Dragging of anchors

[30boat]

On bigger anchors - I reiterate, no-one who has posted has made any mention that they wished they had bought a bigger anchor, though some did upsize when they bought.

I see the reasoning and logic behind a bigger anchor (and it is so convincing) but other than 'gut feel' I have seen no objective evidence that it is necessary. I also appreciate the comment 'no-one complains of having too big an anchor when it is blowing 50 knots' - but this is subjective - though it, importantly, might allow you to relax (how anyone relaxes when its blowing 50 knots - I'm not sure, maybe buy a bigger anchor??!!)

Even those members with a modern anchor of the size recommended for their yacht and those few who have gone 'undersize' have not declared that their anchor has dragged (as a result of being undersized). I would value any refinemment of member's experiences - ie a recommended or undersized modern anchor dragged and you wished to had bought a bigger model).

Currently I am of the harsh view that buying oversize, some buy 2 x the size, simply increases the pension fund of anchor makers (or more likely the pension fund of distributors) and might make you feel better.

Jonathan

In my opinion a bigger anchor may have one advantage.It'll be harder to shift by a light boat (relative to its size) once properly burried so will resist changes of direction better. Which brings me to the Danforth and Fortress anchors.How good are they when it comes to resetting after a change of direction of the pull? This is a genuine question,I honestly don't know and as we have a lot of mud around here I'm interested in the reply.

 

[Robin]

I upsized over the recommended not so much for the extra weight per se but the extra surface area and because a one size down same model on a previous boat had worked well thus considered we were really two sizes up. But temper this with the fact that he larger size was a) in stock and b) was on sale for the same price of the size recommended for our current boat. I could have bought two such Deltas for the price of one Spade ( expensive in the USA) which was my first thought but had never tried one of those whereas I had many years of satisfactory experience with the Delta in Europe.

 

[JumbleDuck]

On bigger anchors - I reiterate, no-one who has posted has made any mention that they wished they had bought a bigger anchor, though some did upsize when they bought.

Of course not. Nobody says that they wish they had bought a better hifi or a faster car either. That's people for you.

 

[Neeves]

In my opinion a bigger anchor may have one advantage.It'll be harder to shift by a light boat (relative to its size) once properly burried so will resist changes of direction better. Which brings me to the Danforth and Fortress anchors.How good are they when it comes to resetting after a change of direction of the pull? This is a genuine question,I honestly don't know and as we have a lot of mud around here I'm interested in the reply.

As I imply - I understand the argument for the bigger anchor - but I have yet to see objective, or quantitative reasoning.

Something to factor into the equation

For a given load a smaller anchor will set more deeply, being smaller it has less resistance to diving. It will have an identical 'holding capacity' at that point to a bigger one loaded in the same seabed with the same load. So in a straight line pull - both have been loaded to say 300kg - about the load generated by a 30hp diesel but the smaller one will be deeper. But smaller boats have smaller engines, usually, so if you have an overly big anchor its going to be difficult to set a big anchor deeply - unless you have wind (and when the wind comes it might be from a different, totally, direction).

If the wind increases and exerts loads of 500kg in the same straight line as the 300kg engine load (which is a pretty strong wind!) then the both anchors (big and small) will continue to dive. (A 15kg anchor will not reach maximum diving depth until maybe 2000kg load in a decent sand seabed).

Now what influences these anchors that might make them 'drag'? If they receive more load, more wind (in the same direction), they will dive more deeply - but the distance in doing so will be measured in cm, not metres - so not disastrous dragging. If they meet a foreign object, buried shell - they might drag - but both are as likely to meet a shell as the other??

The more deeply set anchor will have more chain buried and as the chain buries it is less susceptible to moving during veering - the chain itself when buried adds to holding capacity. So if you can suggest why the little one will drag, lose its holding capacity - remembering it is set more deeply and seabeds become more dense with depth (less water, more compaction) you then answer some of the questions.


Some evidence I have - a bit focussed but its all I have, or all I can think of currently:

In the Chesapeake mud tests Fortress set one of their anchors quite deeply (I think the shackle was at about 11' into the mud) with the fluke set at 45 degrees, this is their optimum for setting in mud. Most anchors have a fluke angle of around 32 degrees.

When Fortress came to retrieve the anchor, by pulling with the winch vertically, the retrieval load and the setting load was almost identical. It took some time and patience to retrieve the anchor. Basically once deeply set the anchor was immovable - it would not matter where the wind (which is commonly in snatch loads, not a sustained pull) came from it would not move.

I see a tendency for owners of Fortress to have 'big' anchors (much bigger in surface area than their main steel, bower, or primary anchor) - because they are light they think they should have, or can have, bigger and at the back of their mind they think, not unreasonably, that the Fortress will be their back up storm anchor. We are guilty of this - our FX 23 has a much bigger surface area than our primary steel anchor. People also complain that their Fortress trips too easily, I wonder if many Fortress are simply too big and/or are not set sufficiently deeply to maximise their effectiveness. Fortress had some stellar results with a smaller anchor rather than their bigger one - its worth looking at the tests. In our case - we would deploy our Fortress when we have a forecast advising wind direction with a high degree of confidence even though it might not be set deeply - any wind which will be in the same direction as the set direction will simply bury it more deeply - no veer, so not much chance of tripping because it is shallow set. Having looked at the depth of set we achieve with engine power - it would not surprise me if, should we have a 90 degree wind shift - if the Fortress did not trip - it is simply too shallow set with out engine power - maybe we should have had a smaller Fortress? as well as a bigger one???

I'm just flying balloons = shoot them down.

Jonathan

 

[Delfin]

As I imply - I understand the argument for the bigger anchor - but I have yet to see objective, or quantitative reasoning.

Something to factor into the equation

For a given load a smaller anchor will set more deeply, being smaller it has less resistance to diving. It will have an identical 'holding capacity' at that point to a bigger one loaded in the same seabed with the same load. So in a straight line pull - both have been loaded to say 300kg - about the load generated by a 30hp diesel but the smaller one will be deeper. But smaller boats have smaller engines, usually, so if you have an overly big anchor its going to be difficult to set a big anchor deeply - unless you have wind (and when the wind comes it might be from a different, totally, direction).

If the wind increases and exerts loads of 500kg in the same straight line as the 300kg engine load (which is a pretty strong wind!) then the both anchors (big and small) will continue to dive. (A 15kg anchor will not reach maximum diving depth until maybe 2000kg load in a decent sand seabed).

Now what influences these anchors that might make them 'drag'? If they receive more load, more wind (in the same direction), they will dive more deeply - but the distance in doing so will be measured in cm, not metres - so not disastrous dragging. If they meet a foreign object, buried shell - they might drag - but both are as likely to meet a shell as the other??

The more deeply set anchor will have more chain buried and as the chain buries it is less susceptible to moving during veering - the chain itself when buried adds to holding capacity. So if you can suggest why the little one will drag, lose its holding capacity - remembering it is set more deeply and seabeds become more dense with depth (less water, more compaction) you then answer some of the questions.


Some evidence I have - a bit focussed but its all I have, or all I can think of currently:

In the Chesapeake mud tests Fortress set one of their anchors quite deeply (I think the shackle was at about 11' into the mud) with the fluke set at 45 degrees, this is their optimum for setting in mud. Most anchors have a fluke angle of around 32 degrees.

When Fortress came to retrieve the anchor, by pulling with the winch vertically, the retrieval load and the setting load was almost identical. It took some time and patience to retrieve the anchor. Basically once deeply set the anchor was immovable - it would not matter where the wind (which is commonly in snatch loads, not a sustained pull) came from it would not move.

I see a tendency for owners of Fortress to have 'big' anchors (much bigger in surface area than their main steel, bower, or primary anchor) - because they are light they think they should have, or can have, bigger and at the back of their mind they think, not unreasonably, that the Fortress will be their back up storm anchor. We are guilty of this - our FX 23 has a much bigger surface area than our primary steel anchor. People also complain that their Fortress trips too easily, I wonder if many Fortress are simply too big and/or are not set sufficiently deeply to maximise their effectiveness. Fortress had some stellar results with a smaller anchor rather than their bigger one - its worth looking at the tests. In our case - we would deploy our Fortress when we have a forecast advising wind direction with a high degree of confidence even though it might not be set deeply - any wind which will be in the same direction as the set direction will simply bury it more deeply - no veer, so not much chance of tripping because it is shallow set. Having looked at the depth of set we achieve with engine power - it would not surprise me if, should we have a 90 degree wind shift - if the Fortress did not trip - it is simply too shallow set with out engine power - maybe we should have had a smaller Fortress? as well as a bigger one???

I'm just flying balloons = shoot them down.

Jonathan

I think the existence of gravity makes a pretty good argument for a heavier anchor, although clearly design can trump absolute weight, as Fortress demonstrates.

 

[Delfin]

I just have never understood the argument against carrying the largest anchor you can manage. Like Steve Dashew, I know my hook is in the ballpark when people walk by and I hear them say "holy shite, look at the size of the anchor." A smaller anchor likely won't drag either, until it does, and when it does, a heavier hook would be more likely to stay put.

 

[Neeves]

I just have never understood the argument against carrying the largest anchor you can manage. Like Steve Dashew, I know my hook is in the ballpark when people walk by and I hear them say "holy shite, look at the size of the anchor." A smaller anchor likely won't drag either, until it does, and when it does, a heavier hook would be more likely to stay put.

Theory, and there have been lots of work and it all comes to the same conclusion, that for a given design an increase in surface area will increase performance (not weight). It is not totally accurate in real life as increasing wight sometimes results in the need to use a thicker or thinner steel plate than theory demands - because that size of plate is not actually made. The fact that Rocna quote surface area suggests they also think it is a critical measure of performance.

I choose as an example a Rocna, as they quote surface area.

A 25kg Rocna has a surface area of 1415 sqcm. The next size up is a 33kg model with a 1695 sqcm fluke. So for a 32% increase in weight you produce a 14% increase in surface area. The increase in surface area results in a linear increase in holding capacity, double area and you have slightly less than double increase in hold. Load on the yacht is proportional to square of wind speed I find it difficult to believe anyone can tell the difference as a very small increase in wind speed will soon account for that increase in surface area (I'm sure someone can do the maths). To make a real difference you need to, say, double surface area - and then you might notice the difference.

To increase surface area, of the 25kg model, by 50% would need a 40kg anchor (1945cmsq) and to double surface area would need a 70 kg model (2690 cmsq).

An alternative would be have 2 x 25kg models - it would double surface area and would allow deployment of an acceptable 'light' anchor most of the time. For those, like us, with only one bow roller the second anchor, available to be deployed when the weather dictated, could be alloy (in fact you could have 2 alloy (+ a steel primary) and triple the available surface area and still only carry 45kg). You do need to be prepared to deploy the second anchor (in a 'V' or fork) but you do not need to have a larger than necessary (most of the time) anchor sitting on the bow roller. And because you have 2, or more, primary anchors you can optimise design to cater for different seabeds, say a fluke for thin mud (that will also work in sand) and convex/concave that will work in medium density weed and pebbly bottoms.

Jonathan

 

[geem]

Theory, and there have been lots of work and it all comes to the same conclusion, that for a given design an increase in surface area will increase performance (not weight). It is not totally accurate in real life as increasing wight sometimes results in the need to use a thicker or thinner steel plate than theory demands - because that size of plate is not actually made. The fact that Rocna quote surface area suggests they also think it is a critical measure of performance.

I choose as an example a Rocna, as they quote surface area.

A 25kg Rocna has a surface area of 1415 sqcm. The next size up is a 33kg model with a 1695 sqcm fluke. So for a 32% increase in weight you produce a 14% increase in surface area. The increase in surface area results in a linear increase in holding capacity, double area and you have slightly less than double increase in hold. Load on the yacht is proportional to square of wind speed I find it difficult to believe anyone can tell the difference as a very small increase in wind speed will soon account for that increase in surface area (I'm sure someone can do the maths). To make a real difference you need to, say, double surface area - and then you might notice the difference.

To increase surface area, of the 25kg model, by 50% would need a 40kg anchor (1945cmsq) and to double surface area would need a 70 kg model (2690 cmsq).

An alternative would be have 2 x 25kg models - it would double surface area and would allow deployment of an acceptable 'light' anchor most of the time. For those, like us, with only one bow roller the second anchor, available to be deployed when the weather dictated, could be alloy (in fact you could have 2 alloy (+ a steel primary) and triple the available surface area and still only carry 45kg). You do need to be prepared to deploy the second anchor (in a 'V' or fork) but you do not need to have a larger than necessary (most of the time) anchor sitting on the bow roller. And because you have 2, or more, primary anchors you can optimise design to cater for different seabeds, say a fluke for thin mud (that will also work in sand) and convex/concave that will work in medium density weed and pebbly bottoms.

Jonathan

Jonathan, this is exactly what we do. We have a Spade of the correct size for our yacht as determined by the manufacturer. We carry two lightweight aluminium anchors. The largest and heaviest of which we set in a fee with the main anchor. We set this second anchor on all rode. We use the rib to pull the yacht forward so we lie to the second anchor most of the time. As this anchor/rode is loaded, it then starts to pull on the main anchor/chain and we get great snubbing and very little movement even in very strong gusts. Works very well

 

[JumbleDuck]

For a given load a smaller anchor will set more deeply, being smaller it has less resistance to diving. It will have an identical 'holding capacity' at that point to a bigger one loaded in the same seabed with the same load.

I don't think that follows at all. I would expect bigger anchors to hold better than smaller ones, all else being equal.

 

[Neeves]

I don't think that follows at all. I would expect bigger anchors to hold better than smaller ones, all else being equal.

If you load an anchor, whether big or small to, say 300kg, then if you leave it and come back to measure it - the hold will still be 300kg. Whether its big or small there is nothing to influence it or make changes. The bigger anchor has a higher ultimate holding capacity (so it will hold better - given the chance), but unless the anchor is very small, or the yacht very big - then you will never reach that maximum. The 300kg is actually a dragging capacity - the load needed to get it to drag, but even if it moves 1cm - it is moving and its what load is needed to drag an anchor that is of interest. As the anchor is dragged it compacts seabed, creating more resistance but it also dives into a more resistant seabed - but it needs extra load to develop that extra hold - more engine power or more wind.

A bigger anchor can set to a higher, ultimate, load - but that's not the case as I described. A 15kg anchors in a nice sand seabed can develop a holding capacity of around 2,000kg - a yacht using a 15kg anchors would be very unlikely to be subject to that load (something will be ripped out first) and if its an 8mm G30 then the chain will stretch. There is apparently a huge safety factor (reduced in a questionable seabed)

Jonathan

 

[Neeves]

Jonathan, this is exactly what we do. We have a Spade of the correct size for our yacht as determined by the manufacturer. We carry two lightweight aluminium anchors. The largest and heaviest of which we set in a fee with the main anchor. We set this second anchor on all rode. We use the rib to pull the yacht forward so we lie to the second anchor most of the time. As this anchor/rode is loaded, it then starts to pull on the main anchor/chain and we get great snubbing and very little movement even in very strong gusts. Works very well

Thanks for the post

I could have used Spade as my example as Spade also quote surface area (and thus also think it an important part of the specification). It was easier to use Rocna - no other reason.

But I'm glad we are not alone!

One factor about setting in a 'V'. There is a tendency in a wide 'V' to have the load on one rode, or one anchor, then the other. Each anchor is then subject to maximum load of all the yacht. This is mitigated as the 'V' reduces veering, in the same way a snubber bridle reduces veering - and as veering results in the major snatch loads - then each anchor is subject to a lower load, than it would be were it used alone. There must be balance, too narrow (better shares the load), too wide (increased reduction of veering) but I do not know what it is, we work at about a 30 degree angle, roughly equal length rodes (one is nylon and chain).

Jonathan

 

[NormanS]

I don't think that follows at all. I would expect bigger anchors to hold better than smaller ones, all else being equal.

This is getting scary. I agree with you 100%.

Jonathon has a catamaran, where every ounce counts, so he doesn't want to carry a heavy anchor. That's fair enough, but in this case I think he is wrong in trying to persuade the rest of us to have light, and therefore, smaller anchors. Obviously Fortress is an exception here, as being made of aluminium alloy, it is much lighter than the equivalent in steel.

(I have a Fortress, an FX23. It weighs about 7.something kg. It was bought to replace a 20kg Danforth, and is roughly the same size. I'm not sure if the Fortress is classified by the pundits as a New Generation anchor, or not. It shouldn't be, as it is just a Danforth made in a different material, but as Jonathon has pointed out, in tests in particular conditions, it beat all the competition.)

I wonder why no other anchor makers have tried making their designs using lighter materials? Maybe they feel that weight matters?

 

[vyv_cox]

I don't think that follows at all. I would expect bigger anchors to hold better than smaller ones, all else being equal.

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.

 

[vyv_cox]

I wonder why no other anchor makers have tried making their designs using lighter materials? Maybe they feel that weight matters?

They have. I know a liveaboard yachtsman, boat about 40 ft, who uses an aluminium Spade. He tells me it holds just as well as the steel version. Unfortunately it has suffered from galvanic corrosion between the aluminium and the lead tip weight (pics on my website) and the anchor has now been replaced with a steel one. My little saying - ''anchors are sold by weight but hold by area'. ISTR several other makers who supply aluminium anchors to the same design as their steel ones.

Edit: It's a HR 42

 

[doug748]

Interestingly Evans is the only yachtsmen I know who replaced his Rocna, or any modern anchor, with Bruce (or pre-modern), or Bruce copy. I think he carries 2 - but my memory might be wrong (on the numbers).

O a slightly different tack, I spied Tom Cunnliffe's boat over the summer and see he still has his awd anchor, a Bruce. Probably for the same reason as the rest of us, it does the job and it's paid for.

Much of the anchor angst in these threads is due to failure to appreciate the other bloke's point of view. If fact you can say this about many of our other rows. The average channel sailor never has to put up with 30 kts in an open anchorage with dodgy holding; so the performance of his anchor, or chain, above this level is irrelevant to him. Before poor weather sets in we have long retired to a marina or muddy river which are invariably only a few hours away.

 

[Neeves]

This is getting scary. I agree with you 100%.

Jonathon has a catamaran, where every ounce counts, so he doesn't want to carry a heavy anchor. That's fair enough, but in this case I think he is wrong in trying to persuade the rest of us to have light, and therefore, smaller anchors. QUOTE]

Apologies,

I'm not trying to persuade anyone. I'm trying to offer an alternative view. Keep wrapping my knuckles if I appear to be selling something!

Most production yachts now are getting lighter (as a function of length) - or weight is being better distributed. Many yacht owners are weight conscious. I'm simply trying to suggest there are other options and going big with anchors is not the only route to security.

I am seriously concerned that the common mantra, underlined by the very credible links posted above (a few posts ago) recommending bigger anchors are considered the only option (to security) and the post that Vyv had made, No 373, become more common place. I do believe that going one size up is neither here nor there, its just extra money (unnecessary in my book) but going too far in the bigness concept and you might have issues as Vyv describes.

The alloy anchors of which I am aware are the FOB Light, Manson's racer, Fortress, Guardian - these are all fluke types, or Danforth types and then Spade's alloy (exactly the same as the steel version - so slightly concave) and Anchor Right's alloy Excel (exactly the same as its steel version so convex). There may be others? I can vouch for the effectiveness of the Fortress, Spade and Excel but have never used the Racer, Guardian nor been aware that I have even seen an FOB light.


I am happy to debate big or small - but the theme remains the same, dragging and harmony. I'm here to learn.

edit - just seen the last post: In many places we go - there are simply no marinas - we do not have that option. I'm guessing, maybe its changed - the same is true of the north west coast of Scotland. But I agree - most people out for the weekend, see the 30 knot forecast and stay snuggled up, one day I hope they have the time and enthusiasm to venture out - and have read this thread! Many of the posts are full of good ideas and advise - I have learnt, which is what its about. close edit.

Jonathan

 

[noelex]

Theory, and there have been lots of work and it all comes to the same conclusion, that for a given design an increase in surface area will increase performance (not weight). It is not totally accurate in real life as increasing wight sometimes results in the need to use a thicker or thinner steel plate than theory demands - because that size of plate is not actually made. The fact that Rocna quote surface area suggests they also think it is a critical measure of performance.

I choose as an example a Rocna, as they quote surface area.

A 25kg Rocna has a surface area of 1415 sqcm. The next size up is a 33kg model with a 1695 sqcm fluke. So for a 32% increase in weight you produce a 14% increase in surface area. The increase in surface area results in a linear increase in holding capacity, double area and you have slightly less than double increase in hold. Load on the yacht is proportional to square of wind speed I find it difficult to believe anyone can tell the difference as a very small increase in wind speed will soon account for that increase in surface area (I'm sure someone can do the maths). To make a real difference you need to, say, double surface area - and then you might notice the difference.

To increase surface area, of the 25kg model, by 50% would need a 40kg anchor (1945cmsq) and to double surface area would need a 70 kg model (2690 cmsq).

An alternative would be have 2 x 25kg models - it would double surface area and would allow deployment of an acceptable 'light' anchor most of the time. For those, like us, with only one bow roller the second anchor, available to be deployed when the weather dictated, could be alloy (in fact you could have 2 alloy (+ a steel primary) and triple the available surface area and still only carry 45kg). You do need to be prepared to deploy the second anchor (in a 'V' or fork) but you do not need to have a larger than necessary (most of the time) anchor sitting on the bow roller. And because you have 2, or more, primary anchors you can optimise design to cater for different seabeds, say a fluke for thin mud (that will also work in sand) and convex/concave that will work in medium density weed and pebbly bottoms.

Jonathan

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.

Not content with just Vryhof's conclusions (Vryhof are very well respected and do a lot of scientific testing but their anchors are much bigger than ours) Professor Knox went on to measure the effect on smaller models. He tested various sizes of Bruce and Delta anchors. His conclusion was that indeed holding capacity was proportional to weight (in other words if you double the weight you double the holding capacity) not surface area as you are suggesting.

This is from his comprehensive test reported in Practical Boat Owner in July and August 2002.

Here is a link: it a very good read (as is the July PBO and in August 2011 Proffessor Knox makes some brief mention to the same thing, but sorry I don't have links)

http://www.spade-anchor.com/IMG/pdf...s_ancres_-_PRACTICAL_BOAT_OWNER__partie_2.pdf

 

[JumbleDuck]

If you load an anchor, whether big or small to, say 300kg, then if you leave it and come back to measure it - the hold will still be 300kg. Whether its big or small there is nothing to influence it or make changes. The bigger anchor has a higher ultimate holding capacity (so it will hold better - given the chance), but unless the anchor is very small, or the yacht very big - then you will never reach that maximum. The 300kg is actually a dragging capacity - the load needed to get it to drag, but even if it moves 1cm - it is moving and its what load is needed to drag an anchor that is of interest. As the anchor is dragged it compacts seabed, creating more resistance but it also dives into a more resistant seabed - but it needs extra load to develop that extra hold - more engine power or more wind.

I don't understand what you are getting at. First you say that an anchor set so it can take a load of 300kg will still take a load of 300kg. Then you say that depending on size it may hold more. Then you say that even at 300kg the anchor is dragging

A bigger anchor can set to a higher, ultimate, load - but that's not the case as I described. A 15kg anchors in a nice sand seabed can develop a holding capacity of around 2,000kg - a yacht using a 15kg anchors would be very unlikely to be subject to that load (something will be ripped out first) and if its an 8mm G30 then the chain will stretch. There is apparently a huge safety factor (reduced in a questionable seabed)

And I still don't understand what you're getting at. Surely we want anchors to be able to take larger loads than those we use to set them? You've previously said that most people go up a size when they buy a modern anchor; now you seem to be saying that larger anchors are no good, or hold too well, or don't set, or something.

 

[Neeves]

Noelex,

There is no contradiction. Surface area is proportional to weight, Ultimate holding capacity is proportional to weight and/or surface area. Its easier to measure weight, you just sit the anchor on a set of scales (or if it big, under a load cell). Surface area takes more time. Guess what most people use? It is also difficult to define what is the 'active' surface area of a Bruce. Its fluke is quite strongly concave - how do you account for the 'working' area of the fluke. A Rocna and Spade are easier (and Fortress easier still). They are geometric shapes and are flat, or flattish.

Vryhof in an earlier edition of their treatise used to provide logarithmic graphs of surface area vs holding capacity - they changed it in more recent editions to weight to holding capacity. I have both editions

Conveniently Rocna and Spade have measured surface area for each of their anchors.

Jonathan

edit I've just checked some independent research conducted using data from Vryhof and Bruce - 'The weight of anchor does not play a role in anchor capacity and trajectory except as it correlates to the size of the anchor. Identical anchor geometries with different anchor weight have almost identical curves and shows that anchor weight has small or no difference on anchor capacity'. Basically an alloy Spade or alloy Excel performs identically to their steel counterparts (of the same surface area). To put this another way, you cannot increase the holding capacity of a Spade by swapping from alloy to, double the weight, steel - the capacity will stay the same. close edit

 

[Neeves]

I don't understand what you are getting at. First you say that an anchor set so it can take a load of 300kg will still take a load of 300kg. Then you say that depending on size it may hold more. Then you say that even at 300kg the anchor is dragging



Sorry JD, my skills with the English language need to be honed:

If you set an anchor, say with your 60hp engine(s) to 300kg then as long as any future load does not go beyond 300kg then the anchor will not move, even at 300kg it will not move. Now if we use the same engines, at full throttle, we have a further 300kg we can impose on the anchor it will 'drag' or set further to the limit of that 600kg - it will then hold the yacht steady. If further loads remain below 600kg it will be stable in the seabed - if the wind produces a higher load it will drag a bit further, or set a bit further - but it might move only 3cm. I may have misused the word drag, my fault entirely.



And I still don't understand what you're getting at. Surely we want anchors to be able to take larger loads than those we use to set them? You've previously said that most people go up a size when they buy a modern anchor; now you seem to be saying that larger anchors are no good, or hold too well, or don't set, or something.

But going back to the set at 300kg - yes the anchor will take more load (if its a 15kg modern anchor it has the potential to drag, or set further, until its holding 2,000kg). So if the wind picks up that anchor set at 300kg - by the engines - has still the potential to hold more - but to do so it will be loaded more (it will not hold more just because its sitting there). it will not hold more until its loaded more, it needs to compact more seabed, or continue to dive into a stronger more cohesive part of the seabed, until the seabed it compacts is holding that extra load.

An anchor when it sets compacts seabed, but modern anchors dive - they disappear (that Fortress whose shackle was 11' below the seabed) as they dive they compact and with depth the seabed becomes more dense. 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.

Large anchors are only questionable if the load applied is insufficient to get them to set, securely. If they are not set then they are just lumps of steel sitting on the seabed - with a sharp toe ready to catch in those baked bean tins (or ribbon grass). I think Vyv's example makes the point.

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.