Anchor Scope Illustrated

MurrayM said:

It's amazing how any aspect of anchoring can get peoples panties in such a knot. Must be because the variables are essentially infinite. Then again, heated debate is understandable when considering the advice I got from a fellow today who said, "Just let out 20% more line than how deep it is. So, if you're anchoring in 70 feet, let out about 90 feet of line. Also, clip a 10 pound weight near the end of your chain...that'll keep the anchor down".

Click to expand...

Much of the "panties in a knot" comes from when someone thinks their method is the only one that makes sense and has all the "right stuff"..when obviously most cruiser's anchoring habits are all over the map...so will their ground tackle be.

Especially when one want's to argue something like "that's the only benefit" when many posters in many threads have posted why they like or dislike dozens of feature s of all chain or mixed rodes. Most of the "all chain" gang have repeatedly responded that their snubbers are in effect a "mixed rode" setup....but when someone is conviced their's is the only way...disussing all other setups just falls on deaf ears.

Nsail,
No there isn't. I have thought about this in the past but not to any great depth. But earlier today talking about catenary I thought about it a bit more and yes as far as I can see now it may mean that lots of chain may be reducing anchoring performance.

Consider a chain fastened to two cars. How much tension will it take to pull that chain up off the ground and make it reasonably straight? If the chain is what trawler skippers usually use anchoring at 5-1 scope as they say they do and the water was 43' deep it would require about 250' of rode. If that rode was 5/16" chain and the chain was fairly straight How much tension do you suppose it would be exerting on the 2 cars? I'm thinking hundreds of pounds but it could even be more.

Now lets imagine a trawler at anchor w the wind blowing hard. There is a lot of tension on the rode. The boat is pulling on the rode because of the wind and current (if there is any). The pull is transmitted to the anchor. But also the weight of the chain (200lbs?) is pulling on the boat and the anchor. Perhaps hundreds of pounds. So the anchor needs to resist the tension resulting from the weight of the chain AND the pull of the boat in the wind.

Nylon anchor line weighs nothing compared to chain so it would seem to me that chain on the anchor rode may have 100 or perhaps several hundred pounds of pull being exerted on the anchor that would not be there if the rode were nylon.

This pull from chain weight is probably more negative than the catenary is positive. Especially in extreme conditions. So as long as you can get an anchor to set it looks like chain would help dislodge it and the more tension on the rode the greater the force that is trying to break out the anchor.

I remember Marin swore by his "all chain rode" and thought his Bruce anchor was the reason he dragged. Perhaps he wouldn't have dragged had he had nylon?

Wow Eric! Thanks for that answer. I really didn't expect you to fully explain it, as I had a feeling it would be a long explanation.

At any rate, I now understand the theory and it gives me something to think about.

Nsail said:

Wow Eric! Thanks for that answer. I really didn't expect you to fully explain it, as I had a feeling it would be a long explanation.
At any rate, I now understand the theory and it gives me something to think about.

Click to expand...

Something to think about, indeed...
I'm sorry - tried to resist, but just had to come in here. Long live the lively anchor stousch.
I certainly would recommend re-thinking it - long and hard, and you as well my dear Eric, old friend, because I think your theory is just wrong.
With all due respect your analogy of pulling the rode taut between two vehicles is just not comparable. The protection the heavier chain rode gives is from the catenary, and the extra tension/load it takes to lift it off the seabed, which if all chain is a lot - way more than a comparable length of synthetic rope rode. But this protection of the anchor from being subjected to a direct pull on the shank, only lasts up to the moment it is finally pulled straight. When you reach that point, then whether the rode is nylon or chain, you just have the pull of the boat transmitted straight to the anchor, and the composition of the rode is almost irrelevant, as it is then just becomes one very strong 'string' pulling on the anchor. It is then that the weight of the anchor, and how deeply it has bedded in becomes the critical factor, but not until that point, because until then it has really just been locating the end of the rode.
Sure up till then it has been resisting the weight of the rode, but until the rode is effectively lifted clear of the bottom, it really has not been feeling much of the actually pull of the boat, just the weight of the rode, and then not so much until nearly all is lifted because of the friction between that still on the bottom and the mud etc still gripping that chain. And, don't forget, the pull is never constant, and the second there is any let-up, the weight of the rode pulls large lengths of it back to the bottom. So it stands to reason, the more weight in the rode, within reason, the longer it will be and the larger the force must be before the anchor is subjected to the full load of boat which includes the force needed to lift the rode, surely. The lighter the rode, the earlier it will be pulled effectively straight and the full pull of the boat go on the anchor.
To take the comparison one step further, the ultimate full force the combined elements are capable of transmitting to the anchor, once the rode is straight, by definition must include the effort/tension required to lift the rode off the seabed, so one can subtract that from the ultimate force of the pull, meaning the net force the boat can exert is the total force minus the force to straighten the rode...which relates directly to its weight...you do the math, as you US guys say...
My case rests....

With PeterB on this one..he is using the same explanation (tho not in an automaton way but in a independent thinking way) that millions of boaters and seaman who have gone before also agree with.

I know better too.... so until someone can explain with vectors and forces that chain is evil...I'm willig to go with the millions of boaters who think chain helps keep an anchor...well...anchored....maybe not in every situation..but in what we really see on a regular basis.

As I posted before (and so did Peter)....LONG before the chain becomes bar tight...nylon will be the same,,,the only difference is it will stretch...they will be exactly at the same angle and exerting the same force (shock forces may vary)....

The holding power of the anchor is based on bottom makeup, fluke area/resistance and angle of pull (based on scope)....not anything to do with rode makeup.

Being the devil's advocate...the ONLY advantage of a nylon rode is that it absorbs shock loading...but that is remedied in an all chain rode with a snubber...

See...now doesn't that sound really dumb? All experienced cruisers (no matter what kind of rode they use) know better...there are other advantages to nylon rodes...if you think about them.

It sure looks like Peter B is the only one here who has read some of the many studies performed on mooring dynamics for the shipping and offshore industries.

There are no more mysteries, this subject is so critical to the offshore industry that a huge amount of money and effort has gone into understanding how a mooring works for all materials and depths.

Has anyone ever noticed that chain is not a single continuous strand? It is collection of individual segments that function independently until a certain system shape is obtained ... think about it.

I can understand your thinking Eric, but I think Peter's clearly worded points out weigh yours.

Still - I dont have all chain, due to lack of a suitable windlass, but try to make up for it with extra length nylon rode and a beefy anchor.
I defimitely think on the first 20 or 30 ft it is critical for chain, not only to keep the pulling angle low, but to minimise possible damage of the rode from chaffing on reefs.

Nsail said:

Wow Eric! Thanks for that answer. I really didn't expect you to fully explain it, as I had a feeling it would be a long explanation..

Click to expand...

I wouldn't accept this answer, literally, if I were you.

Rock climbing ropes have a "fall rating", which is the number of hard leader falls one can safely withstand before the rope should be retired because it has become unsafe due to deformation/damage to the fibres in the rope. Is there an equivalent "storm rating" for nylon rode? I've never seen one mentioned...

MurrayM said:

Rock climbing ropes have a "fall rating", which is the number of hard leader falls one can safely withstand before the rope should be retired because it has become unsafe due to deformation/damage to the fibres in the rope. Is there an equivalent "storm rating" for nylon rode? I've never seen one mentioned...

Click to expand...

I believer there is a "stretch" limitation...once it reaches that the "breaking and or working loads" are supposed to be reduced or I forget if the rope is no longed considered "serviceable"

Because the "stretch" limitation should come from a given load...I would think a rough calculation would do the same thing...can't find a quick and better explanation...but I know I have seen it when discussing marine line, anchor lines, etc...

OK nix on the cars.
Lets say you're down in the harbor and it's low tide. It's in Alaska so it's an 18' tide. You've found a pair of docks supported by pilings and they are 200' apart. You attach a chain to a piling on the north side and the south side and use a come-along to pull the chain up to the point where it's fairly straight. Perhaps 6" of catinary. Or Droop in the chain. Now there's hundreds of pounds of tension on the chain exerted to each link and to the two pilings the ends are attached to.

Now lets say the tide comes in. Is there any difference in the physics that was present at low tide? I'd say yes but not much. I think there is a small difference in the weight of the chain because water is much more dense than air. One could hang an anchor above the water and then let down until it's submerged and experience the difference. Very very little though so the tension on the ends of the chain are about the same.

The only difference between my example on the beach w the pilings and a boat at anchor is the substitution of a good solid anchor at one end of the chain and a boat at the other end w a wind blowing to bring about the necessary pull on the boat end of the chain to pull the chain up so there's 6" of catinary.

The forces are all the same and it hardly made any difference at all when the tide came in.

Now the question is whether or not there is a lot of difference in tension w the chain or w equivalent nylon line. I'd confidentially say the chain creates far far more tension.

At anchor the pull on each end of the rode from the weight of the rode is real. Part of it is from the drag of the wind or/and current and the other part is from the weight of the rode like the chain between the pilings. Lets say the pull from the weight of the chain is several hundred pounds. The pull on the anchor w nylon rode is very little compared to the chain so by using nylon instead of chain there is (I'm resisting the temptation to use the word "obviously") much less pull on the rode. So the boat using nylon under the same dynamic conditions has considerably less pull on the rode from the weight of the rode itself so can experience more drag from wind on the boat to reach the point where the anchor will break out. Clearly anchoring w a line rode is more effective.

Shock load

Eric

For me and my experience any issue I have with anchoring is the shock load presented in some situations. I call it shock load.

That's when the boat comes hard against the anchor , releases , then comes hard against the anchor. I have not had any issue with that in an exposed wind way but in the sea scape or waves created by wind. The movement of the boat can bounce the anchor off the bottom from the shock pull.

The more weight you have on the rode the better the chance that shock load can be recovered from IMO.

As linked before in these discussions and again by Capt Phil in post #12

If you can stand going through the whole article...well if you can't here's it's bottom line....

Rode - Dynamic Behavior

Obviously, the Long Chain + Short Nylon Line is the winner, except for small boats that have on-board weight problems. Actually, there is no boundary between the mixed-rode versions: one can choose any Chain/Nylon mix inside a wide range, say, from 40/60 to 80/20, with no significant performance differences (test it yourself with our spreadsheet!). Practically, this involves having 2 rode elements at one's disposal before attempting to anchor:

Which to me could easily be an all chain rode for ease in handling and a long enough snubber (which has been discussed). Or the other way around if you carry enough chain...but it CERTAINLY alludes to that SOME chain is beneficial over nylon straight to the anchor and generally more than the mantra of at least as much chain as the boat length unless you always anchor in very shallow water (which many have agreed that chain doesn't do all that much more laying on an exposed mudflat)

They show at least some of the physics and calculations...not some backyard banter that doesn't jive with what I learned in several physics classes.

I don't know if my anti-chain theory is right but just for the record I've never anchored w no chain at all but on the other hand I've never anchored w more than 15' either. Usually I anchor w about 3' of 3/8ths chain and 10' of 5/16ths. The rest (425') is nylon.

Re my chain theory I'm going to go back through all the comments to see if I can find what it is that shows/proves that I'm wrong. It would seem after hundreds of years of anchoring ships and boats the best type of rode would not only be well established but unquestionably sound. No room for questions or improvements.

But as I see it the weight of the rode pulls on the anchor so the lightest rode should deliver the highest performance.

manyboats said:

OK nix on the cars.
Lets say you're down in the harbor and it's low tide. It's in Alaska so it's an 18' tide. You've found a pair of docks supported by pilings and they are 200' apart. You attach a chain to a piling on the north side and the south side and use a come-along to pull the chain up to the point where it's fairly straight. Perhaps 6" of catinary. Or Droop in the chain. Now there's hundreds of pounds of tension on the chain exerted to each link and to the two pilings the ends are attached to.

Click to expand...

Do the math, Eric ...

If you are talking 3/8 (10mm) "high test" chain you will more than likely break a piling long before you reduce the catenary to 6 inches.

It would take about 18,000 pounds of tension to straighten it out that much. And while we are looking at the numbers ... that force is about 3 times the allowable working load and almost a ton over the breaking load.

RickB said:

Do the math, Eric ...

If you are talking 3/8 (10mm) "high test" chain you will more than likely break a piling long before you reduce the catenary to 6 inches.

It would take about 18,000 pounds of tension to straighten it out that much. And while we are looking at the numbers ... that force is about 3 times the allowable working load and almost a ton over the breaking load.

Click to expand...

Some of the same that was mentioned in the link Phil posted...they made a point that if the chain was straight and shock loaded (If I remember correctly) it was something like 3x the working load of the chain and more than the holding power the average anchor would break out at.

Also...unless you anchor bow and stern and pull both anchor chains bar tight...not sure how that example applies to any reality.

My experience is when I pick up an anchor in shallow water....I'm deadlifting that anchor...usually the chain stays on the sand and the boat stays where it is a lot of the time. The chain is overcoming at least a little wind and tide yet there's NO pull on the anchor I am holding...so it would seem to me that until the chain or any kind of rode is lifted and stays that way...no substantive forces on the chain/rode are transmitted to the anchor because the boat is free to move and not tightly chained to an immovable object...

For instructional use only... I already knew the answer to the real question.

Can't pull the chain "bar tight" or straight.
I suspected as much all the time. I'm glad someone finally cleared that one up.

The whole point being that there will be plenty of catinary in any anchoring situation w chain to dampen the shock loads on the rode. But catinary or not the chain will add to the tension on the rode to both boat and anchor thus increasing the load on the anchor ..... all else being the same. So anchoring w chain is less effective.

manyboats said:

So anchoring w chain is less effective.

Click to expand...

You will never get the chain straight in Eric's peculiar example. It would take around 100,000 lbs to reduce the catenary to 1 inch.

Like I wrote earlier, there are hundreds if not thousands of papers and studies about mooring dynamics. There really isn't much left to be discovered ...

Oops, I just read Eric's last post, I was very very wrong about that discovery thing.

Amazing...unless you tie off the stern of your boat ....the "energy" you seem to think is "load" becomes "stored" energy in a slacking chain. This is because the boat moves forward creating more catenary until the chain all falls to the bottom or starts to lift again. It's stored or potential energy...the same as in a spring ...and actually the same as a nylon rode.

Read that article Phil linked!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Or not and happy anchoring your way.....

manyboats said:

Can't pull the chain "bar tight" or straight.
I suspected as much all the time. I'm glad someone finally cleared that one up.
The whole point being that there will be plenty of catinary in any anchoring situation w chain to dampen the shock loads on the rode. But catinary or not the chain will add to the tension on the rode to both boat and anchor thus increasing the load on the anchor ..... all else being the same. So anchoring w chain is less effective.

Click to expand...

Eric, would you please go back to my post #34, and read it all again (or maybe for the first time, if you missed it), very carefully. Then see if you still feel that last statement could even come close to be true..?

But, to take your not very similar example of the two piles with the chain stretched between. To make it similar to the anchoring situation, one of the piles is not bedded in - it's floating free...get the real picture..? So what happens is the floating pile keeps getting drawn towards the fixed pile, and can't really exert any real pull on the fixed pile until it has been pulled away far enough to lift all that weight of chain, which takes a much larger force than if it was just a nylon line connecting them. The second that pull lessens it is inexorabley drawn back towards the fixed pile, which in this case is the anchor...right...? So most of the time there is not much force on that fixed pile (anchor), because the floating pile is drawn back all the time by the weight in the chain pulling mainly downwards, (operative word here), not horizontally. I'll spare you the vector discussion, but do you see it now..?

Peter B said:

Something to think about, indeed...
I'm sorry - tried to resist, but just had to come in here. Long live the lively anchor stousch.
I certainly would recommend re-thinking it - long and hard, and you as well my dear Eric, old friend, because I think your theory is just wrong.
With all due respect your analogy of pulling the rode taut between two vehicles is just not comparable. The protection the heavier chain rode gives is from the catenary, and the extra tension/load it takes to lift it off the seabed, which if all chain is a lot - way more than a comparable length of synthetic rope rode. But this protection of the anchor from being subjected to a direct pull on the shank, only lasts up to the moment it is finally pulled straight. When you reach that point, then

"whether the rode is nylon or chain, you just have the pull of the boat transmitted straight to the anchor, and the composition of the rode is almost irrelevant,"

as it is then just becomes one very strong 'string' pulling on the anchor. It is then that the weight of the anchor, and how deeply it has bedded in becomes the critical factor, but not until that point, because until then it has really just been locating the end of the rode.
Sure up till then it has been resisting the weight of the rode, but until the rode is effectively lifted clear of the bottom, it really has not been feeling much of the actually pull of the boat, just the weight of the rode, and then not so much until nearly all is lifted because of the friction between that still on the bottom and the mud etc still gripping that chain. And, don't forget, the pull is never constant, and the second there is any let-up, the weight of the rode pulls large lengths of it back to the bottom. So it stands to reason, the more weight in the rode, within reason, the longer it will be and the larger the force must be before the anchor is subjected to the full load of boat which includes the force needed to lift the rode, surely. The lighter the rode, the earlier it will be pulled effectively straight and the full pull of the boat go on the anchor.
To take the comparison one step further, the ultimate full force the combined elements are capable of transmitting to the anchor, once the rode is straight,

"by definition must include the effort/tension required to lift the rode off the seabed, so one can subtract that from the ultimate force of the pull, meaning the net force the boat can exert is the total force minus the force to straighten the rode...which relates directly to its weight...you do the math, as you US guys say..."


My case rests....

Click to expand...

I'm quoting Peter as I think his response to my theory is most revealing, accurate and well thought out.

Peter wrote;
" whether the rode is nylon or chain, you just have the pull of the boat transmitted straight to the anchor, and the composition of the rode is almost irrelevant"
It is not "irrelevant" because the chain pulls very strongly at both ends of the rode. The pull on the boat can only be proportional to the wind or current. But the pull on the anchor end of the rode limits the holding power of the anchor as it's already preloaded. Then the pull of the boat must be added to the chain induced load. The sum total is greater than what it eould be w a light rode.

Peter wrote;
"by definition must include the effort/tension required to lift the rode off the seabed, so one can subtract that from the ultimate force of the pull, meaning the net force the boat can exert is the total force minus the force to straighten the rode...which relates directly to its weight...you do the math, as you US guys say...

No Peter you need to ADD the force that is the pull on the anchor from the weight of the chain. Is there any information/argument that can show me that shows this is not true?

To make them easier to find I've spaced out Peter's comments.


Yes Peter now I see your post (I was writing mine)
As you can see I've been studying it and now I have another to look at or study if need be ... probably. Thanks. I was hoping you'd post again on this.

I'm with PeterB, Psneeld and millions other boaters who put our confidence in chain. The chain will keep the pull on the anchor parallel with the bottom in higher winds than nylon alone. It will also resist abrasion and bury in the mud better than nylon. With chain, I can achieve the same hold as nylon while using less scope than would be required for nylon only rode. In equal scopes, there's no competition...chain wins.

A nylon rope is good if one doesn't have a windlass and needs to keep weight at the bow minimal. (That's what I used in sailboating days.) Otherwise, chain is bettah. Since I have a windlass with 200 feet of chain and the Coot's bow is still "light," I've no need for a rope rode.

Nevertheless, a nylon snubber seems like a good idea.

This thread almost has me missing the "Rocna or single vs twin screw" debates.

Sailor of Fortune said:

This thread almost has me missing the "Rocna or single vs twin screw" debates.

Click to expand...

Hah hah...hello sailor...which side are you on then..?

manyboats said:

I'm quoting Peter as I think his response to my theory is most revealing, accurate and well thought out.

Peter wrote;
" whether the rode is nylon or chain, you just have the pull of the boat transmitted straight to the anchor, and the composition of the rode is almost irrelevant"
It is not "irrelevant" because the chain pulls very strongly at both ends of the rode. The pull on the boat can only be proportional to the wind or current. But the pull on the anchor end of the rode limits the holding power of the anchor as it's already preloaded. Then the pull of the boat must be added to the chain induced load. The sum total is greater than what it eould be w a light rode.

Peter wrote;
"by definition must include the effort/tension required to lift the rode off the seabed, so one can subtract that from the ultimate force of the pull, meaning the net force the boat can exert is the total force minus the force to straighten the rode...which relates directly to its weight...you do the math, as you US guys say...

No Peter you need to ADD the force that is the pull on the anchor from the weight of the chain. Is there any information/argument that can show me that shows this is not true?

Click to expand...

Good arguments - You had me questioning my conclusions for a while. I had to simplify things to get my head around it.

For simplicity, let's say the wind & current are pushing the boat and creating a pull of 10 pounds on the chain at the bow of the boat, but due to gravity and friction have yet to transfer a pulling force on the anchor.

Wind increases and the pull on the bow is 50 lbs. At this point the chain is partly off the bottom and just begins pulling a force on the anchor. Pull on the anchor is 1 pound.

Wind increases more, and the pull on the bow is 100 pounds. The chain is fully off the bottom. The pulling force on the anchor would be less than 100 pounds. How much less would be dependant on the overall average slope of the chain. I'm sure there is an equation to calculate the difference but my math isn't that good.
With all nylon rode, 100 pounds at the bow would equate to much closer to 100 lbs at the anchor.

I can give you proof without math.
Take a 4 foot length of chain and stretch it out between two hands (or fish scales if you want accuracy). Holding both ends level; there is equal pull on both ends. Move one end higher than the other; you will feel more weight pulling from that end, and the lower end will decrease. More angle = more difference. Doesn't matter if the chain is slack or tight, same result.

This equates to less pull on the anchor with chain. A small difference in shallow water, but a bigger difference in deep water. Heavier chain = more difference. Does this prove that an all chain rode holds better? Not necessarily, but it is an argument in favour. Lots of other variables out there.
The extra weight on the bow could do some harm.
(I wouldn't want to put an end to this argument)

AusCan said:

For simplicity, let's say the wind & current are pushing the boat and creating a pull of 10 pounds on the chain ...

Click to expand...

Finally ... what is still missing is the fact that with adequate scope there is some amount of chain lying on the bottom. Assuminmg the bottom is flat, the catenary ends at the point the chain touches the bottom. As long as some length of chain lies on the bottom, there is no strain on the anchor.

For purposes of calculating strain and catenary a chain is considered the same as wire or even rope, but, once the chain lies on the bottom, the section not in the catenary is merely a flexible collection of independent weights that may be lifted to contribute to the dynamics of the vessel by adding to or taking from the length of chain between the fixed point on the boat and the point where the chain touches bottom.

AusCan,
Oh good. A thinker has stepped up to the plate.

AusCan wrote;
"I can give you proof without math.
Take a 4 foot length of chain and stretch it out between two hands (or fish scales if you want accuracy). Holding both ends level; there is equal pull on both ends. Move one end higher than the other; you will feel more weight pulling from that end, and the lower end will decrease. More angle = more difference. Doesn't matter if the chain is slack or tight, same result."

Proof w/o the math! my style.
Yes you've blown a significant hole in my theory. I hadn't thought of that and I feel stupid that I hadn't seen it. So my rode weight theory would hold less water (PI) at short scope than stretched out. True. And w little wind there may be no pull at all on the anchor w the chain. The chain would be holding the boat in place. Fishermen in Alaska do this. They drop their super heavy chain on the bottom and it stays vertical all night and they basically have swing amounting to their boat length and that's it. They may as well have a piling there to tie to. So w all chain anchoring w short scope and in little wind only a small anchor would be needed. Of course a 1/4" nylon line and any anchor that would set would also hold the boat under these underwhelming conditions.

AusCan wrote;

"Wind increases more, and the pull on the bow is 100 pounds. The chain is fully off the bottom. The pulling force on the anchor would be less than 100 pounds. How much less would be dependant on the overall average slope of the chain. I'm sure there is an equation to calculate the difference but my math isn't that good.
With all nylon rode, 100 pounds at the bow would equate to much closer to 100 lbs at the anchor."

The number is prolly way too low but it dosn't matter. At a 5-1 scope (most everybody uses 5-1 right?) and w enough pull on the chain to raise the anchor off the bottom there is indeed plenty of pull. Remember the numbers Rick B mentioned a few posts ago?
AusCan wrote that the difference between the pull on the anchor and the pull on the boat difference would be determined by "the overall average slope of the chain." The angle of the chain would be very different in his example between the angle at the boat and the angle at the anchor where the chain just came off the bottom. I don't think the angle of the chain is involved here. I think the difference is totally dependent on the scope angle. In my mind it gets even more muddy here. It looks like the amount of pull difference should be greatest just after the chain left the bottom. Due mostly to the weight of the chain. Or is it the angle of pull (chain to anchor or chain to boat) or is it both? I'll bet on chain weight. I'm sure it also has to do w the size and weight of the actual chain involved.
But it seems nobody here (except me) anchors at 3-1 anyway so we need to focus on a scope of 5-1 to have much relevance. And at 5-1 just pulling the chain off the bottom involves much more pull. Probably many times as much. So the difference between boat pull and anchor pull at 5-1 is very small both just off the bottom and even less in a blow. And in a blow of course is where any reduction in anchoring performance can be very important or even threatening. And in a blow w maximum pull on the rode and anchored at 5-1 the difference between chain weight pull on the bow and the anchor is minimal. Of course the greatest amount of holding power loss due to chain would be at 7 or 10-1 scope and w max pull on the rode.

So AusCan I think you have put a dent in my theory but mostly under anchoring conditions that require only the lowest level of performance. With demanding or storm type anchoring conditions the difference of pull on the anchor w chain or line is great.

But still the weight of the chain reduces anchor performance by preloading the anchor. And 95% of the extra pull on the anchor would not be there w nylon rode.

Excepting those who choose to anchor in hurricanes (why) or in areas with big swells and lots of fetch:

Some of us anchor in areas where the shore line and other boats limit the scope from ideal to real. With an oversize anchor and all chain I've no difficulty with about 3:1 or very often 2:1 scope. A few years ago Steve Dashew commented on his world wide Windhorse travels with a very big Rocna, all chain and 2:1 or 3:1 scope much of the time.

Yesterday I noted a 70' Princess setting about 5:1 at high tide and very quickly shortening it to about 3:1 as the tide dropped 15' putting his (and those around him) vessel in danger.

My experience is that if you want to go by the "scope book", rely upon rope rode and have a light anchor, your anchor location choices dwindle (very quickly) in many/most places around the world.