Loading capability of anchor versus anchor chain

[noelex]

Thanks for posting, Mathias.

As the ratio of total chain weight to anchor weight is greater than 2/0.6 or 3.5/0.75 for most long distance cruising boats the formula suggests the chain provides similar or even greater holding power than the anchor in conditions where most or all of chain is deployed.

However, I do not think this formula is accurate or relevant for our sized vessels. Large ships have always anchored significantly differently to smaller sailing yachts, relying much more on chain and less on the anchor. There are scale factors that need to be considered.

 

[MathiasW]

Thanks for posting, Mathias.

As the ratio of total chain weight to anchor weight is greater than 2/0.6 or 3.5/0.75 for most long distance cruising boats the formula suggests the chain provides similar or even greater holding power than the anchor in conditions where most or all of chain is deployed.

However, I do not think this formula is accurate or relevant for our sized vessels. Large ships have always anchored significantly differently to smaller sailing yachts, relying much more on chain and less on the anchor. There are scale factors that need to be considered.

Indeed, our setup is very different. The ratio is more like 12/0.75 for modern anchors. But even 3.5/0.75 is perhaps misleading. It is not the ratio of anchor to chain as such, since it all depends on the length of chain on the seabed - the Lc factor in the 2nd term. It would only be this ratio if the total weight of the anchor chain lying on the seabed is the same weight as that of the anchor. But for this, a huge amount would have to be lying around. In my case, a 35 kg Spade anchor, so ok 32 kg or what have you in water, and a 10 mm chain, meaning 2 kg/m, this matching weight would only be achieved by 64 metres of chain lying on the seabed and doing nothing else but lying there. And even then the anchor would strongly dominate with a factor probably even exceeding 12/0.75...

I guess we usually do not do that...

 

[noelex]

Indeed, our setup is very different. The ratio is more like 12/0.75 for modern anchors. But even 3.5/0.75 is perhaps misleading. It is not the ratio of anchor to chain as such, since it all depends on the length of chain on the seabed - the Lc factor in the 2nd term. It would only be this ratio if the total weight of the anchor chain lying on the seabed is the same weight as that of the anchor. But for this, a huge amount would have to be lying around. In my case, a 35 kg Spade anchor, so ok 32 kg or what have you in water, and a 10 mm chain, meaning 2 kg/m, this matching weight would only be achieved by 64 metres of chain lying on the seabed and doing nothing else but lying there. And even then the anchor would strongly dominate with a factor probably even exceeding 12/0.75...

I guess we usually do not do that...

While the chain above the seabed will not be contributing anything to the holding power. The formula does not make this distinction and uses the total length of deployed chain. This simplification also applies to the anchor side of the equation where the total anchor weight is used despite the fact that parts the anchor such as the shank contribute little to the holding power once the anchor is set.

Yacht anchor chains are above the seabed more often than chain on large ships, so this simplification does not work well and I think this is part of the reason the formula does not give the correct answer on our sized vessels.

 

[Neeves]

While the chain above the seabed will not be contributing anything to the holding power. The formula does not make this distinction and uses the total length of deployed chain. This simplification also applies to the anchor side of the equation where the total anchor weight is used despite the fact that parts the anchor such as the shank contribute little to the holding power once the anchor is set.

Yacht anchor chains are above the seabed more often than chain on large ships, so this simplification does not work well and I think this is part of the reason the formula does not give the correct answer on our sized vessels.

Ordinarily you are correct that the shank contributes little to hold but if the wind is veering side to side then the shank acts as a vertical fluke (and the more of it that is buried the greater the effect). I, for one, would not ignore the size of the shank (in terms of side elevation) as the vertical plate is going to deter the anchor from also moving from side to side as a moving anchor is one whose hold is compromised.

To me - when the chips are down and you want maximum hold then the chances are all the chain is off the seabed - forget the chain (except as a means of joining anchor to yacht) and rely on anchor characteristics (and an ability to absorb energy - in the form of catenarary or elasticity). As I tried to illustrate in post 13 - catenary is a finite resource, elasticity last much longer (and you can always 'add' elasticity without the need to deploy more chain, which may be impossible anyway)

Jonathan

 

[MathiasW]

While the chain above the seabed will not be contributing anything to the holding power. The formula does not make this distinction and uses the total length of deployed chain. This simplification also applies to the anchor side of the equation where the total anchor weight is used despite the fact that parts the anchor such as the shank contribute little to the holding power once the anchor is set.

Yacht anchor chains are above the seabed more often than chain on large ships, so this simplification does not work well and I think this is part of the reason the formula does not give the correct answer on our sized vessels.

Hmm, you are right. Also in the full paper they do not say what Lc actually means. Too bad. The only sensible thing is for it to be the length of chain lying on the seabed.

But even without it, with a modern anchor where Ka / Kc = 12 / 0.75 = 16, it would mean that the total chain weight would need to be 16 times larger than the anchor weight to match the load capabilities of the anchor. In my case 16 * 32 kg, which for a 10 mm chain would mean 256 metres of this chain are required for it to have the same nominal holding power as my anchor - according to this formula, at least.

I do not have that much chain on board...

 

[Neeves]

I do not have that much chain on board...

Maybe you need a bigger boat

But just think with 256 metres of chain, according to the formula, you don't need an anchor. We have all been wrong - the chain will hold you.

Mathias - do you really use 10mm chain?

Jonathan

 

[MathiasW]

Yes, I use 10 mm chain. The vessel normally is fitted with 12 mm chain as per shipyard, but I "down-graded" to duplex 10 mm, so that I could accommodate a longer chain with the same total weight...

And on the meaning of Lc... Since in their table they give different values for Lc depending on the type of seabed, it can only mean that Lc refers to the chain length on the seabed. Why, otherwise, would that Kc coefficient be different when the chain is hanging over one type of seabed or another, without actually touching it...

But it is far from clear.

 

[TNLI]

Maybe you need a bigger boat

But just think with 256 metres of chain, according to the formula, you don't need an anchor. We have all been wrong - the chain will hold you.

Mathias - do you really use 10mm chain?

Jonathan

Mine Gott! 256m of 10mm chain, he is going to need a new bank manager or credit card company for that lot. It would cost around 2.5K great British pounds!

 

[Stemar]

It would cost around 2.5K great British pounds!

Getting up there in pounds avoirdupois too!

 

[NormanS]

While you guys are busy calculating, and worrying about ever more meaningless and abstruse formulae, I'll just anchor in my usual simple way, and relax.......?

 

[MathiasW]

Never mind that. Here is a document from the US Navi:

Taylor R and Valent P, 1984. Design Guide for Drag Embedment Anchors, Naval Civil Engineering Laboratory (USA), TN No N-1688.

They explicitly work out the length of the chain that remains on the seabed, L_s, and then do a similar analysis as in the Korean paper for the frictional force this part of the chain causes. (Their variable s is the catenary part of the chain, and L_t is the total length.)

They also note that using the chain as a measure to take on part of the load is not a cost-effective way and it is better to invest in a larger anchor instead...

It is kind of embarrassing to have to keep pushing such a trivial point, but apparently, it is not a no-brainer...

Cheers, Mathias

 

[MathiasW]

PS: According to their definition of symbols, Taylor et al use L_s as the length of chain on the seabed, and L_c as the length of chain cutting into the seabed, which presumably is even less. So, maybe the Koreans have used the same definition implicitly and index 'c' does not stand for chain, but rather for cutting.

 

[NormanS]

PS: According to their definition of symbols, Taylor et al use L_s as the length of chain on the seabed, and L_c as the length of chain cutting into the seabed, which presumably is even less. So, maybe the Koreans have used the same definition implicitly and index 'c' does not stand for chain, but rather for cutting.

I rest my case. ?

 

[Neeves]

Never mind that. Here is a document from the US Navi:

Taylor R and Valent P, 1984. Design Guide for Drag Embedment Anchors, Naval Civil Engineering Laboratory (USA), TN No N-1688.

They explicitly work out the length of the chain that remains on the seabed, L_s, and then do a similar analysis as in the Korean paper for the frictional force this part of the chain causes. (Their variable s is the catenary part of the chain, and L_t is the total length.)

They also note that using the chain as a measure to take on part of the load is not a cost-effective way and it is better to invest in a larger anchor instead...

It is kind of embarrassing to have to keep pushing such a trivial point, but apparently, it is not a no-brainer...

Cheers, Mathias



View attachment 129058View attachment 129059

Most of us, virtually everyone, thought the comment that 'its the chain not the anchor that holds the vessel' was dangerous nonsense. The danger being someone, a neophyte, might think it correct. It is a no brainer - the evidence focussing at the anchor - is vast and there is no evidence supporting the contention.

If you google Taylor and NCEL you will find a whole hose of technical papers all focussed to anchors and anchoring. Taylor holds a patent for one of the anchors used by the US Navy, he consulted for oil rig companies, knew Peter Bruce personally. Much recent work on anchors - was done by Taylor and his colleagues years ago. I am biased and was introduced to him some years ago and rely on his comments.

Jonathan

 

[thinwater]

Indeed, our setup is very different. The ratio is more like 12/0.75 for modern anchors. But even 3.5/0.75 is perhaps misleading....

AT LEAST 12:1. I thought that was too obvious to even mention.

A 35-pound anchor sand or good mud will be more like 40:1 A 35-pound anchor will only hold 12:1 in the worst imaginable bottom.

It's like this. The ONLY case that matters in terms of ultimate holding is when it is blowing, and in that case the chain will be off the bottom or virtually so. The 20 feet that are still on the bottom could be pulled with a pinky finger.

The other case that matters is during wind and tide shifts. In that case it is sideways drag, and the math is much different. It is the much stronger case for chain.

---

We don't ponder things like anchoring dynamics or rigging loads because we really need to. We don't need to do crossword puzzles or sail or read fiction. We just enjoy it. The other reason we don't need to do it is that if we just follow manufacturer and ABYC guidelines, someone else did the figuring for us, a good bit on the conservative side. The only people who need to think about it either anchor in very tough conditions, very poor bottoms, or want to minimize weight. Those folks need to understand a bit more.

 

[Neeves]

We don't ponder things like anchoring dynamics or rigging loads because we really need to. We don't need to do crossword puzzles or sail or read fiction. We just enjoy it. The other reason we don't need to do it is that if we just follow manufacturer and ABYC guidelines, someone else did the figuring for us, a good bit on the conservative side. The only people who need to think about it either anchor in very tough conditions, very poor bottoms, or want to minimize weight. Those folks need to understand a bit more.

A slight amendment.

You should not look at chain or anchor, or shackle etc in isolation - its a matched package.

We don't need to ponder anchor and chain size because its already available. Anchor makers have spread sheets and supplier of chains also have spread sheets. These spread sheets have been honed over decades and are tweaked by the Classification Societies (or vice versa). If these spread sheets were wrong, too small, we would have a number of yachts on beaches. We don't have, many, yachts on beaches, chains don't break - the anchor makers and Classification Societies seem to have it 'about right'. The spread sheets look crude - because the suppliers make it simple and, for example, for chain there are only 4 common sizes catering for yachts from 16' long to 50' long.

If anchor or chain spread sheets were wrong the legal profession would have a field day and chain and anchor makers would spend their time in courts of law. If you don't believe me - quote me case law where chain makers or anchor makers have been in court because their chain or anchor has been 'too' small.

It merits note that the spread sheets were developed before lightweight chains, aluminium anchors, anchors made from high tensile steels and elastic ropes were available - and common place. The historic spread sheets need some modification to accomodate technical developments and innovation. Fortress are the exception - and they have their own 'personalised' spread sheets - which are accepted by the Classification Societies (or Fortress would not be common place on Government vessels).

Now we have seen that there is a considerable absence of any quantitative knowledge, certainly in terms of the rode (and I suspect in terms of anchor size) on this forum - yet armchair experts appear to hold sway and recommend larger anchor (without an iota of evidence and despite the fact yachts do not end up on beaches - and most owners use the anchor supplied at commissioning which are sized according to the spread sheets). The same can be said of chain - many use oversized chain - yes it offers more catenary (but more is finite) and there are now other means that can be used to replace catenary. It is said that long term cruisers use oversized anchors (this may be true) - but evidence - zero.

If you probe the armchair experts, like this thread has tried to do, you will soon determine the claims lack any objective basis and in fact in some cases the claims are based on a false premise (but no need to go there - unless provocation develops).

But sad but true - the idea of heavy chain being necessary or the bigger anchor being essential is now so ingrained it will be years before we can return to objectivity being accepted.

Jonathan

 

[thinwater]

^^ I was just responding to the common "over thinking this" responses that show up on threads of all sorts. We could just ignor the details and go sailing. I fact, that is exactly what we do one relaxing days! Other days we puzzel over whatever strikes our fancy.