How much anchor chain?

[thinwater]

Just a point of accuracy

The tension on the anchor is not dictated by the scope but by the angle of orientation of the shackle. The shackle angle is dictated by the burial depth of the anchor and the shear strength of the seabed. The shackle angle has little to do with scope.

Now that should raise a few comments

Jonathan

I wonder, how deep does the anchor have to bury for this to be true in a practical sense? A few inches? A foot?
* some anchors don't bury
* some bottoms don't let them

For the most part, I've only seen a few brands do this, and only in rather soft mud. Interesting when you see it, though.

 

[Neeves]

I wonder, how deep does the anchor have to bury for this to be true in a practical sense? A few inches? A foot?
* some anchors don't bury
* some bottoms don't let them

For the most part, I've only seen a few brands do this, and only in rather soft mud. Interesting when you see it, though.

I've been measuring shackle angles but I have insufficient data to draw any conclusions - except its real, the angles are higher than you might think and the angles are independent of the scope (that idea of keeping chain on the seabed). Its slow - you need to set the anchor (or anchors) in the intertidal zone and dig it out carefully so as not to disturb the shackle angle - and then measure (and photograph) - you then need to wait for the tide to return so that you can float off. But that old much quoted foundation of 'gut feel' does not work nor does adhering to outdated religions - we need to move forward, nd occsionally look back (so as we keep our feet on the ground (or seabed).

I know shackle angle can be predicted and calculated, if you know the shear strength and depth of burial. I am lucky to have some calculations but I, personally, cannot measure the angle in mud - simply because mud has insufficient consistency to allow me to dig down to the shackle and ensure it remains undisturbed. I can dig down in firm mud and nice clean sand. I suspect weed, with roots, would also defeat me.

If you profess to the belief that having the chain on the seabed and the tension angle should be zero then (you are barking up the wrong tree) having the shackle lifted by 10 degrees would be a negative and if it were 20 degrees - you would never sleep. But advocating the religion of the horizontal tension is simply too simplistic. Modern anchors, some or most of them, operate and develop hold with quite short scopes - though that does not necessarily mean they are 100% secure (there are other factors such as yawing and hobby horsing).

I'm not entirely sure why you might think the shackle angle only varies in soft mud - that lacks logic - its the shear strength of the seabed (and size of the shackle and chain and anything else buried) that determines the shackle angle - and the shear strength of hard sand will be higher than that of mud.

I pointed out that some anchors, I mentioned Fortress (and Danforth) bury such that the shackle (and shank) bury last but modern anchors, Rocna, Spade, Supreme, Excel, Ultra all bury the shackle and toe almost simultaneously (the shackle lags slightly). The critical factor is if the seabed does not allow the anchor to bury then the shear strength will be high - draw your own conclusions.

There is nothing you can do about the angle of the shackle (or not currently) we have been living with it for decades (well 2 decades) - but it undermines the idea that we must have the chain on the seabed to ensure the tension angle is zero - its not zero, it has little or nothing to with scope, that 5:1, 7:1 etc - its all about shackle angle. The US Navy know this, Vryhof knows this, Bruce knows this - live with it.

Shackle angle for older designs did not matter - the anchor did not bury (or not easily and then only buried when the fluke had already buried) and the shackle was one of the components to bury last - chain on the seabed then was important - we have moved on but the cult of catenerary and scope lives on deeply embedded in the psyche of some - even though we have anchors that perform differently.

There is a Luddite in everyone.

I am not saying scope is not important - I am saying that the mantra needs to be qualified.

I am also not saying the this is bad - I did mention - modern anchors do not drag consequently - maybe its not important, maybe we can manipulate the angle and make our anchors better? (Bruce have their boosters.....)

Using small chain, a wire trace, smaller shackles, smaller or no swivels (a Boomerang, or Norman's bent link come to mind).

What we need is the religion of power setting and use of decent snubbers to replace the outdated faiths of catenary and the chain on the seabed.

If you get it right - do as Norman does - you can chuck and forget it (once you have the right rode and power set).

Jonathan

And for those who worship at the alter of catenary please read this article again

http://www.mysailing.com.au/cruising/the-art-of-snubbing-in-the-nicest-possible-way

Please pay some attention to the images, specifically 'scroll' to the last image and look at the graphs. If you don't understand the graphs or think they are wrong - please comment - don't ignore tham - open your mind.

And maybe we can put catenary to bed - once and for all, time.

 

[Refueler]

What always amuses me is the lengths people will go to prove their PoV ... and so often quote another site or blog ...

9x out of 10 ... such sites are from other boaters based on what they observe / experience with THEIR ways to do the job, full of fancy bits and bobs that give it air of expert science. But in reality their PoV ...

I cite again the idiot who wrote page after page in books and online about testing different anchors ... rope rodes ... chain rodes ... he spent ages at it, a significant amount of energy and effort. Trouble was his fundamental mistake was that he was using a tractor on a beach and not taking into account all the other factors that come into play. It did not take long for him to be quiet ..... once he grasped the error ...

As I mentioned before ... if anyone wants to start a long thread and argument ... that has no conclusion other than I do it my way and I'm not going to change ... Just put the word ANCHOR or chain in the title !!

 

[Poey50]

I
What we need is the religion of power setting and use of decent snubbers to replace the outdated faiths of catenary and the chain on the seabed.

Brilliant post as usual but, please, no new religion. I'm absolutely here for the evidence-based anchor atheism.

 

[Poey50]

As I mentioned before ... if anyone wants to start a long thread and argument ... that has no conclusion other than I do it my way and I'm not going to change ... Just put the word ANCHOR or chain in the title !!

Apart from not starting the thread, Isn't this essentially your position? That you are not going to change your mind regardless of the evidence. False belief is always more dangerous than genuine ignorance since the latter opens up space for new learning.

 

[vyv_cox]

Apart from not starting the thread, Isn't this essentially your position? That you are not going to change your mind regardless of the evidence. False belief is always more dangerous than genuine ignorance since the latter opens up space for new learning.

Good post and oh so true. I had used a Delta for 20 years and thought NG anchors were expensive marketing. Until my Delta dragged through soft mud in 50 knots of wind. I bought a Rocna that has survived similar conditions successfully.

 

[NormanS]

As long as everyone is happy with their methods and beliefs, and anchoring results, what does it matter?

 

[RupertW]

As long as everyone is happy with their methods and beliefs, and anchoring results, what does it matter?

It doesn’t really but this is a subject that literally keeps us awake at night and when people with little experience or knowledge like Refueler give counter-factual advice to newbies then bad decisions get made. And I’ve certainly had to change my mind with the evidence from threads like this years ago, and I’m sure I will do so again when new designs and materials for rode and anchor come along.

 

[Poey50]

As long as everyone is happy with their methods and beliefs, and anchoring results, what does it matter?

Any opinion is as good as any other until reality smacks you in the face. On a lee shore you may be on the edge of your anchor dragging and, if it doesn't, then reality has spared you and your belief that all is fine with your setup remains intact. Then one day reality may not be as kind. (See Vyv's anecdote.) The scientific method is a way of testing opinions against reality without personal risk and may spare us that ultimate smack.

 

[thinwater]

I've been measuring shackle angles but I have insufficient data to draw any conclusions - except its real, the angles are higher than you might think and the angles are independent of the scope (that idea of keeping chain on the seabed). Its slow - you need to set the anchor (or anchors) in the intertidal zone and dig it out carefully so as not to disturb the shackle angle - and then measure (and photograph) - you then need to wait for the tide to return so that you can float off. But that old much quoted foundation of 'gut feel' does not work nor does adhering to outdated religions - we need to move forward, nd occsionally look back (so as we keep our feet on the ground (or seabed).

I know shackle angle can be predicted and calculated, if you know the shear strength and depth of burial. I am lucky to have some calculations but I, personally, cannot measure the angle in mud - simply because mud has insufficient consistency to allow me to dig down to the shackle and ensure it remains undisturbed. I can dig down in firm mud and nice clean sand. I suspect weed, with roots, would also defeat me.

If you profess to the belief that having the chain on the seabed and the tension angle should be zero then (you are barking up the wrong tree) having the shackle lifted by 10 degrees would be a negative and if it were 20 degrees - you would never sleep. But advocating the religion of the horizontal tension is simply too simplistic. Modern anchors, some or most of them, operate and develop hold with quite short scopes - though that does not necessarily mean they are 100% secure (there are other factors such as yawing and hobby horsing).

I'm not entirely sure why you might think the shackle angle only varies in soft mud - that lacks logic - its the shear strength of the seabed (and size of the shackle and chain and anything else buried) that determines the shackle angle - and the shear strength of hard sand will be higher than that of mud.

I pointed out that some anchors, I mentioned Fortress (and Danforth) bury such that the shackle (and shank) bury last but modern anchors, Rocna, Spade, Supreme, Excel, Ultra all bury the shackle and toe almost simultaneously (the shackle lags slightly). The critical factor is if the seabed does not allow the anchor to bury then the shear strength will be high - draw your own conclusions.

There is nothing you can do about the angle of the shackle (or not currently) we have been living with it for decades (well 2 decades) - but it undermines the idea that we must have the chain on the seabed to ensure the tension angle is zero - its not zero, it has little or nothing to with scope, that 5:1, 7:1 etc - its all about shackle angle. The US Navy know this, Vryhof knows this, Bruce knows this - live with it.

Shackle angle for older designs did not matter - the anchor did not bury (or not easily and then only buried when the fluke had already buried) and the shackle was one of the components to bury last - chain on the seabed then was important - we have moved on but the cult of catenerary and scope lives on deeply embedded in the psyche of some - even though we have anchors that perform differently.

There is a Luddite in everyone.

I am not saying scope is not important - I am saying that the mantra needs to be qualified.

I am also not saying the this is bad - I did mention - modern anchors do not drag consequently - maybe its not important, maybe we can manipulate the angle and make our anchors better? (Bruce have their boosters.....)

Using small chain, a wire trace, smaller shackles, smaller or no swivels (a Boomerang, or Norman's bent link come to mind).

What we need is the religion of power setting and use of decent snubbers to replace the outdated faiths of catenary and the chain on the seabed.

If you get it right - do as Norman does - you can chuck and forget it (once you have the right rode and power set).

Jonathan

And for those who worship at the alter of catenary please read this article again

http://www.mysailing.com.au/cruising/the-art-of-snubbing-in-the-nicest-possible-way

Please pay some attention to the images, specifically 'scroll' to the last image and look at the graphs. If you don't understand the graphs or think they are wrong - please comment - don't ignore tham - open your mind.

And maybe we can put catenary to bed - once and for all, time.

Mostly, I wanted to hear more.

I did not mean to imply I only though soft mud affected the shackle angle, only that is where deep burying is most common. In fact, firm material would have an effect at less depth.

Yes, the shackle angle depends on burying. But burying depth is affected by scope. The math on shackle angle during the burying process is not independent of the angle at the seabed. In other words, an anchor set at 10:1 scope (angle at the sea floor) will generally bury deeper than one set at 4:1 scope, even though the shackle angle on both will end up about the same. Anchor set at 7:1 scope (Fortress, for example) will hold at full capacity at 3:1 once well-set, but it will not set at 3:1 in most beds. That said, once set at long scope, we are back to your point; scope matters less. But if we assume some of the setting is done by the storm rather than by power setting, then you need longer scope (depending on catenary and depth--I don't intend to unpack all that just now).

You used the US Navy example of a Fortress anchor set at 45 degrees. When it catches, the anchor is POWERFUL at that angle. But in most seabeds it is also unreliable at 45 degrees, prone not to setting at all. I've tried this and so have others. Thus, the 45 degree example is an exaggeration for cruisers. But I do believe the shackle angle can be 30 degrees or so, equivalent to about 3:1 scope.

I have performed and have studied tests pulled at steadily reducing scope. Although the angle varies, holding capacity falls as the scope (angle at sea bed) passes below about 3 or 5:1, depending on the anchor. The drop off can be slight (Fortress) or extreme (Mantus and Claw).

Finally, this logic (shackle angle) only works if you KNOW the anchor is buried. Some places you can dive or see it, but others you cannot. Of course, isn't this nearly always the case with anchoring?

I think the practical answer is in the middle. Scope helps, but well set anchors can deal with some uplift. Catenary exists, but depends on depth and has limits. Chain reduces chafe considerably, but yawing only a bit once the wind picks up. We're not ludites and tradition is not all wrong, only a little out of date.

----

A slight change in topic. The below data was taken between 8.3:1 scope and 5:1 scope. In general, anchors show failure or drop off at about 6:1 scope, but notably, the Fortress 45 degree just keeps going up. But most are failing a 6:1. Not my data, but I have similar results and have seen other similar data (it always varies with the bottom). This is in very soft mud with layers of shells down deep.
https://www.boatus.com/seaworthy/assets/pdf/anchor-test.pdf

 

[Stemar]

Brilliant post as usual but, please, no new religion. I'm absolutely here for the evidence-based anchor atheism.

Who was it that said, "There are no atheists in a storm at sea"?

 

[thinwater]

... And I liked the "evidenced-based anchoring atheist" comment!

What I have learned from anchor testing is that by the time you add variable bottoms, waves, yawing, wind shifts, and the range of depths and boats, it's bloody complicated! And that is why we settle on traditional, overweight strategies. They have a certain robustness. More chain than we need. A bigger anchor. Snubbers. Perhaps a riding sail or kellet. It all helps. Sometimes it's good seamanship. Sometimes it's a bit silly.

 

[thinwater]

Who was it that said, "There are no atheists in a storm at sea"?

Oh really? I've never understood that statement at all. The fallacy should be, well, obvious to a small child.

 

[noelex]

The issue of rode weight effect on anchor performance is an interesting question that has important implications for the equipment chosen.

I think we need to look at some of the broader aspects of anchor performance to arrive at the truth.

For example, in the very early stages of setting, the anchor chain weight helps the anchor develop the very important initial bite. If you try setting the anchor with all rope rode, for example, the almost neutral weight of rode (in water) means that before the anchor has even moved or adopted the correct setting position the angle of the rode will correspond to the scope angle (for example at a scope of 5:1 this would be 12°). A heavy, all chain rode will be on the seabed during these initial stages and the horizontal pull considerably helps the anchor initially bury. Light chain will be something between these two extremes.

Modern anchors are very good at initially digging in, but in difficult substrates such as weed and hard sand, failure to initially grab is a very common primary cause of the anchor failing. In an ideal medium mud substrate almost any anchor (design and size) is likely to be OK. It is anchoring in more challenging circumstances where the best equipment and techniques are needed and here a heavier rode does have advantages.

When subject to a wind shift is another time when anchor performance can commonly fail. The speed and momentum of the yacht during a significant wind shift will be greater for boats with an all rope rode compared to boats using all chain rode. This is easily observed (and cursed at in a crowded anchorage ) from the surface. The anchor has to rotate more rapidly. Boats using very lightweight chain will be between the two examples.

I think we need some more research to reach some firm conclusions, but to totally dismiss rode weight as an aspect effecting anchor performance is wrong. The above are just a couple of important areas where chain weight has some role.

 

[RupertW]

The issue of rode weight effect on anchor performance is an interesting question that has important implications for the equipment chosen.

I think we need to look at some of the broader aspects of anchor performance to arrive at the truth.


When subject to a wind shift is another time when anchor performance can commonly fail. The speed and momentum of the yacht during a significant wind shift will be greater for boats with an all rope rode compared to boats using all chain rode. This is easily observed (and cursed at in a crowded anchorage ) from the surface. The anchor has to rotate more rapidly.

....

I think we need some more research to reach some firm conclusions, but to totally dismiss rode weight as an aspect effecting anchor performance is wrong. The above are just a couple of important areas where chain weight has some role.

I’m not convinced by your first example that rode weight helps the initial dig in but it’s very plausible and worth some thought - my only counter thought at the moment is that the anchor won’t dig in until it has some force applied which will start to straighten the rode anyway but, interesting.

Your second one I don’t think is right the way you state it. I think there are two separate behaviours - the first absolutely right one is that in a very light shifting wind boats lying to chain will move it around the seabed far less than a rope rode. But once the wind is maybe 5 knots or more the differences diminish and disappear by the time the wind is anywhere close to dragging. And that leads to the second behaviour of an anchor having to reset when the wind changes and is strong enough to upend or unearth the anchor. At those wind speed I can’t see how rope or chain will make any difference as the angle will be scope based not light-wind catenary.

Your overall point is so true about most of us doing all things we have learnt to be correct and then some of the old myths just in case we are wrong.

 

[AntarcticPilot]

Who was it that said, "There are no atheists in a storm at sea"?

Nor in foxholes, as verified by an ex soldier who had served in Afghanistan.

 

[thinwater]

The issue of rode weight effect on anchor performance is an interesting question that has important implications for the equipment chosen.

I think we need to look at some of the broader aspects of anchor performance to arrive at the truth.

For example, in the very early stages of setting, the anchor chain weight helps the anchor develop the very important initial bite. If you try setting the anchor with all rope rode, for example, the almost neutral weight of rode (in water) means that before the anchor has even moved or adopted the correct setting position the angle of the rode will correspond to the scope angle (for example at a scope of 5:1 this would be 12°). A heavy, all chain rode will be on the seabed during these initial stages and the horizontal pull considerably helps the anchor initially bury. Light chain will be something between these two extremes.

Modern anchors are very good at initially digging in, but in difficult substrates such as weed and hard sand, failure to initially grab is a very common primary cause of the anchor failing. In an ideal medium mud substrate almost any anchor (design and size) is likely to be OK. It is anchoring in more challenging circumstances where the best equipment and techniques are needed and here a heavier rode does have advantages.

When subject to a wind shift is another time when anchor performance can commonly fail. The speed and momentum of the yacht during a significant wind shift will be greater for boats with an all rope rode compared to boats using all chain rode. This is easily observed (and cursed at in a crowded anchorage ) from the surface. The anchor has to rotate more rapidly. Boats using very lightweight chain will be between the two examples.

I think we need some more research to reach some firm conclusions, but to totally dismiss rode weight as an aspect effecting anchor performance is wrong. The above are just a couple of important areas where chain weight has some role.

Well put.

I use a ridiculously light set-up on my Farrier trimaran. It's a light boat, sails very fast, and I don't cruise it. But the tackle is very secure because it is tuned to the boat. I even use a light kellet sometimes so I can swing like all-chain. But all-chain would be absurd on this boat.

On my cruising cat I used all-chain for the reasons you suggest. Light chain, only 40 meters (enough for my waters 98% of the time) backed with rope, and a modern anchor. A windlass did the work and it had a nice stability about it.

No one truth. A continuum.

 

[Refueler]

There's an age old joke regarding Ships Officer Aural Examination :

Cadet is sitting there with Examiner asking questions ....

Q. : You are at anchor and wind increases, Master is incapacitated, you have to make decision what to do. You are concerned that ship may drag anchor ... what do you do ?

A. : Pay out more cable (note for yachties ... cable is ships term for chain)

Q. : Wind gets up more ?

A. : Pay out more cable ..

Q.: Wing gets up even more ??

A. : Pay out more cable ...

Q.: Where are you getting all this cable from ?

A.: Same place as your blo**y wind !!

 

[noelex]

You make some good points Rupert. I dont think this stuff is simple. A couple of comments:

my only counter thought at the moment is that the anchor won’t dig in until it has some force applied which will start to straighten the rode anyway but, interesting.

If you have an all chain rode and the chain is the normal recommended size for your boat (not thin and lightweight G70) the portion of the chain close to the anchor will stay on the seabed until quite late in the setting process.

Photographs taken during the setting process with little chain catenary are completely accurate and realistic, but this is at the late stage of setting when full reverse power is being applied to complete the final set of the anchor. In the early stages when the anchor is starting to dig in up to the stage where it reasonably buried, the chain will be on the seabed giving a zero angle and therefore giving the anchor the best hope of starting to bury well. With an all rope rode the rode will immediately rise above the seabed and adopt the scope angle (12° for 5:1) and the resulting upward force on the shank makes the anchor’s job harder. Lightweight chain will be between these two examples.

In good substrates the anchor will bite and start to initially dig in satisfactorily with this high rode angle, but it does make the anchor’s job harder. The setting distance will be longer if the rode angle is higher, and for example in weed there is more of risk that the fluke will be obstructed by broken and torn out weed, reducing the quality (and ultimately the maximum holding ability of the anchor).

Your second one I don’t think is right the way you state it. I think there are two separate behaviours - the first absolutely right one is that in a very light shifting wind boats lying to chain will move it around the seabed far less than a rope rode. But once the wind is maybe 5 knots or more the differences diminish and disappear by the time the wind is anywhere close to dragging.

In only 5 knots a boat anchored with all chain rode will not move (or at least move very little) if the wind changes direction. The boat will swing around to face into the wind, but the bow will be held in roughly the same position by the chain. It takes much more wind than this for the chain to start sliding along the seabed so that the boat moves to the new position in the swing circle. The amount of wind when this occurs and speed of transition depends on the substrate (for example it requires more wind in thick weed) and the weight of the chain, as well as scope etc, but it is well above 5 knots.

Below is an example from our Vesper anchor plot. The breadcrumb trace show the slow transition of boat position with a 180° change in the wind direction. This was in about 20 -25 knots of wind from memory. This type of slow transition gives the anchor plenty of time to shuffle. With an all rope rode the boat position would have changed very rapidly leaving little time for the anchor to rotate. In addition if the wind change is rapid, the boat can build considerable momentum increasing the chance that the anchor is going to to flip (although this is unlikely) rather than the more secure “shuffle” or slow rotation.

 

[noelex]

Here is an example of an Ultra anchor rotating to a new wind direction. You can see the puffs of sand produced as the anchor slowly rotates. It is not doing a great job, with a very large list, but to be fair it was not set very well to begin with.

The chain is along the seabed so the anchor is being pulled at 0°, helping the anchor stay engaged with the seabed as it rotates. If this was on an all rope rode, the rope would be elevated at the scope angle (12° for 5:1), as with even a small force the only slightly negatively buoyant rope will have zero catenary.

If using all rope rode this 12° upwards force angle on the shank would increase the chance that the anchor will completely break out during the rotation.

Chain is very heavy. It is worth considering “spending” this weight elswhere when practical such as increasing the size of the anchor. Generally this will result in more benifits. However, I think it is wrong to percieve the weight is the chain as someting that has no value.