noelex
Well-Known Member
That is very noble of you Vyv.
Anchor Scope
- Thread starter LadyInBed
- Start date
That is very noble of you Vyv.
pmyatt
Well-Known Member
Will this be in a single or double bed - with the lights on or off ? ? ?
Neeves
Well-Known Member
Quite correct that they went to wire and/or Dyneema for weight saving but Vryhof somewhere in their manual have a graphs illustrating the negative effects of chain over wire.
I was sceptical, I tried it, with 6mm wire, 8mm and 12 mm chain, from our cat. And measured setting depth (string with float attached to fluke). It was a very fiddly test, constantly changing chains, resseting next to the previous set etc. The differences are stunning! I'm not recommending wire, I'm not actually recommending going to thinner chain (unless you go to G7) but I am suggesting that blindly advocating heavier rodes (by use of a bigger chain diameter) has some fundamental downsides that have not been aired.
Our research was not original, I discussed this with John Knox some 4 or 5 years ago with regard to the validity of testing anchors with a wire or chain rode (JK uses chain, others use wire). He went off and compared I think 4mm wire vs 8mm chain and found the same result - with wire the anchor will set much more deeply.
I might recommend a short wire strop if I knew more about wire and swaging (and longevity) - but in the absence of knowledge I simply mention the phenomena.
Jonathan
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KellysEye
Well-Known Member
> I am suggesting that blindly advocating heavier rodes (by use of a bigger chain diameter) has some fundamental downsides that have not been aired.
The only thing I can think of is if you have heavy chain in a lightweight boat the bows would be down, but lightweight boats don't need heavy chain, heavy displacement boats do but the weight is fine. What other downsides are there, please air them I'm intrigued?
The only thing I can think of is if you have heavy chain in a lightweight boat the bows would be down, but lightweight boats don't need heavy chain, heavy displacement boats do but the weight is fine. What other downsides are there, please air them I'm intrigued?
Neeves
Well-Known Member
Larger chain reduces the efficiency of an anchor. Though if, as I think in your case, the anchor is only a back up it does not really matter.
Jonathan
Jonathan
Neeves
Well-Known Member
This is what Vryhof say:
Mooring line
An anchor connected to a wire rope mooring line will penetrate deeper
than the same anchor connected to a chain mooring line (fig. 2-05 and fig.
2-06). This is caused by the higher lateral resistance (penetration resistance)
along the chain mooring line. This effect is noticeable in all soil conditions,
but especially in very soft clay where very deep penetration can be
obtained.
This is a simple comparison of chain and wire, but the same is true of thin chain and thicker chain.
A comparison of wire, thin chain 8mm and coarse chain 12mm and their effects on the setting of a modern anchor (as used on some yachts) will be published in Practical Sailor in the next couple of months. I'll post the link when available.
Jonathan
Mooring line
An anchor connected to a wire rope mooring line will penetrate deeper
than the same anchor connected to a chain mooring line (fig. 2-05 and fig.
2-06). This is caused by the higher lateral resistance (penetration resistance)
along the chain mooring line. This effect is noticeable in all soil conditions,
but especially in very soft clay where very deep penetration can be
obtained.
This is a simple comparison of chain and wire, but the same is true of thin chain and thicker chain.
A comparison of wire, thin chain 8mm and coarse chain 12mm and their effects on the setting of a modern anchor (as used on some yachts) will be published in Practical Sailor in the next couple of months. I'll post the link when available.
Jonathan
GHA
Well-Known Member
any reliable data to back this up?how does it work in a blow when the chain is off the seabed?
Neeves
Well-Known Member
The larger the chain the less deeply set is the anchor. On the basis that holding capacity is a function of depth of set then the deeper the anchor is set the better. In mud and fine sand it is possible that the seabed can squeeze though the links but in a hard seabed, one with weed or with stones and pebbles then the surface area of the link becomes large (in effect solid) and is a major barrier to the anchor diving and developing optimal holding capacity. Even in the 'open' link scenario a metre of chain has a lot of surface area - the only way an anchor dives and develops depth is by pulling that chain down, the bigger the chain the more difficult to pull down.
I'm not a great believer that catenary disappears in a blow, its still there but not much. On the basis that most people carry a finite amount of chain then in 40 knots (and in fact at much less wind) its all off the seabed and the difference in pull angle of a 10mm chain and 12mm chain (for the same yacht) will be imperceptible in terms of anchor performance - I'd rather have a well set anchor than rely on a 1 or 2 degree difference in pull. Practical Sailor have the data, wait till they publish.
An answer is not to rely only on the catenary but to reply on a snubber, for a 35' yacht a 10m snubber will absorb all the energy of that yacht moving (yawing at anchor) at 1 knot and does not need to 'use' any of the catenary. The reality is the yacht's energy is shared between catenary and snubber - but as the caternary straightens the snubber will take a larger share. Relying on the catenary alone means you will be experiencing shock loads, certainly, at 40 knots. Shock or snatch loads are much more damaging than the same load applied in slow increments. Again the data on snubbers will be available from Practical Sailor in a couple of months, again it is their data.
There are some nice little videos on the Bruce Anchor website, demonstrating their anchors in a glass tank with a transparent artificial seabed. I copy the links, but might have got it wrong - they are the first 2 vids of 3 on the website. I recall they had better vids but they seem to have updated the site. Its not that good but if you look at the rode (just when they stop pulling, you will see it develops a slight reverse catenary, this is the rode not being pulled down evenly and offering resistance to the anchor diving. I appreciate its not a yacht anchor but only oil rig anchor makers can afford this sort of demo!
Bruce Omni-Directional Dennla Demonstration [4.8MB]
Drag Embedment Near Normal Load Anchor (Dennla) [4.5MB]
Jonathan
I'm not a great believer that catenary disappears in a blow, its still there but not much. On the basis that most people carry a finite amount of chain then in 40 knots (and in fact at much less wind) its all off the seabed and the difference in pull angle of a 10mm chain and 12mm chain (for the same yacht) will be imperceptible in terms of anchor performance - I'd rather have a well set anchor than rely on a 1 or 2 degree difference in pull. Practical Sailor have the data, wait till they publish.
An answer is not to rely only on the catenary but to reply on a snubber, for a 35' yacht a 10m snubber will absorb all the energy of that yacht moving (yawing at anchor) at 1 knot and does not need to 'use' any of the catenary. The reality is the yacht's energy is shared between catenary and snubber - but as the caternary straightens the snubber will take a larger share. Relying on the catenary alone means you will be experiencing shock loads, certainly, at 40 knots. Shock or snatch loads are much more damaging than the same load applied in slow increments. Again the data on snubbers will be available from Practical Sailor in a couple of months, again it is their data.
There are some nice little videos on the Bruce Anchor website, demonstrating their anchors in a glass tank with a transparent artificial seabed. I copy the links, but might have got it wrong - they are the first 2 vids of 3 on the website. I recall they had better vids but they seem to have updated the site. Its not that good but if you look at the rode (just when they stop pulling, you will see it develops a slight reverse catenary, this is the rode not being pulled down evenly and offering resistance to the anchor diving. I appreciate its not a yacht anchor but only oil rig anchor makers can afford this sort of demo!
Bruce Omni-Directional Dennla Demonstration [4.8MB]
Drag Embedment Near Normal Load Anchor (Dennla) [4.5MB]
Jonathan
NormanS
Well-Known Member
OK, I know ypu are determined that bigger chain causes what you call a reverse catenary. This means that the length of chain in question is being pulled firmly down against the seabed, increasing the holding power of the rig. So where is the loss?
How much difference in penetration would there be between say, 8mm and 10mm? A lot? A little? An imperceptible amount?
In normal anchoring, some of the chain is, by design, buried in the seabed. This means that as the boat yaws the buried chain hardly moves, ensuring that the direction of pull on the anchor is constant. This, and the enhanced catenery, is why traditional anchor chains are much heavier than the required strength would suggest. I accept that light-weight, high-windage boats like catamarans, have to make other arrangements, because they cannot cope with the weight, but for heavy boats, there is nothing wrong with heavy gear.
How much difference in penetration would there be between say, 8mm and 10mm? A lot? A little? An imperceptible amount?
In normal anchoring, some of the chain is, by design, buried in the seabed. This means that as the boat yaws the buried chain hardly moves, ensuring that the direction of pull on the anchor is constant. This, and the enhanced catenery, is why traditional anchor chains are much heavier than the required strength would suggest. I accept that light-weight, high-windage boats like catamarans, have to make other arrangements, because they cannot cope with the weight, but for heavy boats, there is nothing wrong with heavy gear.
GHA
Well-Known Member
And also how much increase in peak holding power would there be?
GHA
Well-Known Member
It's nothing to do with belief, it's fairly basic physics. At least one area about anchoring with few variables which can be predicted with confidence.
?????? that doesn't mean anything. If the bottom end of the snubber is made off to a solid point then all the energy from the moving boat will be transferred to the rope, if the lower end moves at all then some energy will be transferred to whatever moves. The idea of not needing to "use" is meaningless. There is no choice in the matter.
Strong scent of conformation bias around here.....
Bruce video here..
http://www.bruceanchor.co.uk/OmniDirectionalDennla.html
Neeves
Well-Known Member
There is a body of opinion suggesting bigger chains are some sort of panacea. That catenary, greater weight, is the answer to a yachtsmen's prayer. Oddly there are people who disagree.
I am suggesting that there are factors (other than cetenary) when considering chain weight that might come into the equation. If I was able to give you the answers I would. I understand you want certainty (which is difficult, as you will know, with anchoring). But maybe its something that merits thinking about
If the chain is pulled down causing a reverse catenary then the load on the anchor is not horizontal - which is the whole basis of the argument for the increased chain weight.
So we have the idea - weight chain keeps the load horizontal when in reality the opposite happens - there might be a contradiction? Which might merit thought?
On the other side of the Atlantic there is a whole slew of people suggesting downsizing chains, for heavy cruising yachts, and replacing with G7 chain - I'm ambivalent, but willing to explore and try to discuss the ideas.
Jonathan
I am suggesting that there are factors (other than cetenary) when considering chain weight that might come into the equation. If I was able to give you the answers I would. I understand you want certainty (which is difficult, as you will know, with anchoring). But maybe its something that merits thinking about
If the chain is pulled down causing a reverse catenary then the load on the anchor is not horizontal - which is the whole basis of the argument for the increased chain weight.
So we have the idea - weight chain keeps the load horizontal when in reality the opposite happens - there might be a contradiction? Which might merit thought?
On the other side of the Atlantic there is a whole slew of people suggesting downsizing chains, for heavy cruising yachts, and replacing with G7 chain - I'm ambivalent, but willing to explore and try to discuss the ideas.
Jonathan
Neeves
Well-Known Member
GHA,
Thanks for the correct link to the video
Jonathan
Thanks for the correct link to the video
Jonathan
noelex
Well-Known Member
The deeper an anchor buries the better the holding. To burry the anchor also has to pull some chain under the surface. The force required to do this subtracts rom the the total "diving force".
Worse, the chain force is at the end of the shank forcing the anchor into a shallower angle of attack (effectively decreasing the tip weight)
In strong wind, which is really all we are intersted in when discussing the holding power of anchors, most of the chain will be off the bottom. The small section of chain that is pulled down with a deeply set anchor will contribute very little to the holding. Burry some chain on the beach and try it.
I think for all boats it is useful to think how we maximise holding for a given weight. (Although cost, size and abrasion resistance are also factors)
A 500KG oil rig anchor with 500m of 20mm chain would hold any recreational boat in any conditions,and in depth of anchorage, but this is not practical. If we can distribute the weight in the most efficient way we will achieve the "highest bang for the buck". There is naturally some disagreement on the ideal distribution of weight, but I think it is a concept worth considering even for a heavy displacement cruising yacht.
GHA
Well-Known Member
There is a body of opinion suggesting bigger chains are some sort of panacea. That catenary, greater weight, is the answer to a yachtsmen's prayer. Oddly there are people who disagree.
I am suggesting that there are factors (other than cetenary) when considering chain weight that might come into the equation. If I was able to give you the answers I would. I understand you want certainty (which is difficult, as you will know, with anchoring). But maybe its something that merits thinking about
If the chain is pulled down causing a reverse catenary then the load on the anchor is not horizontal - which is the whole basis of the argument for the increased chain weight.
So we have the idea - weight chain keeps the load horizontal when in reality the opposite happens - there might be a contradiction? Which might merit thought?
On the other side of the Atlantic there is a whole slew of people suggesting downsizing chains, for heavy cruising yachts, and replacing with G7 chain - I'm ambivalent, but willing to explore and try to discuss the ideas.
Jonathan
I think the old idea of chain weight actually making much difference when you want it most is eroding quickly, thanks to the web & sites like the excellent Alain Fraysee
http://alain.fraysse.free.fr/sail/rode/rode.htm
The idea of thicker chain making a difference is a bit harder to quantify in such a chaotic system such as anchoring in a storm, I suspect other factors would possibly eclipse it but no data, just a hunch. As for chain, if I was to spend the money now I think I'd go for lots smaller chain and a bigger anchor. Even though this one is slightly over. But many factors at play, you don't want to be one sailing round the anchorage with no decent chain weight to slow you down, but on the other hand if the windlass does have a grumpy moment pulling up 40m lighter chain by hand and just having to winch up the final bit would be a lot easier. Many things to factor in, peak loading is only one of them.
KellysEye
Well-Known Member
>Larger chain reduces the efficiency of an anchor. Though if, as I think in your case, the anchor is only a back up it does not really matter.
I think the opposite is true because with light chain there is less weight in the catenary thus will pull on the anchor in less wind than heavy chain. What is the proof of what you are saying, all anchors are efficient if dug in properly.
I think the opposite is true because with light chain there is less weight in the catenary thus will pull on the anchor in less wind than heavy chain. What is the proof of what you are saying, all anchors are efficient if dug in properly.
Neeves
Well-Known Member
Personal experience when testing specifically to look at the phenomena, report from John Knox who was looking at the idea, Vryhof statement and video from Bruce anchor.
But I am sure you are correct.
But I am sure you are correct.
Andrew G
Well-Known Member
Jonathan - you are being very patient. I gave up pages ago. (Did you read my article referenced in posts #18, 39 and 45 )?
KellyE - I don't want to appear to be impolite but there is NO SUCH THING AS CATENARY HELPING THE ANCHOR. There would be if the links were all welded together but since they are all hinged joints then the chain weight/catenerary has no primary benefit. The main factors are in the holding of the anchor in the ground conditions, the length of the rode (and hence the angle to the ground) and snubbing.
In case you don't know, Jonathan has done a number of documented tests over several years. He, I and many other commentators are relying on maths, physics and, in my case, 40+ years of Civil Engineering - it is not a matter of "thinking" or "feeling".
In lighter conditions catenerary and a heavy rode (or even a kellet) may reduce snatching and minimise swing - BUT NOT IN EXTREME CONDITIONS. As I've said before - piano wire would perform almost identically (assuming it had sufficient tensile strength).
If you havn't already, I really do recommend reading the references cited in post #18. All the best, Andrew.
KellyE - I don't want to appear to be impolite but there is NO SUCH THING AS CATENARY HELPING THE ANCHOR. There would be if the links were all welded together but since they are all hinged joints then the chain weight/catenerary has no primary benefit. The main factors are in the holding of the anchor in the ground conditions, the length of the rode (and hence the angle to the ground) and snubbing.
In case you don't know, Jonathan has done a number of documented tests over several years. He, I and many other commentators are relying on maths, physics and, in my case, 40+ years of Civil Engineering - it is not a matter of "thinking" or "feeling".
In lighter conditions catenerary and a heavy rode (or even a kellet) may reduce snatching and minimise swing - BUT NOT IN EXTREME CONDITIONS. As I've said before - piano wire would perform almost identically (assuming it had sufficient tensile strength).
If you havn't already, I really do recommend reading the references cited in post #18. All the best, Andrew.
Neeves
Well-Known Member
Hi Andrew,
I did read the article, way back in post 18, and was only disappointed that you did not provide it for wider coverage in Oz. Why you allowed something so good to be hidden away carries modesty too far. I was also slightly, if not very, embarrassed that I had not read it before. But threads are a window onto the rest of 'a' society, not necessarily typical, but gives an idea what and how people think. I largely only get involved in threads where I have invested some time - interestingly the threads indicate where some clarification, or more detail, might be useful. Kelly and his firm belief in catenary, I'd read his sparring with Vyv Cox some time ago, (and its implications) were part of my motivation for looking at the effects of bigger chain.
I'm mulling over whether to install 6mm G7 (still on our Lightwave) as our new anchor chain for our next trip to Tasmania. Putting our money where my mouth is. Its as near to piano wire as I can get Amongst other things, need to sort out a new gypsy, which is something of a disincentive.
I too will be giving up - we are to take to the water and the internet can become a bit onerous.
Jonathan
I did read the article, way back in post 18, and was only disappointed that you did not provide it for wider coverage in Oz. Why you allowed something so good to be hidden away carries modesty too far. I was also slightly, if not very, embarrassed that I had not read it before. But threads are a window onto the rest of 'a' society, not necessarily typical, but gives an idea what and how people think. I largely only get involved in threads where I have invested some time - interestingly the threads indicate where some clarification, or more detail, might be useful. Kelly and his firm belief in catenary, I'd read his sparring with Vyv Cox some time ago, (and its implications) were part of my motivation for looking at the effects of bigger chain.
I'm mulling over whether to install 6mm G7 (still on our Lightwave) as our new anchor chain for our next trip to Tasmania. Putting our money where my mouth is. Its as near to piano wire as I can get
Amongst other things, need to sort out a new gypsy, which is something of a disincentive.I too will be giving up - we are to take to the water and the internet can become a bit onerous.
Jonathan
Andrew G
Well-Known Member
Thanks Jonathan. Should I tart it up for CH? Cheers, Andrew