Dragging of anchors

[JumbleDuck]

There is no contradiction. Surface area is proportional to weight,

It's not. In general, surface area is proportional to weight^2/3, because weight and volume scale with size³ while surface area scale with size².

That, of course, assumes that larger anchors are simply scaled up versions of smaller ones. That may be the case for Danforths and Bruces, to my eye, but CQRs, for example, get proportionally much bigger breasts (that seems to be the word for the sides of a plough) as they get larger.

 

[JumbleDuck]

But if they do not move, they cannot compact nor dive - so they need extra load (say from the wind) to generate that extra holding capacity -and when that extra load is applied - they move, they need to move because the previous compaction was only sufficient to hold 300kg. The movement might only be centimeters - but they move.

I think you are conflating two aspect of solid mechanics here. It may be that movement of the anchor reconfigures the substrate in which it is embedded or it may be that it simply compresses the existing configuration a bit. The former, if it happens, may be significant. The latter isn't, unless you worry about the anchor stretching as it's loaded.

In any case, systems like this rarely behave in a linear force-displacement way. It's much more common for there to be jerks, because of non-linearities in material behaviour. Anchor sets, load increases, everything stays as it is, shearing planes get established, anchor moves easily, substrate reconfigures, anchor stops moving, shear planes disappear, anchor stays put until even higher load establishes new shear planes.

A friend of mine who researches the solid mechanics of subsea engineering has promised me some references to works on the solid mechanics of small anchors. I must chase him up on that.

I see no issue, except I think it unnecessary, with going up one size - its neither here nor there. But going up a size, and thinking that the increase in weight reflects a commensurate increase in performance is wrong.

There may, I grant you, be issues with getting a large anchor to set. As long as it sets, though, I cannot imagine any circumstances under which a large anchor would let go before a small one of the same type.

Incidentally, Fortress anchors were The New Thing, years ago, various tests of them were published. They did very well indeed against Danforths, but that was because the comparisons were always weight-based. If you compared them by physical size, performance was very similar. The advantages of a Fortress therefore seem to be (a) that you can have a very much more effective main anchor for the same weight and (b) you can have a very much lighter kedge anchor for the same size.

A titanium spade could be fun.

 

[Neeves]

It's not. In general, surface area is proportional to weight^2/3, because weight and volume scale with size³ while surface area scale with size².

That, of course, assumes that larger anchors are simply scaled up versions of smaller ones. That may be the case for Danforths and Bruces, to my eye, but CQRs, for example, get proportionally much bigger breasts (that seems to be the word for the sides of a plough) as they get larger.

I should not have used the word proportional - I should have said, maybe, surface area is a function of weight.

Jonathan

 

[Fortress Anchors]

Incidentally, Fortress anchors were The New Thing, years ago, various tests of them were published. They did very well indeed against Danforths, but that was because the comparisons were always weight-based. If you compared them by physical size, performance was very similar. The advantages of a Fortress therefore seem to be (a) that you can have a very much more effective main anchor for the same weight and (b) you can have a very much lighter kedge anchor for the same size.

JumbleDuck,

Tests between Danforth and Fortress have typically compared anchors of similar physical size, and not similar weight. As examples, in 1989 the US Navy tested a 47 lb Fortress FX-85 along with a 90 lb Danforth, and they were almost identical in size. The FX-85 achieved a pull of 10,200 lbs in a sand / clay bottom when the test was stopped, while the 90 lb Danforth pulled out at 9,000 lbs due to structural damage.

During the more recent Chesapeake Bay testing, the 21 lb FX-37 and 35 lb Danforth, which are also close in physical size, both achieved comparable performance when the FX-37 was set at the 32° angle, while at the 45° angle for soft mud, the FX-37 was significantly higher.

 

[Neeves]

JD

There might be jerks - but when you set an anchor any jerks are 'lost'. Even if you test with a winch and load cell, there maybe jerks - but that can be the anchor crushing oyster shells. Commonly the winch is powerfull and I suspect the jerks are lost in the running averages of the data. I think our anchors do reconfigure the substrate, they would have to in order to squirrel their way through the seabed, and it is possible to see the seabed 'moving' when the anchor is well buried but moves under load.

If there is data on soil mechanics with relation to small anchors it is very likely focussed at the oil industry. Oil rig anchors are considerably more efficient than ours so scaled down versions might not accurately reflect what we have. There has been a lot of work conducted in Universities in Houston (unsurprisingly) and Perth, here in Australia.

I was going to quote a Fortress/Danforth comparison but hesitated because the Fortress is not an exact copy of a Danforth. I do not even know if they are have equal sizes. The other problem is that Fortress have sharpened up the fluke edges and altered/improved the shank profile making it appear much more efficient. Some results I have seen show the Fortress to be better (rather than similar) for 'similar' sizes in comparison to the Danforth. The other problem is that Tie Down, who make the Danforth, make a number of versions of the Danforth and when results are quoted no-one differentiate between the various models. The other advantage quoted for a Fortress is that it disassembles - but an anchor in pieces does not seem much use to me - and probably languishes unloved in a corner, better assembled. edit I have a suspicion Fortress suggest their model sets more quickly than a Danforth - but I do recall seeing any data on that, close edit

But the comparison is there - a Fortress weighs about half that of a Danforth of the same size but has a similar holding capacity - (not as close in design as the Excel and Spade) but it is surface area that is dictating hold, not weight.

Given the costs people will pay for a stainless anchor and your belief that people are motivated by fashion then maybe your idea of a titanium Spade is not so ludicrous.

I missed the introduction of the Fortress, not quite sure why, as we campaigned a racing yacht (in Hong Kong) at around that time and I would have dropped the 2 CQRs in a hurry had I known. But there again I kept the CQRs and we still have them - slightly forlorn now.

Jonathan

 

[JumbleDuck]

I should not have used the word proportional - I should have said, maybe, surface area is a function of weight.

The hold vs weight graph presented is straight, though.

 

[JumbleDuck]

JGiven the costs people will pay for a stainless anchor and your belief that people are motivated by fashion then maybe your idea of a titanium Spade is not so ludicrous.

I was serious! A titanium Spade with the same holding power as a steel CQR would be perhaps half the size, an eighth of the volume and a sixteenth of the weight. Titanium is much more expensive, of course ($20/kg vs $0.10/kg for steel) but since the material cost is a negligible part of the cost of an anchor, that shouldn;t matter too much.

 

[Delfin]

It's not. In general, surface area is proportional to weight^2/3, because weight and volume scale with size³ while surface area scale with size².

That, of course, assumes that larger anchors are simply scaled up versions of smaller ones. That may be the case for Danforths and Bruces, to my eye, but CQRs, for example, get proportionally much bigger breasts (that seems to be the word for the sides of a plough) as they get larger.

And because surface area goes up less than the increase in weight, the larger anchor is more likely to penetrate the subsurface than a lighter anchor. The easy way to understand this is to think about two identical anchors, one made of aluminum and one of depleted uranium. Under what circumstances would the aluminum anchor with precisely the same dimensions penetrate better than the heavier doppelganger? Answer: None, because earth's gravity exists.

Now, that doesn't mean that different anchor designs don't offer different balances of surface area and weight, the Fortress being a prime example. The Manson Ray has less surface area to weight than does the Rocna, which may explain why the Ray penetrated in Starzinger's beach test better than the Rocna. But take two Rays, one heavier and one lighter and the heavier will always penetrate better than the lighter. I just don't see any physical basis for thinking otherwise.

 

[Sybarite]

But the comparison is there - a Fortress weighs about half that of a Danforth of the same size but has a similar holding capacity - (not as close in design as the Excel and Spade) but it is surface area that is dictating hold, not weight.

A word of caution. As the tests have shown, lightweight anchors such as the Fortress hold virtually as well as the best of NG anchors - BUT they are often damaged in the test eg bent flukes and cannot be used again.

Apparently a very small distorsion ruins the anchor's ability to reset. (Not my findings but as indicated in a test.)

 

[JumbleDuck]

And because surface area goes up less than the increase in weight, the larger anchor is more likely to penetrate the subsurface than a lighter anchor. The easy way to understand this is to think about two identical anchors, one made of aluminum and one of depleted uranium. Under what circumstances would the aluminum anchor with precisely the same dimensions penetrate better than the heavier doppelganger? Answer: None, because earth's gravity exists.

I think the argument is that all anchors need to be pulled a bit to set, and that a small boat may not pull a big heavy anchor enough for it to set. It's not an impossible argument, but it's not an argument that would bother me much because if the wind and forces increase the anchor will presumably do its stuff.

 

[Delfin]

I think the argument is that all anchors need to be pulled a bit to set, and that a small boat may not pull a big heavy anchor enough for it to set. It's not an impossible argument, but it's not an argument that would bother me much because if the wind and forces increase the anchor will presumably do its stuff.

I think that is correct. I realize this is heresy to some, but I am not a big fan of backing down on anchors until they have settled for a while and already been 'nudged' into the seabed from the force of wind and/or tide. I have seen too many people just plowing a groove thinking they are helping to set the hook when if they just let the darn thing alone for a bit, they'd not waste their time, never mind the seabed.

And if one is a proponent of dropping the hook and then motoring around the bay in reverse, if you can't move the heavier anchor with the motor because it is so big, is this an indication that it's going to drag, or an indication the vessel's mass is going to be held quite nicely by the bigger hook?

 

[NormanS]

This has all got way over my head. I'm out!

 

[sailaboutvic]

This has all got way over my head. I'm out!

Norman , best to keep to the other anchor Thread, much easier to understand.

 

[Robin]

This has all got way over my head. I'm out!

As the CQR said to the chain pulling it's leg in shallow waters

 

[Neeves]

That is probably true but I have witnessed a very different situation when it comes to initial set. A 32 ft boat with a 20 kg Rocna had great difficulty getting the anchor into a slightly difficult seabed, hard sand and weed. This was a very experienced skipper, using adequate scope, who tried repeatedly to get his anchor in. It seemed that the boat could not pull sufficiently strongly for the size of anchor. We have anchored here many times, 34 ft with 15 kg Rocna, and never had a problem.

In terms of size, I go back to this description.

But without ignoring the debate I would still desire any comments on modern anchors dragging, or not. I'm not so fascinated with pre- modern anchors dragging - a reason why respondents bought their modern version is because their pre-modern did drag (I hoping the purchase was not made because anyone thought it fashionable) - so everyone has a story, or 2 to tell - but I suspect they are very similar. I'm still trying to identify where members might think anchor makers might direct their gaze were they to consider tweaking the design, or (do we really need another new anchor) looking at another model.

Jonathan

 

[Neeves]

I was serious! A titanium Spade with the same holding power as a steel CQR would be perhaps half the size, an eighth of the volume and a sixteenth of the weight. Titanium is much more expensive, of course ($20/kg vs $0.10/kg for steel) but since the material cost is a negligible part of the cost of an anchor, that shouldn;t matter too much.

And you would save on the galvanising costs!

edit So if you copied a 15kg Spade (and assume for this exercise it has no lead, as i do not know how much lead they use), then if we are only going to use a sixteenth part of the metal (and assume a little wastage) then the material costs are only $20 (as its only going to weigh, excluding the lead), 1kg. I have nasty suspicion you might need a bit more than one sixteeenth of the metal to achieve the strength - but your logic still looks interesting. But whether you will become rich in marketing your idea - I'm even less sure. Does titanium really come in plate at that sort of price?

close edit

Jonathan

 

[Neeves]

The hold vs weight graph presented is straight, though.

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

 

[JumbleDuck]

I'm still trying to identify where members might think anchor makers might direct their gaze were they to consider tweaking the design ...

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

Manson, you need to tweak your design

Why?

Someone says their anchor dragged?

Was it one of ours?

No idea, but it was a modern one.

Don't let the door hit your arse on your way out, will you.​

 

[JumbleDuck]

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.

Why would that be?

 

[Deleted member 36384]

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.