to be an important factor.

We are entering new ground.

It should also be remembered that the vast majority of boats rarely anchor in strong wind conditions, even more so in recent years with the large number of protected marinas and the desire for creature comforts.

The minutiae of the detail on CF anchoring threads is about being able to anchor safely and reliably in strong wind conditions. It will take a long time for reports of chain failures (if there are any) to become apparent with modern anchors and fine chain.

I think it is a better approach to analyse some of the engineering rather than relying on historical data points most of which have been collected in different circumstances.
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