RichardS
N/A
I covered all this ground way back in post #8 but made my little impact (my life story)
I used the terms "related to" rather than "roughly proportional to" but I think we all know what we mean.
Richard
A contradiction with bigger anchors
- Thread starter Neeves
- Start date
I covered all this ground way back in post #8 but made my little impact (my life story)
I used the terms "related to" rather than "roughly proportional to" but I think we all know what we mean.
Richard
Neeves
Well-known member
I covered all this ground way back in post #8 but made my little impact (my life story)
I used the terms "related to" rather than "roughly proportional to" but I think we all know what we mean.
Richard
Yes, I like 'related'.
Jonathan
edit
And I did like your post #8, pointed to the obvious error of the focus on weight (which was never the issue) an excellent example of diplomacy, not inflammatory - but missed. Its the internet.
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noelex
Well-known member
The evidence points to holding power being roughly proportional to anchor weight (if we compare anchors of the same design and material).
You said exactly the same thing in one of your recent magazine articles on anchoring, so I am not sure what has lead to the change of heart:
"A final argument could be suggested that bigger anchors will work better. This is valid and roughly twice the weight will produce twice the hold" (Jonathan Neeves Cruising Helmsmen magazine November 2015).
For experimental results you can look at the work done by Prof Knox. His conclusions are:
"The maximum holding of an anchor, as recommended by manufacturers, is proportional to its weight. This is precisely what I found by direct experiment" (Prof Knox PBO magazine August 2002).
You can see for both the tested Delta and Bruce, as you double the weight you double the hold:
Or you can look at the results from the Vryhof tests. These are for very large anchors. Their results indicate doubling the anchor size increases the hold by 92%. Their formula for hold verses anchor weight is:
Ultimate holding capacity= A*(W)0.92 where W is weight and A is constant based on the anchor design and substrate. In graph form it looks like this. The slope is not quite 1.0, but is very close:
Or you can look at results published by the anchor manufacturers. This is from Rocna, showing the anchor manufacturer expects a 20kg anchor to have double the holding power of the 10kg model. (The 44 lb Rocna model holds 1122 lbs in this substrate the 22 lb model hold 1/2 of this or 561 lbs)
Holding
Power(lbs) .......Weight (lbs)
229 .................... 9
331 .....................13
561 .....................22
841 .....................33
1122 ...................44
1402 ...................55
1861 ...................73
2244 ...................88
3085 ...................121
3927 ...................154
6196 ...................243
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Neeves
Well-known member
The evidence points to holding power being roughly proportional to anchor weight (if we compare anchors of the same design and material).
You said exactly the same thing in one of your recent magazine articles on anchoring, so I am not sure what has lead to the change of heart:
"A final argument could be suggested that bigger anchors will work better. This is valid and roughly twice the weight will produce twice the hold" (Jonathan Neeves Cruising Helmsmen magazine November 2015).
For experimental results you can look at the work done by Prof Knox. His conclusions are:
"The maximum holding of an anchor, as recommended by manufacturers, is proportional to its weight. This is precisely what I found by direct experiment" (Prof Knox PBO magazine August 2002).
You can see for both the tested Delta and Bruce, as you double the weight you double the hold:
Or you can look at the results from the Vryhof tests. These are for very large anchors. Their results indicate doubling the anchor size increases the hold by 92%. Their formula for hold verses anchor weight is:
Ultimate holding capacity= A*(W)0.92 where W is weight and A is constant based on the anchor design and substrate. In graph form it looks like this. The slope is not quite 1.0, but is very close:
Or you can look at results published by the anchor manufacturers. This is from Rocna, showing the anchor manufacturer expects a 20kg anchor to have double the holding power of the 10kg model. (The 44 lb Rocna model holds 1122 lbs in this substrate the 22 lb model hold 1/2 of this or 561 lbs)
Holding
Power(lbs) .......Weight (lbs)
229 .................... 9
331 .....................13
561 .....................22
841 .....................33
1122 ...................44
1402 ...................55
1861 ...................73
2244 ...................88
3085 ...................121
3927 ...................154
6196 ...................243
John Knox says in his article where he looks at anchor weight vs performance that he used different rodes for different anchors and different weights, but did not define what he used with which. So some are chain and I think at the other extreme some are nylon. If you can re-interpret his results - please feel free. Interestingly Knox is suggesting a Delta has the same hold as a Rocna, of the same size, comparing your 2 sets of data.
The Rocna data you quote, I wonder if you can define the source and how the data was collected. I know of no-one who has ever tested and reported on testing a 100kg leisure anchor. The other odd factor is the holding capacity being quoted is not something to be too proud of, without further data. A 15kg anchor with a holding capacity of about 400kg would not hold our catamaran in 30 knots - not the sort of anchor nor data I would wish to rely on. This is in complete contrast to data from most other tests, for example Sail, where a 15kg Rocna will return holding capacities of 2,000kgs. I accept that different seabeds will return different holds - so I'm interested in who did the work, which seabed and why the data has not been widely aired.
The Vryhof data is of the best anchors around and they have an efficiency of about 0.93. These are anchors that have enjoyed massive investment and research yet you are suggesting that our anchors are actually better than oil rig anchors. So even though our anchors are compromised as they need ballast or extra thickness in the fluke and need to be designed to fit to a bow roller etc - they are more efficient than oil rig anchors. One would think on this basis the oil rig industry would simply make bigger anchors like ours - as you suggest ours are so much better. it would save millions in R&D. Based on the 0.93 efficiency, double weight and you will roughly double hold, but to be more precise you will increase hold by 1.86 times. If you treble weight you will increase hold by 2.79 times. If you take all of Fortress tests and they have tested most of their anchors multiple times in both sand and mud and the data is freely available, if you take the time to search it is possible to plot weight against hold. The relationship is an efficiency of 0.83 (vs 0.93 for the best of an oil rig anchor). This implies if you double weight then hold will increase 1.66 times or treble weight and hold increases 2.49 times. This is fairly constant for both mud and sand, but the base line for mud is lower. The oil rig anchor industry have tested Fortress and their data is in agreement with the analysis of data from Fortress and US Navy tests of the Fortress. Moreover the oil rig anchor industry think the Fortress a very efficient design. Like an oil rig anchor a Fortress is mostly fluke. The data is not available, or I have not seen it but the oil rig anchor industry put the Bruce at an efficiency of about .65, double weight and hold increases by about 1.3 times. Treble weight and hold about doubles. This latter data was in an early version of the Vryhof manual, about 10 years ago - I don't know if they still quote in the current version.
I'd put our modern anchors somewhere between a Bruce and a Fortress, say an efficiency of 0.75. This suggests double weight and hold increases about 1.5 times and to double hold you will need to almost treble weight. Some of 'our' anchors might be slightly more efficient than others, the Super SARCA and Danforth are 'mostly' fluke and they might be slightly better than say an Excel or Supreme - but I don't think any of us will tell the difference.
Jonathan
Why the change of heart. I found the focus on weight in conflict with the idea that surface area was critical, so I asked around (over some considerable time). Spade made a particular point of stressing surface area (as have Rocna) - I took the time to collate all the data from Fortress tests from the early 90's, I spoke with Vryhof R&D, I searched published data from Vryhof, Bruce and those universities that specialise in drag impediment anchors, for example Southampton (UK), Houston, Perth (WA Oz) and I talked with John Knox - and modified my views.
J
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noelex
Well-known member
No. It is important to understand that the rule of thumb that doubling the anchor weight doubles the hold is a rule that is only applicable when comparing the same anchor mode made of the same material.
Prof Knox tested the Delta and the Rocna (among others)and reported the results in the August 2011of PBO.
The Rocna did considerably better than the Delta as you would expect. The Rocna had over double the holding capacity (2.6x and 2.7x) when compared to the equivalent Delta.
Incidentally in this 2011 test he also tested several sizes of anchors with multiple pulls.
There were two sizes of Spade, three sizes of Delta, two sizes of Rocna, two sizes of Manson Supreme and two sizes of CQR.
All these anchors exceeded the rule of thumb that doubling the anchor size doubles the anchors holding capacity. Suggesting that if anything this rule is conservative.
Of all the anchors tested only the Bruce did (very slightly) worse than this rule predicted.
The entire PBO article can be downloaded here:
http://www.spade-anchor.com/IMG/pdf/2011_-_Test_et_Comparatif_Ancres_-_PRACTICAL_BOAT_OWNER.pdf
Neeves
Well-known member
John's work on comparing anchor weights, of the same design, goes back years. I have had his original and full data for some time now.
As I say - the suggestion is that leisure anchors are more efficient than oil rig anchors, I find that very difficult to believe.. Actually I do not believe it at all. Nor do I believe that a Fortress is not more efficient than a Spade, with its ballast pocket, nor Rocna with its roll bar and beefed up toe etc. I also do not believe that the Bruce developed in 1970 has an efficiency of 1.0 yet anchors developed and honed over the last 45 years only have an efficiency of 0.93. Bruce anchors are still used in the oil rig industry but they are considered to be grossly inefficient today. I also find it difficult to accept the idea the Vryhof's efficiency data on Bruce is wrong and that of John's correct and based on better testing.
It needs to be underlined that a bigger anchor will have more ultimate hold and that increased ultimate hold is related to size and weight. The hold developed is a function of windage (and as its the same yacht) and the hold will be identical for both anchors (large or small) under the same wind conditions, no more no less. The only time the 'extra' hold is advantageous is when the ultimate hold of the smaller anchor is exceeded. In a previous thread those that bought the recommended sized anchors and those few of us who bought slightly smaller made no report of our modern anchors dragging suggesting they had never been tested to their ultimate hold - and thus had we had bigger anchors the 'extra' hold would never have been 'used'.
There seems to a concept that somehow a bigger anchor magically is safer, magically develops more hold - this is a fallacy. The tension in the rode is a function of the yacht and wind - and its the tension that results in hold. The idea it allows you to use a shorter scope is interesting as all the video I see at short scope would discourage me from ever using the idea - possibly your recommendations are based on hard won experience - in which case I wish you would share it.
However this is thread drift.
If you wish to commence a thread on holding capacity of anchors - feel free. If you wish to describe in detail your experiences in using a bigger anchor in tight anchorages at short scope - I, for one, will be very interested. Personally I find it dangerous that people make fairly controversial suggestions without fully testing the practice themselves.
This thread is querying why if the owner buys a bigger anchor (and I make the assumption he buys this because he is going to achieve better hold) does he not buy a stronger chain (not necessarily bigger but stronger). One might extend this to wonder why proponents of the advice of buying the biggest anchor you can carry do not mention chain strength at the same time. To me buying an anchor with a potential for, say ,25% more hold should raise a query as to whether the chain is still sufficiently strong and doubling the hold raises much bigger questions. Either people simply do not think about it - which raises some questions about the credibility of the proponents (who ought at least make mention) - or possibly that current chain 'practice' is factorially too strong.
Jonathan
As I say - the suggestion is that leisure anchors are more efficient than oil rig anchors, I find that very difficult to believe.. Actually I do not believe it at all. Nor do I believe that a Fortress is not more efficient than a Spade, with its ballast pocket, nor Rocna with its roll bar and beefed up toe etc. I also do not believe that the Bruce developed in 1970 has an efficiency of 1.0 yet anchors developed and honed over the last 45 years only have an efficiency of 0.93. Bruce anchors are still used in the oil rig industry but they are considered to be grossly inefficient today. I also find it difficult to accept the idea the Vryhof's efficiency data on Bruce is wrong and that of John's correct and based on better testing.
It needs to be underlined that a bigger anchor will have more ultimate hold and that increased ultimate hold is related to size and weight. The hold developed is a function of windage (and as its the same yacht) and the hold will be identical for both anchors (large or small) under the same wind conditions, no more no less. The only time the 'extra' hold is advantageous is when the ultimate hold of the smaller anchor is exceeded. In a previous thread those that bought the recommended sized anchors and those few of us who bought slightly smaller made no report of our modern anchors dragging suggesting they had never been tested to their ultimate hold - and thus had we had bigger anchors the 'extra' hold would never have been 'used'.
There seems to a concept that somehow a bigger anchor magically is safer, magically develops more hold - this is a fallacy. The tension in the rode is a function of the yacht and wind - and its the tension that results in hold. The idea it allows you to use a shorter scope is interesting as all the video I see at short scope would discourage me from ever using the idea - possibly your recommendations are based on hard won experience - in which case I wish you would share it.
However this is thread drift.
If you wish to commence a thread on holding capacity of anchors - feel free. If you wish to describe in detail your experiences in using a bigger anchor in tight anchorages at short scope - I, for one, will be very interested. Personally I find it dangerous that people make fairly controversial suggestions without fully testing the practice themselves.
This thread is querying why if the owner buys a bigger anchor (and I make the assumption he buys this because he is going to achieve better hold) does he not buy a stronger chain (not necessarily bigger but stronger). One might extend this to wonder why proponents of the advice of buying the biggest anchor you can carry do not mention chain strength at the same time. To me buying an anchor with a potential for, say ,25% more hold should raise a query as to whether the chain is still sufficiently strong and doubling the hold raises much bigger questions. Either people simply do not think about it - which raises some questions about the credibility of the proponents (who ought at least make mention) - or possibly that current chain 'practice' is factorially too strong.
Jonathan
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noelex
Well-known member
Anchor efficiency is a completely different concept. An anchor's efficiency is its holding power divided by its weight.
An anchor can have very low efficiency but still follow the rule of thumb that its holding power is roughly proportional to its weight.
So a 10kg anchor that has an ultimate holding capacity of 100 kg in a particular substrate is less efficient than an alternative design 10kg anchor that can hold 300kg, but if we increase both anchor sizes to 20 kg and measure holding powers of 200 kg and 600 kg respectively then both anchors (in this case) are following the rough rule of thumb of doubling their holding power when the weight is doubled, despite very different efficiency.
It is not magic. A bigger anchor has a higher ultimate holding capacity (assuming the same substrate as well as anchor design and construction material) than a smaller model.
I am very surprised anyone would argue against such a simple and self evident concept.
Neeves
Well-known member
Possibly you might like to discuss the thread and not semantics?
Jonathan
Jonathan
