Anchor Scope

[vyv_cox]

>

The catenary is the curve in the chain before the chain lying on the bottom, there is no pull load on the anchor when there is still chain on the bottom..

Why on earth would you think that? It is so simple to prove that you are incorrect. Take a short length of chain, anything will do, bike chain, plastic chain or best marine grade. Hold one end of it at your waist and walk, in any direction. The first bit of chain will form a shallow catenary, looping down to the lowest point (the ground). The rest of it will simply drag along the ground, while lying on it. It works equally well with rope, hose pipe, sausages or anything else.

 

[lustyd]

Ah I see where you're going wrong. There is a load on the anchor all the time regardless of whether there is chain on the seabed, and many forum members have said this over and over again. Boats drag at East Head all the time with their chain on the ground. It's easy to demonstrate by just trying to anchor your boat without an anchor. Until you try this and record for our amusement then please stop talking nonsense on the forum!

>So are you saying the anchor doesn't hold the end of the catenary still until all the chain is off the seabed, or are you saying that the catenary reduces the load on the anchor while there is still chain on the seabed? Physics would suggest that either way the anchor won't move if it has the correct holding power and that when the wind gets up the chain is irrelevant anyway so why keep mentioing it?

The catenary is the curve in the chain before the chain lying on the bottom, there is no pull load on the anchor when there is still chain on the bottom. How can chain be irrelevant? Trust me boat anchors do move I've seen many boats drag and two on the rocks when the wind gets up because they have put too little chain or don't carry enough for strong winds or haven't put out a second anchor out.

>the chain does not go "bar tight" before the boat drags in the same way that it does when there is a blow and the anchor is dug in and holds me.

All the boats I've seen drag have had bar tight chain.

 

[lustyd]

Why on earth would you think that? It is so simple to prove that you are incorrect. Take a short length of chain, anything will do, bike chain, plastic chain or best marine grade. Hold one end of it at your waist and walk, in any direction. The first bit of chain will form a shallow catenary, looping down to the lowest point (the ground). The rest of it will simply drag along the ground, while lying on it. It works equally well with rope, hose pipe, sausages or anything else.

Laughed out loud at this. What gauge of sausage have you tested?

 

[vyv_cox]

Laughed out loud at this. What gauge of sausage have you tested?

No Chinese-made yet, so far as I know. Supermarket and locally made beef and pork comply.

 

[oldbilbo]

How did you settle for waist-height? Shirley holding the rode at shoulder height returns a different result? And what's more, if you used SWMBO's string of sausages, how did you compensate for the wee mongrel dawg that attaches itself unshakably to the end, digs in its paws, and pulls....?

Er, have a preference for pork and leek myself..... Calibrated, of course....

 

[TimBennet]

. . . .there is no pull load on the anchor when there is still chain on the bottom.

I'm afraid that's completely and utterly wrong.

You can prove it yourself; take your anchor chain to a beach that is backed by a fairly high sea wall. Lay your chain out across the beach and up the sea wall. Now start pulling the chain tight from the top of the wall with a friend holding the end where the anchor would be. You will be having a right old 'tug of war' long before the all the chain is off the sand.

If the load on the 'anchor' by the chain being pulled exceeds the holding power of the anchor, it will drag, Whether all the chain is off the bottom is irrelevant. The only differences with a bar tight straight chain are the snatch loads are transmitted fully to the anchor and the angle of pull might be less helpful. The research would suggest that this 'bar tight' situation happens at lower loads than traditionally thought.

 

[NormanS]

I'm afraid that's completely and utterly wrong.

You can prove it yourself; take your anchor chain to a beach that is backed by a fairly high sea wall. Lay your chain out across the beach and up the sea wall. Now start pulling the chain tight from the top of the wall with a friend holding the end where the anchor would be. You will be having a right old 'tug of war' long before the all the chain is off the sand.

If the load on the 'anchor' by the chain being pulled exceeds the holding power of the anchor, it will drag, Whether all the chain is off the bottom is irrelevant. The only differences with a bar tight straight chain are the snatch loads are transmitted fully to the anchor and the angle of pull might be less helpful. The research would suggest that this 'bar tight' situation happens at lower loads than traditionally thought.

Yes, but nearly as much fun as the sausages.

 

[KellysEye]

>Ah I see where you're going wrong. There is a load on the anchor all the time regardless of whether there is chain on the seabed, and many forum members have said this over and over again. Boats drag at East Head all the time with their chain on the ground. It's easy to demonstrate by just trying to anchor your boat without an anchor. Until you try this and record for our amusement then please stop talking nonsense on the forum!

It isn't nonsense, dive down on the chain in the bottom and lift it you will find there is no pull on the anchor - been there done that. If the anchor drags with chain on the ground it hasn't been set properly. In the Caribbean I always dived on boat anchors that could drag down on us, around 10% weren't set properly, CQRs on their side and spade anchors not dug in properly, if I saw that we would move.

Let us know what you find when you dive on the chain on the bottom.

 

[lustyd]

>Ah I see where you're going wrong. There is a load on the anchor all the time regardless of whether there is chain on the seabed, and many forum members have said this over and over again. Boats drag at East Head all the time with their chain on the ground. It's easy to demonstrate by just trying to anchor your boat without an anchor. Until you try this and record for our amusement then please stop talking nonsense on the forum!

It isn't nonsense, dive down on the chain in the bottom and lift it you will find there is no pull on the anchor - been there done that. If the anchor drags with chain on the ground it hasn't been set properly. In the Caribbean I always dived on boat anchors that could drag down on us, around 10% weren't set properly, CQRs on their side and spade anchors not dug in properly, if I saw that we would move.

Let us know what you find when you dive on the chain on the bottom.

I've dived on anchors plenty of times - that's how I know how wrong you are!

Why would you move just because the anchor wasn't set, surely in your crazy world you could have removed the anchor and been perfectly safe?

 

[Neeves]

Kelly'I'

I think what you are saying is that when you dive on your anchor, with some or much of of the chain on the seabed, then there is no load on the anchor. I would like you to elaborate as to how you know this. The only way to prove this would be to lift the anchor off, or out of, the seabed (and frankly a well set anchor would be impossible for anyone to move). As if the anchor is in the seabed I'm not sure how you know there is no load. I can believe that you can lift a link or 2 and it 'feels' load less - but that's because the anchor is actually taking the load. But to lift enough chain off the seabed and take whatever load the yacht might be imposing would require strength and diving skills (if not equipment) well beyond most.

However If there is enough chain on the seabed (and the dangling part a minor part of the whole) then the friction between seabed and chain will be sufficient to take the load off the anchor, but this will only occur at low wind speeds, like 5 knots, and is not really relevant.

Our practical experiments suggest that 30m of 8mm chain at 5:1 will all be off the seabed at around 15-20 knots, do not recall precise figure, or a tension in water of 75kg - really its what happens beyond 75kg or 20 knots that is what concerns most people. In fact most people could not care less the mechanics of chain and anchor upto 15 or 20 knots its what happens afterwards as the catenary slowly disappears, straitens, (and there is no friction in the system, apart from chain on water?) that is of most interest.

I appreciate that if I carried 300m of 12mm chain, for a 6t yacht, there would be different maths involved (same theory but bigger numbers) but I like to work a bit closer to reality - though my idea of reality is often called into question But I do note many people use minimal chain and nylon as a mixed rode, where there is no abrasion, suggesting that chain (or weight, or catenary) is largely, or wholly, irrelevant - and a piano wire would suffice (to quote Andrew), were it strong enough.

Jonathan

 

[Andrew G]

KellysEye - please read all that I say below. This is a serious subject and I would not want less experienced readers to get on the wrong track. BUT some of your basic premises are wrong.
What I attempt to explain is based on the theoretical and experimental work by many others, reviewed by peers, is well documented and is supported by 99% of mariners – I’m afraid you are in the 1%.
If you disagree please cite your calculations, experiments and documentation. Mine are listed on posts 18 and 49 above.
Anchoring is a system, with a number of components playing a role. Each has to be strong enough to support the others and need not be so strong as to be wasteful:
1) The anchor transfers the load from the vessel to the ground, it needs to be matched to the ground conditions and be large enough and be designed and constructed to hold the load. (Its design includes the need to re-set, be able to be handled, fit the bow etc).
We agree that it must be set properly to work.
2) At the bow the load needs to be transferred to the vessel by a snubber, chain lock, cleat etc.
3) The rode transfers the load from the vessel to the anchor.

What 1% do not realise is:
a) THE ONLY ROLE FOR THE RODE IS TO TRANSFER THE LOAD FROM THE VESSEL TO THE ANCHOR. IT PLAYS NO MATERIAL PART IN TRANSFERRING LOAD TO THE GROUND. (Sorry for shouting but this is where you are wrong). Different types of rode do behave differently, eg rope vs chain, but their role is the same – to transfer the load between the vessel and the anchor.
b) In extreme conditions there is essentially NO CATENERY. The rode is in a straight line from the bow to the anchor with little or no rode on the ground. (If the anchor digs in there will be some where it exits the hole and if it is not yet extreme there may be a meter or so at the anchor. In lighter conditions there will be quite a lot on the ground).
c) Since a chain (or rope) is a series of hinged joints it transfers no vertical force TO the anchor (it just cannot!) – ITS WEIGHT DOES NOTHING FOR THE ANCHOR. It does transfer a vertical component of the load vector to the anchor but that is UP, NOT DOWN. That is why the angle to horizontal needs to be minimised.
d) If you could dive on an anchor and lift the chain it was NOT EXTREME.
e) The reason one pays out more rode in stronger winds/tides is to reduce the angle of the rode to horizontal AT THE ANCHOR – this is vital for improving holding.
f) I agree that, having payed out rode, if the anchor is not coping one needs another anchor, preferably in-line (and when you do get ashore get a better/bigger anchor for next time).
g) One doesn’t see piano wire in use as a rode because it is NOT strong enough, it can’t be handled without kinking or cutting hands, can’t be winched in or stowed etc. BUT IF IT WERE STRONG ENOUGH IT WOULD BE JUST AS EFFECTIVE AS CHAIN IN TRANSFERRING THE LOAD TO THE ANCHOR.
For those within the 1%, or teetering please:
- read the references cited in posts 18 and 49 above; and
- not anchor up-wind of Brut.
Cheers, Andrew

 

[pmagowan]

Just an observation from my recent cruise in Scotland in gales. I was testing a new anchor (Rocna from CQR) so was keeping a close eye. In really bad weather all the boats in the anchorage were sailing around on their windage. This allowed the anchor chain to retain its catenary most of the time with just occasional snatching when it straightened. It is like the boats convert some of the force of the wind into forward motion similar to when sailing and thus take load off the anchor. It strikes me that in this situation the most important thing is protecting from the snatch loads. Some of the boats, like mine, had a snubber. Ours is one of those dodgy looking rubber things on a length of polysomething rope which fits on to the Sampson post and the chain with a snap shackle. I could see the loading being spread out over time by the stretch in the snubber. The anchorage was crowded so I only had about 4/5:1 on this occasion. The end result was that the anchor simply burrows deeper and is harder to extract.

I understand all the complication of the theoretical variables but in my mind you need a trustworthy anchor, enough chain of sufficient strength to prevent compromising your anchors angle of attack, and a method for reducing shock loads (snubber). If, given the scope you have laid out, the anchor, with rode bar tight, still has a working angle of attack then surely more scope, although marginally better, will have significantly diminishing returns. Since we all like to live on the safe side and our anchors are trusted and slightly oversized then we should have a sufficient margin of safety.

There are some semantics in this debate that are perfectly reasonable in a scientific analysis but in practical terms are not really significant enough to give concern. I think most of us just want to avoid dragging. The only circumstance we are really interested in is the one where this may occur, I.e. severe conditions. If our current tackle at a 4:1 scope would need a force 11 to drag then we don't need to worry about wire or catenary or friction of water. Perhaps the only way to know would be to attach suitable equipment and use the engine to repeatedly test loads. IIRC the new generation anchors normally exceed hold test pulls in the magazine trials but they also don't include snubbers and obviously do not predict the load characteristics of your boat in the described conditions.

Get an anchor you trust, use recommended scope or more if possible, use a snubber. What more?

 

[vyv_cox]

In situations such as you describe, very common in bays surrounded by hills, the angle of yaw on boats can be as much as 140 degrees. Dependent upon anchor type, bottom, and maybe the boat itself, this can be enough to cause the anchor to work free, maybe by churning the bottom into soft mud. Reducing the angle of yaw is therefore a useful contribution to staying put. I use my kedge, laid at somewhere between 60 and 90 degrees to the bower, which cuts down yawing remarkably, usually to less than 70 degrees. You need to keep an eye on wind direction but in most gales this isn't an issue.

There is a writeup on this subject that I did for an internet magazine at http://coxengineering.sharepoint.com/Pages/Thoughtsatanchor.aspx

 

[Neeves]

There are some semantics in this debate that are perfectly reasonable in a scientific analysis but in practical terms are not really significant enough to give concern. I think most of us just want to avoid dragging. The only circumstance we are really interested in is the one where this may occur, I.e. severe conditions. If our current tackle at a 4:1 scope would need a force 11 to drag then we don't need to worry about wire or catenary or friction of water. Perhaps the only way to know would be to attach suitable equipment and use the engine to repeatedly test loads. IIRC the new generation anchors normally exceed hold test pulls in the magazine trials but they also don't include snubbers and obviously do not predict the load characteristics of your boat in the described conditions.

Get an anchor you trust, use recommended scope or more if possible, use a snubber. What more?

Actually much of the work you suggest has been done. There are a number of reports on the loads on yachts under different wind conditions and scopes. The work has not be done using engine power because it is insufficient, most of the work has been done using, surprise, surprise - the wind (and measurement made using loads cells etc etc.). There are reports published on snubbers, for your information the rubber snubbers to which you refer are a waste of money and at best transfer 20% of the kinetic energy of the yacht into potential energy (when you really need in 100% under any conditions), the rubber devices are expensive and 10m of 10/12mm of nylon (not polysomething but actual nylon) much, much more effective as they will convert 100% of the kintetic energy of the yawing yacht into potential energy. Comments on wire and friction in water were flippant and simply made to illustrate a point. There are semantics but a thorough understanding of the semantics if carefully translated into real life actually works.

But to suggest, as has been stated, that an anchor is a 'back up' lacks, any, credibility.

Trustworthy anchors are made by trustworthy suppliers - and fortunately there are some around.

As Vyv has commented there are a number of ways of reducing yawing at anchor, a second anchor set at an angle to the primary anchor is one way, a riding sail is another (championed by many on these forum), using a bridle can be helpful, dropping an anchor off the bow so that it simply drags on the seabed is a well used and documented mechanism (and in HM Navy documentation). On one point I might correct Vyv, in 'sheltered' anchorages a yacht can yaw at 180 degrees, well beyond 140 degrees (been there, done that) and the answer is not necessarily an anchor at all but to tie to a tree (and have an anchor, or 2 as well), recently aired on another thread on these forum. There are no set procedures.

Dragging occurs in many conditions, not only strong wind. Any anchor will drag, catch a beer can in the toe - the anchor will drag. Fill your anchor with weed - the anchor will drag. There are no guarantees.

Jonathan

 

[KellysEye]

>THE ONLY ROLE FOR THE RODE IS TO TRANSFER THE LOAD FROM THE VESSEL TO THE ANCHOR. IT PLAYS NO MATERIAL PART IN TRANSFERRING LOAD TO THE GROUND.

You keep misquoting me I have never said the rode transfers load to the ground. I'm getting seriously p*ssed off with your misquotes so I give up. I suggest you start writing fiction novels since you seem good at it.

 

[vyv_cox]

In fact the article I linked does suggest 180 degree yawing. I didn't want to frighten people so I said 140 degrees in the post

 

[KellysEye]

>But to suggest, as has been stated, that an anchor is a 'back up' lacks, any, credibility.

Obviously an anchor is essential and the reason we viewed it as backup is that we always put out more chain than the wind strength required. Thus when the wind really picked up and the chain started pulling on the anchor we knew it was time to do something, more chain or a second anchor. I hope that explains the thinking, maybe backup is the wrong word but I couldn't think of another.

 

[JohnGC]

>But to suggest, as has been stated, that an anchor is a 'back up' lacks, any, credibility.

Obviously an anchor is essential and the reason we viewed it as backup is that we always put out more chain than the wind strength required. Thus when the wind really picked up and the chain started pulling on the anchor we knew it was time to do something, more chain or a second anchor. I hope that explains the thinking, maybe backup is the wrong word but I couldn't think of another.

I've got a lot from this thread, it has been an interesting debate and I know it isn't always easy to express an idea in a forum; I hope my attempt doesn't muddy the waters

I agree that "backup" is an odd way to think of an anchor in this context;

A catenary is formed in a chain, rope or wire when a tensional force with a horizontal component (pull between the ends) is opposed by a vertical force (gravity). Consider what happens if you remove either of these forces;
- No gravity = straight line between the ends.
- No tension = the chain falls to the seabed.

If one accepts this (and assuming the chain is not just piled up on the seabed) then there must always be some force at the anchor.

We can then consider the extremes;
- No wind or current. The friction of the chain on the seabed will be significant and if a lot of chain is on the seabed the tension at the anchor may be close to zero.
- "Extreme" wind. The chain is completely lifted from the seabed. A very small amount of the tension is opposed by gravity so the majority must be opposed by the anchor (I don't think anyone disagrees).

I think the following observation is an example of chain friction being sufficient to hold the boat in very calm conditions. The anchor was set as usual and dug in under engine. The clear water made it possible to see the anchor shank, chain on the seabed and catenary all in a straight line to the boat. Next morning the boat had swung through 180. The anchor had not dragged or reset and the chain lying on the seabed was in a neat U shape. The point to note here is that it was chain friction holding the boat not weight.

Chain resting on the seabed does not create tension in the catenary due directly to its weight (although weight is a factor in friction). This because it is a flexible assembly of links rather than a solid block of steel. Given a pile of chain 50m long at your feet, lift an end 1m and you are holding the weight of 1m, the remaining 49m is supported by the ground. But if you tie the chain into a ball and lift that.....

I'm not fond of the argument that the catenary disappears completely when the rode becomes "bar tight", I find it more helpful to think of it reducing as the tension increases rather than suddenly switching states which seems a bit mystical. After all, a steel bar suspended horizontally between two points has a noticeable catenary if its long and thin. If the bar is short and fat it still has a catenary but you can't see it.

 

[NormanS]

Well, I was anchored in 74 knots of wind (sustained), with a fabricated plough anchor, based on a CQR, but with a shorter shank. It wasn't a particularly pleasant experience, particularly as most of the time was overnight. We didn't drag.
I'm glad that at the time I didn't know that I was doomed if I didn't have a snubber, because I wasn't using one.
I'm glad that I didn't know that my heavy chain would stop my anchor from burying. It didn't, indeed it was very difficult to break out.
I'm glad that I didn't know that my system wasn't getting relief from the chain's catenary, because it obviously was.

So thanks to all for all the fascinating information in this thread. I just wonder how we all survived before the "new generation" anchors arrived on the scene. Maybe we used anchors and gear sized to suit our boats rather than sized for easy handling by the crew.

I should perhaps add, that although the 74 knot occasion was a one-off, we have frequently had gales in the mid 60s.

 

[macd]

we have frequently had gales in the mid 60s.

Sorry, Norman, but you're clearly making that up. After all, everyone knows that if you can remember the 60s, you weren't there.