Dragging of anchors

[Neeves]

No idea, but that is what happens out in the real world, according to people in Holland who build anchors.

Jonathan

 

[Neeves]

Holding capacity is directly related to the shear stress of the soil and weight will be an advantage when shearing the top layer to allow tip penetration. However, If the surface area of the tip is too large then anchor will not shear the soil similarly if it is too light, the titanium anchor example, perhaps. Thereafter, once penetration starts area matters to spread the force on the soil over an area such that Force (pull on anchor) divided by area is less than the soil shear stress = Force to shear the soil divided by unit area sheared of the soil. This is why heavy CQRs fail because their area, at right angles to the shear direction, is relatively small. Thats how I think about it and why I like my Knox. However, I am known to get principles wrong, so quite happy to be educated.

Thanks for joining in BOB, good to have a Knox owner on board.

Surely its a function of weight and sharpness, you could have more weight and a blunter toe of less weight and sharper toe - for the same effect? Most modern anchors have very sharp toes in comparison to their pre-modern elder relatives.

Any observation on your Knox, any weaknesses you might care to mention.

Jonathan

 

[stranded]

Ooohhh my head hurts. Wonder if it might be possible to get views on what combination of anchors we should carry.

We are 44 fin keel mono, probably 13 tons cruising weight. We have a big Bugel on 80m of 10mm chain on the bow, in which we have great faith, a big fortress on 10m of 12mm chain plus a heap of nylon in the stern locker, and a 45lb CQR in the garden. We plan to head to the med then further. We need to ration our 'just in case' kit. I also find manhandling the CQR a pain. Going from most of the comments here about the limitations of CQR, I am inclined to leave ours in the garden. Would that be unwise?

 

[Neeves]

I don't think you'll get very far with that if you insist on mentioning no names. QUOTE]

I'll leave it to the individual to decide how to overcome the dilemma you pose. They are a skilled group and I suspect most would prefer a thread without rancour so I am sure they will devise solution that suits.

Jonathan

 

[Neeves]

Ooohhh my head hurts. Wonder if it might be possible to get views on what combination of anchors we should carry.

We are 44 fin keel mono, probably 13 tons cruising weight. We have a big Bugel on 80m of 10mm chain on the bow, in which we have great faith, a big fortress on 10m of 12mm chain plus a heap of nylon in the stern locker, and a 45lb CQR in the garden. We plan to head to the med then further. We need to ration our 'just in case' kit. I also find manhandling the CQR a pain. Going from most of the comments here about the limitations of CQR, I am inclined to leave ours in the garden. Would that be unwise?

You do not mention what size your Bugel is, I will guarantee my idea of 'big' and your idea of 'big' are very different. But that 10m of 12mm chain for the Fortress looks very heavy, I cannot imagine how you manage it - so I hope you have good health cover, or are a weight lifter.

Jonathan

 

[stranded]

You do not mention what size your Bugel is, I will guarantee my idea of 'big' and your idea of 'big' are very different. But that 10m of 12mm chain for the Fortress looks very heavy, I cannot imagine how you manage it - so I hope you have good health cover, or are a weight lifter.

Jonathan

Sorry - I was being lazy - the Bugel is 25kg. Yes, the 12mm chain was what we inherited - plan to change to 10mm. Its not too bad to handle though as it lives in a locker in the swim platform steps so it is just lift anchor, turn and drop - no need to lift chain, just guide it.

Must admit I quite miss my formative sailing years which was almost invariably on small Sunsails in the Med without windlasses. Had shoulders like a fighting bull at the end of two weeks in 10 - 20m anchorages!

 

[Neeves]

Sorry - I was being lazy - the Bugel is 25kg. Yes, the 12mm chain was what we inherited - plan to change to 10mm. Its not too bad to handle though as it lives in a locker in the swim platform steps so it is just lift anchor, turn and drop - no need to lift chain, just guide it.

Must admit I quite miss my formative sailing years which was almost invariably on small Sunsails in the Med without windlasses. Had shoulders like a fighting bull at the end of two weeks in 10 - 20m anchorages!

The concept of this thread was to try to identify if modern anchors had met their owners expectations (the idea being that people who had moved from a pre-modern anchor to a modern anchor might like to share their conclusions and specifically did their modern anchor drag). As anchor threads often, always, degenerate into slanging matches of 'my anchor is better than you anchor' I tried to keep brand names out of the thread to try to keep harmony. Basically harmony has been possible, with a few exceptions. Many have quoted brand names anyway -without rancour.

I'd have a read through the posts, some of which have been quite illuminating, and maybe draw up some conclusions and post what you have distilled. Members might then comment.

In the meantime you might get comment anyway.

Jonathan

 

[JumbleDuck]

Holding capacity is directly related to the shear stress of the soil and weight will be an advantage when shearing the top layer to allow tip penetration. However, If the surface area of the tip is too large then anchor will not shear the soil similarly if it is too light, the titanium anchor example, perhaps.

The weight of the anchor is trivial compared to the pull on the cable, as the effectiveness of Fortresses shows.

Thereafter, once penetration starts area matters to spread the force on the soil over an area such that Force (pull on anchor) divided by area is less than the soil shear stress = Force to shear the soil divided by unit area sheared of the soil. This is why heavy CQRs fail because their area, at right angles to the shear direction, is relatively small. Thats how I think about it and why I like my Knox. However, I am known to get principles wrong, so quite happy to be educated.

In haste - about to go sailing - but it's more complicated than that. The shearing takes place in at the interfaces between blocks of soil moved by the anchor, not just at the surface of the anchor. In general, though, the more you shear the bigger the force needed.

 

[Neeves]

JD - enjoy the sail and the weekend.

J

 

[geem]

Ooohhh my head hurts. Wonder if it might be possible to get views on what combination of anchors we should carry.

We are 44 fin keel mono, probably 13 tons cruising weight. We have a big Bugel on 80m of 10mm chain on the bow, in which we have great faith, a big fortress on 10m of 12mm chain plus a heap of nylon in the stern locker, and a 45lb CQR in the garden. We plan to head to the med then further. We need to ration our 'just in case' kit. I also find manhandling the CQR a pain. Going from most of the comments here about the limitations of CQR, I am inclined to leave ours in the garden. Would that be unwise?

We are also a 44 fin mono but we weigh 19tons fully loaded. We are currently in the Caribbean. We have 70m of 10mm chain with a 30kg Spade anchor. In addition we have two large aluminium anchors. We don't use any chain on the aluminium anchors due to difficulty of handing them and because in my experience the chain is unnecessary. We do carry lengths of chain for deployment on the anchors but we have never used them. If the second anchor is easy to set you will do it. We are in the habit of laying a second anchor when ever we feel the need to do so. if it was a pain to do we would, I am sure, do it less often. We left our CQR at home. Iam taking to scrap man tomorrow to weigh in along with the Admiralty pattern anchor that was originally supplied as the kedge with out boat in 1980!!

 

[stranded]

Thanks Geem, that's really helpful - especially that you.left your CQR at home - must confess that hinge looks like an accident waiting to happen to my fingers.

 

[Neeves]

Thanks Geem, that's really helpful - especially that you.left your CQR at home - must confess that hinge looks like an accident waiting to happen to my fingers.

If you read back you will find that Geem sometimes sets his alloy anchors to support his primary and might/can/does do this from a dinghy. It would be difficult, to say the least, and dangerous to try to deploy 12mm chain + anchor (even an alloy) from a dinghy. It must be difficult to lower 12mm chain from the deck - without risk of damage to the hull. We do the same thing (alloy in a 'V'), but do use chain, but its only 10m x 8mm and will be replaced with 6mm high tensile chain in the near future (38' x 7t cat). As Geem says - if its difficult or dangerous you would not do it - if its easy, you do do it. We can also deploy the second anchor in the 'V' by motoring forward, but try to do it from the dinghy, its easier to get the anchor in the right place.

Jonathan

 

[Delfin]

Correct, in theory (see below) - but not on the basis that 2 times weight equals 2 times hold.

In fact there is some data that suggests some anchors of a design with which we would be familiar - if you double weight then hold will increase by a factor of 1.3 times. I think this is worst case. But none, not even the best oil rig anchors, achieve a factor of 2.

In practice the plate from which bigger anchors are made, because the stresses increase more than the scaling of size, are larger than scaling would require. So though the external dimensions might accurately reflect scaling the plate used being thicker results in a heavier anchor (as a function of both volume and surface area). Additionally plate comes in standard thicknesses and if the size of plate required lies between 2 standard plate thicknesses responsible manufacturers will use bigger plate. The alternative is to use higher tensile steel - and then scaling can be accurate - but that does not appear to happen in our sector of the industry (you might find it with oil rig anchors). Finally some components of anchors are critical, in terms of strength, for example the shank. Increased thickness of plate in the shank (to maintain adequate strength) can be greater than the need to increase weight of plate in the fluke - but to maintain balance - plate can be unnecessarily increased in the fluke, not for strength but to maintain balance (or if in the design, more lead). (A heavy shank would reduce the ability of the toe to engage quickly).

You can see some of this in chandlers. If you have a chandler with a good range of one design. Lay them all alongside each other and look at the shanks. You will find that 2 or 3 sizes of anchor, from memory 20kg, 25kg and 33kg Rocnas, for example, will use the same thickness of plate for the shank, but the shanks otherwise have the same proportions. Of those anchors with the same thickness of plate, some might be too light and some too heavy (strong) - this is all because there is not the correct steel plate sizes being made (it should have nothing to do with economy, though it might) - and the anchor maker had to make compromises (and add or reduce weight to keep the balance right). The gaps in the size ranges of plate can be surprisingly large.

To add to the complexity smaller anchors, certainly 5kg anchors, tend to be over engineered, they are more susceptible to misuse (they are easy to overload). To return to Noelex posting of John Knox data - I am sure the data is totally accurate, I have considerable respect for John and his meticulous work (and I happen to have the original data for the tests published in PBO). However it is very likely the small anchors he used were not scaled versions of the 16kg anchors he also used and to extrapolate that data, or even to use intermediate points, would not be sensible.

Consequently to think a doubling of weight would double hold is sadly erroneous for 2 reasons - anchors are not that efficient, a different subject (and some are very inefficient) and secondly because weight increases more than area/volume (the weight. balance, safety factors) as anchors are scaled..

If weight were critical, or it was the parameter to provide maximum hold - we would not have alloy anchors. If gravity were a critical holding capacity control - we would not have alloy anchors with the same holding characteristics as their steel counterparts. Equally oil rig anchors would be made from much heavier plate, rather than HT steels. If surface area were not important then lightweight alloy anchors with high fluke areas - like the Fortress - would not be producing such exceptional results in virtually every test conducted - and ignoring test results (for those who question them) no-one would be carrying them as storm anchors and everyone would have at least a heavy Danforth.

Jonathan

Can you think of an example where a heavier anchor has less holding power than a lighter doppelganger?

 

[Neeves]

Can you think of an example where a heavier anchor has less holding power than a lighter doppelganger?

There are few identical copies but if we work on the heavier one being 1.5 times the weight of the alloy, a heavier well made Danforth type will have less hold than a Fortress, a stainless Ultra or Vulcan will have less hold than an alloy Spade etc. Or for a real compariosn a steel Spade and an alloy Spade, or steel Excel and an alloy Excel - the alloy at half the weight has the same holding capacity for the same size, in terms of surface area or volume, as the steel model.

Jonthan

 

[Neeves]

Jonathan, I don't think this is accurate. It certainly does not sound intuitively correct that to double the holding power of a 25kg anchor you need to increase to a 70kg (or more) model of the same design.

I think the evidence supports that holding power for any single anchor design made of the same material is (roughly) proportional to its weight.

Professor Knox noted that Vryhof (who make large oil rig anchors) concluded that ultimate holding capacity was proportional to weight for anchors from 1 to 50 tonnes.

/QUOTE]

Noelex,

I thought it polite to specifically answer your post.

There is a relationship between weight and performance but because no anchor is 100% efficient it is sadly not possible to build an anchor such that if you double weight you double hold. The best that is possible, from the oil industry, is an efficiency of 0.9. A Fortress is about 0.83 and many anchors with which we are familiar have an efficiency of 0.66. Basically, of these latter, if you treble weight you will not even double hold. I suspect our anchors are around the 0.7 efficiency which effectively means if you want to double hold you will need to almost treble weight. So if you want to double the hold of a 25kg Rocna (or any other design - excluding Fortress) you will need their 70kg model.

Much of the efficiency data is on the Vryhof website, or its on an old site that I have a copy of. I have noted that as they update their anchor manual they edit out information that would be useful to us (though possibly not of interest to someone looking at oil rig anchors). From personal correspondence I have some data on 'our' anchors, but it is limited (it does not cover many models). There is some data on the Fortress Chesapeake mud tests but you may need to compute the efficiency yourself.

The fact that the oil industry rate their anchors as 0.9 efficient does not mean they are any use to us - in a smaller version. Oil rig anchors do not need to veer, they do not need to perform in a change of tide, they do not need to sit on a bow roller. The efficiency is simply a straight line pull - but it does undermine the idea that doubling weight will double holding capacity.

The only way I can think to double hold, by doubling weight, would be to deploy 2 anchors in a "Y', with a longish single line central rode and a shortish 'V' at the end. You then double hold. Its not convenient, I do not know of anyone doing it, I do not even knoe if it would work - but the nearest common practice is to deploy in a 'V'.

Jonathan

 

[Delfin]

There are few identical copies but if we work on the heavier one being 1.5 times the weight of the alloy, a heavier well made Danforth type will have less hold than a Fortress, a stainless Ultra or Vulcan will have less hold than an alloy Spade etc. Or for a real compariosn a steel Spade and an alloy Spade, or steel Excel and an alloy Excel - the alloy at half the weight has the same holding capacity for the same size, in terms of surface area or volume, as the steel model.

Jonthan

That actually doesn't address the question. I was asking whether there are any examples where two anchors of the same design would show the heavier to have lower holding power than the lighter.

 

[Neeves]

That actually doesn't address the question. I was asking whether there are any examples where two anchors of the same design would show the heavier to have lower holding power than the lighter.

Sorry, I misunderstood.

There are only 2 anchor designs of which I am aware that come in alloy and steel - Spade and Excel. Fortress are not identical to a Danforth and I do not know if the Racer is identical to the Manson interpretation of the Danforth. From this small population base - no steel version, identical to the alloy version, has a lower holding capacity.

Jonathan

edit, Post #384 appears to have floated around in the ether and then been introduced into the thread - people may have missed it. I did not see it when I looked 12 hours ago - but then I miss lots of things. close edit

 

[Delfin]

Sorry, I misunderstood.

There are only 2 anchor designs of which I am aware that come in alloy and steel - Spade and Excel. Fortress are not identical to a Danforth and I do not know if the Racer is identical to the Manson interpretation of the Danforth. From this small population base - no steel version, identical to the alloy version, has a lower holding capacity.

Jonathan

edit, Post #384 appears to have floated around in the ether and then been introduced into the thread - people may have missed it. I did not see it when I looked 12 hours ago - but then I miss lots of things. close edit

No problem, and I agree with you. Heavier of the same design cannot have lower holding capacity that lighter of the same design. Which begs the question, if two anchors are identical in every way other than weight, is it plausible that the heavier anchor would set less quickly than the lighter? Can't think so, which in a nutshell is the argument for bigger is better. A Fortress made of depleted uranium will set quicker than the same Fortress in aluminum. Gravity dictates that outcome. Having said that, the aluminum Fortress is designed so it doesn't require high weight to achieve high holding even if it might benefit from the weight, but for every other example I can think of, if you add more mass without a proportional increase in surface area, the anchor with more weight per square inch of surface area will set quicker and hold better than the same design that is lighter and has a lower weight per square inch of surface area. Physics dictates this outcome.

 

[Robin]

No problem, and I agree with you. Heavier of the same design cannot have lower holding capacity that lighter of the same design. Which begs the question, if two anchors are identical in every way other than weight, is it plausible that the heavier anchor would set less quickly than the lighter? Can't think so, which in a nutshell is the argument for bigger is better. A Fortress made of depleted uranium will set quicker than the same Fortress in aluminum. Gravity dictates that outcome. Having said that, the aluminum Fortress is designed so it doesn't require high weight to achieve high holding even if it might benefit from the weight, but for every other example I can think of, if you add more mass without a proportional increase in surface area, the anchor with more weight per square inch of surface area will set quicker and hold better than the same design that is lighter and has a lower weight per square inch of surface area. Physics dictates this outcome.

On the use of a Fortress, we have about 20ft of 3/8 or 10mm S/S chain on ours then 150ft of nylon rode. It almost seems a contradiction of ideas to add the chain and it's weight to the lightweight Fortress, but then it may reduce chafe from a rough bottom as well as improve the initial angle of 'tug' when setting albeit that our not very large FX-11 Fortress is only carried as a spare and kedge/come occasional stern anchor and not as a storm anchor or even for setting in a Bahamian moor' two off the bow.

 

[Neeves]

No problem, and I agree with you. Heavier of the same design cannot have lower holding capacity that lighter of the same design. Which begs the question, if two anchors are identical in every way other than weight, is it plausible that the heavier anchor would set less quickly than the lighter? Can't think so, which in a nutshell is the argument for bigger is better. A Fortress made of depleted uranium will set quicker than the same Fortress in aluminum. Gravity dictates that outcome. Having said that, the aluminum Fortress is designed so it doesn't require high weight to achieve high holding even if it might benefit from the weight, but for every other example I can think of, if you add more mass without a proportional increase in surface area, the anchor with more weight per square inch of surface area will set quicker and hold better than the same design that is lighter and has a lower weight per square inch of surface area. Physics dictates this outcome.

Once an anchor engages then weight should not be relevant. There are issues with very light and/or very small anchors engaging in hard seabeds - but once they engage the anchors then dive. One way to overcome this is to have a very sharp toe - which is, I believe, what Mantus have done with their dinghy anchor. The toe is so sharp there is a danger if you are careless to puncture a rib. They appear to have accepted dinghy anchors need to be light and to ensure the anchor engages have sharpened up the toe. Fortress also say something similar of their Guardian. They suggest because the toe is not so sharp and the fluke edges not bevelled that it lascks the same efficiency to slice into harder substrates.

If weight were that advantageous we would have heavy oil rig anchors. The heavy ones, like the old cast Bruce, are very 20th Century. And the old cast or drop forged Danfroths are very inefficient as anchors, the light better engineered modern interpretation is more efficient.

Jonathan