Catenary?

[Neeves]

This a still shot taken from a, poor, video that Bruce made of the operation of one of their anchors. If you look at the rode it describes a reverse catenary, the angle to the right of the screen is less than the angle at the shank. This is a commonly observed phenomena and occurs on our anchors in equal measure. The video was made 'in the lab' using an artificial, semi transparent, seabed and a scale model of the anchor.

Its not the scope that is important but the tension angle at the shank.

When our modern anchors engage they do so with the toe and the shackle end of the shank burying together. As tension is increased on the rode the toe and shackle bury together - the impact of shear strength of the seabed impacts the shackle - immediately.

Scope has little or no effect on the buried shackle angle.

Jonathan

 

[noelex]

On a modern well set anchor with a sensible snubber - it will make no measurable difference.

Increasing the effective scope, improves an anchors holding ability, at least until the diminishing returns of a very long scopes sets in. This is a well established anchoring principle.

 

[geem]

Increasing the effective scope, improves an anchors holding ability, at least until the diminishing returns of a very long scopes sets in. This is a well established anchoring principle.

8:1 being the limit.
Delta's seem to need 5:1 when my Spade needs 3:1 in the same seabed in the same wind with anchors selected off the manufacturers sizing charts. In a big blow I let out up to 5:1 if there is room but mainly for piece of mind since we have never dragged on a set anchor in winds up to 50kts plus

 

[Neeves]

Increasing the effective scope, improves an anchors holding ability, at least until the diminishing returns of a very long scopes sets in. This is a well established anchoring principle.

It is well established that staying in a marina is a good and safe anchoring principal. Most owners need to deal with the real world

Most modern anchors have a holding ability well beyond the ability of an owner to access that hold. A 15kg modern anchor in good holding has a hold of around 2,000kg (commonly measured at 5:1 scope) this hold might be reduced at 3:1, it might be reduced in 'poor holding' but a snatch load of 650kg will scare the pants of the owner - been there done that.

Often there is insufficient room to increase scope, other vessels, rock, or shallows - you can have as much chain as you want - but room is sometimes outside your control.

User a decent snubber and the background to reliance on scope, which is (as you say is well known) shrivels and is completely meaningless.

Most owners are not interested in well established principles - they simply want comfort and no mention of mutiny nor divorce. A snubber is a better answer.

Jonathan

 

[Neeves]

8:1 being the limit.
Delta's seem to need 5:1 when my Spade needs 3:1 in the same seabed in the same wind with anchors selected off the manufacturers sizing charts. In a big blow I let out up to 5:1 if there is room but mainly for piece of mind since we have never dragged on a set anchor in winds up to 50kts plus

But you do use a snubber? and don't (from your post) rely on scope.

Jonathan

 

[Neeves]

Scope is so very 20th century. Chain has been a stalwart (after flax) but nylon (a bit younger) is having a re-birth - overlooked by so many.

Anchoring in wind is all about 'managing' the energy of wind, or the energy that the yacht develops from wind.

You can use the catenary, of which you have more if you (can) increase scope and have a large anchor locker and room to deploy the contents. But chain is heavy (reduces sailing performance) and costs - more than nylon.

This is a simple plot of the ability of nylon, 10mm x 10m and chain 10mm x 30m at 5:1 scope.


Upto around 300 kg of tension, maybe 30 knots there is little difference in performance. Beyond 30 knots - just when you might consider increasing scope - nylon is king - cheap and light.

Its really difficult for me to understand the reliance on 'more' scope, heavier chain, longer chain. It also difficult not to question the benefits of 'scope' when there are such simple and cheap alternatives.

But then I'm a cheap scape and believer in technology (all of it, not just little bits).

The rode needs to be considered as one unit.

Snubbers are only part of the equation - you still need some chain (nylon cannot compete with chain for strength - but you can use smaller, stronger, chain)

Jonathan

 

[geem]

But you do use a snubber? and don't (from your post) rely on scope.

Jonathan

Of course

 

[doug748]

Increasing the effective scope, improves an anchors holding ability, at least until the diminishing returns of a very long scopes sets in. This is a well established anchoring principle.

Yes, I think we can all agree on that. I have not seen short scope or high angles of pull suggested for better holding (or setting) but there is always a first time.

The Panope video in post 92 was instructive. It did not tell us anything we did not know about catenary but illustrated nicely how a chain affects the angle of pull.
Thus with catenary playing no effective part at a force equal to 40 kts, (in the example, though arguably around 30kts). The angle at the anchor was c 12deg; the angle with an imaginary rope of no weight would be c 17deg.

However the nub of the matter is: that with half the maximum load the angle was 7 deg and at lower loads/windspeeds in the (critical) setting range the very significant forces acted horizontally or near enough to make no difference. Indeed a force of around 300 lbs is needed to merely lift the chain off the seabed.

 

[Neeves]

Yes, I think we can all agree on that. I have not seen short scope or high angles of pull suggested for better holding (or setting) but there is always a first time.

The Panope video in post 92 was instructive. It did not tell us anything we did not know about catenary but illustrated nicely how a chain affects the angle of pull.
Thus with catenary playing no effective part at a force equal to 40 kts, (in the example, though arguably around 30kts). The angle at the anchor was c 12deg; the angle with an imaginary rope of no weight would be c 17deg.

However the nub of the matter is: that with half the maximum load the angle was 7 deg and at lower loads/windspeeds in the (critical) setting range the very significant forces acted horizontally or near enough to make no difference. Indeed a force of around 300 lbs is needed to merely lift the chain off the seabed.

At 5:1 scope, 30m of 8mm chain, in air, 84.5 kg of tension is enough to lift the last link. 85kgs is also the tension in a 5:1 scope, 30m of 8mm chain subject to about 17 knots of wind on a 38' cat (with a similar windage to a 45' Bav) - but in seawater. At 30 knots the chain would look straight.

Both are my own measurements.




When we set our anchors, 8kg, aluminium Excel or 8kg aluminium Spade we set at approximately 3:1 - every time. In the few times either anchor does not engage, set and hold under engine - there is something wrong. We set without a snubber, so just chain, but we take the tension off the windlass with a dyneema strop.

Jonathan

 

[noelex]

User a decent snubber and the background to reliance on scope, which is (as you say is well known) shrivels and is completely meaningless.

Jonathan

Anchor threads of late seem to be degenerating into nonsense. There are posts trying to assert that the well established, basic rules of anchoring do not apply.

For anyone inexperienced who is reading this thread in the hope of gaining some knowledge it is important to understand that using a snubber (as you should do with chain rode) does not remove the need to deploy adequate scope.

 

[geem]

Scope is so very 20th century. Chain has been a stalwart (after flax) but nylon (a bit younger) is having a re-birth - overlooked by so many.

Anchoring in wind is all about 'managing' the energy of wind, or the energy that the yacht develops from wind.

You can use the catenary, of which you have more if you (can) increase scope and have a large anchor locker and room to deploy the contents. But chain is heavy (reduces sailing performance) and costs - more than nylon.

This is a simple plot of the ability of nylon, 10mm x 10m and chain 10mm x 30m at 5:1 scope.
View attachment 157355

Upto around 300 kg of tension, maybe 30 knots there is little difference in performance. Beyond 30 knots - just when you might consider increasing scope - nylon is king - cheap and light.

Its really difficult for me to understand the reliance on 'more' scope, heavier chain, longer chain. It also difficult not to question the benefits of 'scope' when there are such simple and cheap alternatives.

But then I'm a cheap scape and believer in technology (all of it, not just little bits).

The rode needs to be considered as one unit.

Snubbers are only part of the equation - you still need some chain (nylon cannot compete with chain for strength - but you can use smaller, stronger, chain)

Jonathan

Anchor threads of late seem to be degenerating into nonsense. There are posts trying to assert that the well established, basic rules of anchoring do not apply.

For anyone inexperienced who is reading this thread in the hope of gaining some knowledge it is important to understand that using a snubber (as you should do with chain rode) does not remove the need to deploy adequate scope.

What is adequate scope? It varies for different anchors in my experience

 

[noelex]

What is adequate scope? It varies for different anchors in my experience

The amount of scope needed to qualify as “adequate”depends on many factors. The quality of the substrate, the expected strength of the wind (and or current), the slope of the seabed (a very important and often ignored factor), the size of the anchor (in relation to the boat size) the anchor design, the depth of water and how this will change, the type of rode used etc etc.

It is also helpful in crowded anchorages to deploy similar amounts of scope to other boats.

Choosing the scope to use is often easy because the situation allows for a very comfortable safety margin, but occasionally in tight anchorages, difficult substrates and strong conditions it requires some judgement to decide if the scope that can be deployed will be adequate.

 

[geem]

The amount of scope needed to qualify as “adequate”depends on many factors. The quality of the substrate, the expected strength of the wind (and or current), the slope of the seabed (a very important and often ignored factor), the size of the anchor (in relation to the boat size) the anchor design, the depth of water and how this will change, the type of rode used etc etc.

It is also helpful in crowded anchorages to deploy similar amounts of scope to other boats.

Choosing the scope to use is often easy because the situation allows for a very comfortable safety margin, but occasionally in tight anchorages, difficult substrates and strong conditions it requires some judgement to decide if the scope that can be deployed will be adequate.

All things being equal. A Delta needs more scope than a Spade for example. I suspect old generation anchors all need more scope. We often set 3:1 but 4:1 with a snubber as part of that 4:1 when we know there will be squalls.
We were in Calatra in Portugal a couple of years ago and watched several yachts with Delta anchors drag over the course of a few days. We were set at 3:1 on our Spade. A local Portuguese guy we got to know had a Delta. He said he set 5:1 with the Delta otherwise it would drag in the seagrass in the anchorage. Friends with Deltas dragged. He would go over and tell people to reset with 5:1. They did and they didn't drag afterwards. I asked him why he didn't buy a better anchor. He said the Delta was cheap

 

[noelex]

All things being equal. A Delta needs more scope than a Spade for example. I suspect old generation anchors all need more scope. We often set 3:1 but 4:1 with a snubber as part of that 4:1 when we know there will be squalls.
We were in Calatra in Portugal a couple of years ago and watched several yachts with Delta anchors drag over the course of a few days. We were set at 3:1 on our Spade. A local Portuguese guy we got to know had a Delta. He said he set 5:1 with the Delta otherwise it would drag in the seagrass in the anchorage. Friends with Deltas dragged. He would go over and tell people to reset with 5:1. They did and they didn't drag afterwards. I asked him why he didn't buy a better anchor. He said the Delta was cheap

Some of the anchor manufacturers produce tables outlying the relationship between holding power and scope. This is the Fortress table for example:

Scope ….. % holding power retained.

10:1 …… 100%

7:1 ……. 85%

5:1 ….... 70%

3:1 ….... 40%

2:1 ….... 10%

These tables make assumptions such as anchoring in average depths so should only be seen as very rough indicators, nevertheless they do provide some guide to the relationship between scope and an anchor’s ultimate holding power.

There are differences between anchor models, but most anchor designs perform in a roughly similar manner to the the information provided by Fortress.

Even if the Spade and Delta respond to the reduction in scope with a similar loss of holding power, your observations are still exactly what I would expect.

This is because even if both the Spade and Delta lose exactly the same percentage of hold when using a short 3:1 scope, the Spade starts out with a higher ultimate holding ability. So a Spade at a scope of 3:1 and therefore retaining (say) 40% of its potential holding ability may hold as well or better than a Delta at 5:1 and therefore retaining (say) 70% of its potential holding ability.

40% of larger number may be greater than 70% of a smaller number.

So anything that increases an anchor’s ultimate holding ability, such as moving to a better design of anchor or a larger model of the same design, will enable you to anchor at a shorter scope with the same security.

This is one of the reasons why I am fan of not only selecting the best anchor design but applying the “Big is better" philosophy rather than "Small is adequate" rule when selecting an anchor for a cruising boat. This is always with the caution that the anchor should be able to be handled comfortably by the boat and crew. Improving the anchor design or selecting a larger version of the same anchor design will enable you to anchor at shorter scopes with the same security. This is exactly as your experience demonstrates. This ability to utilise shorter scopes when this is needed opens up anchoring locations that would otherwise be unsuitable.

 

[thinwater]

[I was typing while Nolex was posting. Different bottom, different anchor, and a different tet protecole I suspect, but the same basic story.]

All of this talk of anchor holding at a given scope with chain (catenary) is uninterpretable chatter, since we can't know the angle of the rode at the bottom. based on the information given.

This chart is a combination of data from:

• Anchor manufacturer manual
• Manufacturer of large stockless anchors
• Testing in the field by me

All of the data fit neatly together, so I think it is correct. I am sure there are oil platform anchors that have different characteristics. Names, such as Spade, Rocna, Mantus, and Viking have been eliminated to avoid controversy. I don't need the aggravation. Anchors ranged from 2 pounds to 1500 pounds, yet they stayed on the same curves. Several testing locations were used, including soft mud (different plot), which didn't actually change the story much.

• Zero catenary. Polyester or Dyneema rode was used. The scope is the scope at the bottom.
• The load is the no-movement value. No creep allowed.
• They are normalized for holding capacity at long scope. thus, even lower holding scoops performed similarly. Same with the Claw (I did not test a Bruce).
• The "high scoop" is actually several times better at short scope, if not normalized. A small difference in angles makes a LOT of difference at short scope.
• Note that they are pretty good at 5:1, but they fall off so fast after than as to be completely unreliable. Data was not collected below 3:1 because the scatter was ridiculous. They just break out. With some catenary, your scope at the bottom is actuall greater and you won't see this. Mostly, it is obvious in shallow water.
• The anchors were set at 70% holding capacity at long scope before the scope was shortened.
• Pivoting fluke anchors can be a bugger to recover if deeply set. Now you know why. They can hold more than 50% straight up.

Bear in mind that is the water is >15 feet deep there is some catenary and the scope at the bottom is not the scope. If the water is >25 feet deep, the difference is usually considerable. If you are a multihull anchored in 6 feet, the difference isn't that much in a good breeze. In many ways, it is as much about how much chain (pounds) you have out than the scope.

Cheers.

 

[geem]

Some of the anchor manufacturers produce tables outlying the relationship between holding power and scope. This is the Fortress table for example:

Scope ….. % holding power retained.

10:1 …… 100%

7:1 ……. 85%

5:1 ….... 70%

3:1 ….... 40%

2:1 ….... 10%

These tables make assumptions such as anchoring in average depths so should only be seen as very rough indicators, nevertheless they do provide some guide to the relationship between scope and an anchor’s ultimate holding power.

There are differences between anchor models, but most anchor designs perform in a roughly similar manner to the the information provided by Fortress.

Even if the Spade and Delta respond to the reduction in scope with a similar loss of holding power, your observations are still exactly what I would expect.

This is because even if both the Spade and Delta lose exactly the same percentage of hold when using a short 3:1 scope, the Spade starts out with a higher ultimate holding ability. So a Spade at a scope of 3:1 and therefore retaining (say) 40% of its potential holding ability may hold as well or better than a Delta at 5:1 and therefore retaining (say) 70% of its potential holding ability.

40% of larger number may be greater than 70% of a smaller number.

So anything that increases an anchor’s ultimate holding ability, such as moving to a better design of anchor or a larger model of the same design, will enable you to anchor at a shorter scope with the same security.

This is one of the reasons why I am fan of not only selecting the best anchor design but applying the “Big is better" philosophy rather than "Small is adequate" rule when selecting an anchor for a cruising boat. This is always with the caution that the anchor should be able to be handled comfortably by the boat and crew. Improving the anchor design or selecting a larger version of the same anchor design will enable you to anchor at shorter scopes with the same security. This is exactly as your experience demonstrates. This ability to utilise shorter scopes when this is needed opens up anchoring locations that would otherwise be unsuitable.

Sorry, but I can't agree with this. The situation where the Delta anchors were dragging was in 20 to 25kts. We are no where near where what anybody would consider testing conditions. They simply don't set or hold well in sea grass.
I have never seen evidence that having a bigger anchor in seagrass provides more hold. You need an anchor design that will pentrate seagrass. Weight over a sharp tip seem to be what is required.
A Fortress is a mud/soft sand anchor that performs poorly in anything but that substrate. What it does in soupy mud has little bearing on what a Spade or Delta does in hard sand/broken coral.
I don't agree with big is better. I have already made a significant improvement in anchoring performance by doubling the holding capacity weight for weight compared to an old generation anchor. In addition, the setting ability of my NG anchor is world's apart from anything such as Bruce, Delta or CQR.
I see no point in oversizing. We deal with some pretty significant weather events and have never had an issue with a Spade other than trying to recover it after a hard blow. It tends to dig in very well.
I actually had a good look at a Mantus anchor, I beleive your chosen anchor, and from an engineering perspective thought it was fairly lightly built. What was alarming was how big it was for its weight ( it had to be relatively lightly built to be so light for its size). Lots of bolts as well. It doesn't look like an anchor that would take much abuse but handy if you one to post it somewhere
That massive roll bar looks like a major hinderence to deep setting but I suppose you could say that about any roll bar anchor. There may be some logic in over sizing a roll bar anchor simple because it may not set so deeply with the resistance of a roll bar and with a Mantus a number of bolt protrusions in a location that would hinder a deep set. Imagine what it would be like to install a number of M10 bolts through your shovel then try digging a hole

 

[noelex]

They simply don't set or hold well in sea grass.

Agreed, the Delta design is not good in thick weed.

I have never seen evidence that having a bigger anchor in seagrass provides more hold.

It is important to select an anchor design that is versatile and has good performance in a wide range of substrates. Weed is a very common, difficult substrate so how the anchor performs in this type of seabed is particularly important. This is one reason why I like the Mantus M1. Its thin, tapered fluke together with the high tip weight is very good at penetrating even thick weed. See the photo below as an example.



However, even an anchor design with good weed performance will not generate as high holding power in thick weed as a more ideal substrate such as thick mud. It is similar to looking at scope. Difficult seabeds produce only a fraction of the holding ability the anchor would produce in a more favourable substrate. Even if the percentage loss stays constant between a smaller and larger model of the same design, increasing the size will increase the absolute holding power.

In simple terms larger anchors have greater ultimate holding ability than smaller anchors of the same design this can be be particularly valuable in difficult substrates.

However, you have indicated before that the design of your boat is not suitable for a larger anchor so I would certainly not encourage you to fit a size that could not be comfortably managed, especially as your anchor is meeting your expectations.

 

[geem]

Agreed, the Delta design is not good in thick weed.


It is important to select an anchor design that is versatile and has good performance in a wide range of substrates. Weed is a very common, difficult substrate so how the anchor performs in this type of seabed is particularly important. This is one reason why I like the Mantus M1. Its thin, tapered fluke together with the high tip weight is very good at penetrating even thick weed. See the photo below as an example.

View attachment 157407

However, even an anchor design with good weed performance will not generate as high holding power in thick weed as a more ideal substrate such as thick mud. It is similar to looking at scope. Difficult seabeds produce only a fraction of the holding ability the anchor would produce in a more favourable substrate. Even if the percentage loss stays constant between a smaller and larger model of the same design, increasing the size will increase the absolute holding power.

In simple terms larger anchors have greater ultimate holding ability than smaller anchors of the same design this can be be particularly valuable in difficult substrates.

However, you have indicated before that the design of your boat is not suitable for a larger anchor so I would certainly not encourage you to fit a size that could not be comfortably managed, especially as you are obviously happy with your existing performance.

I have never said my boat isn't suitable for a larger anchor. In fact, quite the opposite. It would take a far bigger anchor. I just don't see the need.
We carry a couple of large aluminium anchors for different seabeds or to set two anchors should the need arise.
I don't want to carry the extra weight around on the bow for no reason.
What is a difficult seabed? If I can set my anchor and it doesn't drag with the full 86hp in reverse then it's set. If I dive on it and it's disapeared, then it's well set. If the design of the anchor encourages the anchor to set deeper and deeper it will set better with more tension applied.

 

[noelex]

If you are content with the performance of your existing anchor I would not recommend a change.

Personally, my wish list is for an anchor that can reliably hold in hurricane force winds at short scopes in all substrates. I am difficult to please . Sadly this type of performance is not feasible with current anchor technology, at least in an anchor size that could be comfortably managed by my boat, but perhaps one day.

 

[Tranona]

In simple terms larger anchors have greater ultimate holding ability than smaller anchors of the same design this can be be particularly valuable in difficult substrates.

It may - no actually will - have greater ultimate holding power, but that does not mean you can access that holding power.

The flaw in all these arguments about increase or loss in holding power is they are expressed in terms of the holding power you are starting with NOT the potential ultimate holding power. A 50% loss in holding power for a good anchor like like a Spade will always leave you with more holding power than even the maximum holding power of a poor anchor like a CQR given equal conditions.