A contradiction with bigger anchors

[Neeves]

Read any sailing forum on anchors and somewhere, someone will advise, buy the biggest modern anchor that you can safely and efficiently carry. Looking at anchor sizing charts suggestions are go one or 2 sizes bigger - some suggest going to twice the size. The thought process is that the bigger anchor will have less chance of dragging. There is an extension to this advise and thought that if you buy an anchor 25% bigger it will give you 25% more hold as bigger is commonly thought to be a measure of weight.

I choose my numbers with a bit of care (but round the numbers out): but if your smaller modern anchor, say 20kg, has an ultimate holding capacity (accepting this will vary seabed to seabed) of say 750kg then the next size up (same design), say 25kg will have a holding capacity of 1,000kgs.

Why when these recommendations are made and why when people follow these suggestions is the same rule not applied to the chain. If the forum expert recommends the bigger anchor and the owner relies on the recommendation and agrees and expects the need for the greater capacity why not stronger or bigger chain. The small anchor will match the WLL of 8mm G30 chain and the larger anchor will match the limit of WLL of a 8mm G40 chain. If the need is there for the bigger anchor why is the chain (and shackle) ignored in this equation? The numbers get worse, or the mismatch gets worse, if you upsize from a 20kg anchor to a 30kg anchor - and become quite bizarre if you double size/weight to say 40kg.

So if you double weight the suggestion is, on the preceding premise, that there is real need for an almost doubling of hold, which implies - somehow - a doubling of tension. Anchors have finite limits, so do chains.

So to those that bought the modern anchor bigger than the sizing chart, or recommend the same, what was your reasoning and motivation and why did you not think of the chain and shackle.


To make it clear - as most who read my comments will know: I don't believe there is a simple relationship between weight and hold, or not as simple as double weight and you double hold and I am also yet to hear of a modern anchor of the size recommended on the charts dragging in a normal, sand, clay, silt seabed.

Jonathan

 

[Kelpie]

I would have thought that it was about increasing your chances of getting an anchor to hold on marginal seabeds rather than needing higher ultimate holding power in a good seabed.

 

[Neeves]

If the 2 anchors are of the same design then the toe of the 2 anchors should be very similar (arguably the toe of the bigger anchor will be chunkier and more difficult to force into a harder substrate) the fluke is the same shape and style - so why is the bigger one to develop more hold - the ultimate load is provided by the yacht (either engine of windage), so the load is the same.

All the pictures I see of anchors much larger, than recommended on anchor charts, show anchors not particularly deeply set - which is what I would expect. It does not matter if you have a big or small (within reason) anchor - if your engines on windage develops a 400kg tension then that is the hold of the anchor, no more no less. If the wind picks up and windage results in tension of 600kg you will have a hold of 600kg, big or small - it will not be 'more' because you have a bigger anchor. If the ultimate hold, for the small anchor in a specific seabed is, say 750kg then once the windage tension exceeds 750kg - then the big one is advantageous.

But returning to the question - if you have the bigger anchor and the conditions are such that the bigger anchor holds 'too' well then you are in danger of breaching the WLL (and as we have no idea what snatch loads might be) you are in danger of stretching your chain. If the owner is sufficiently concerned about anchor size - why does that concern not extend to the chain and shackle?

Jonathan

 

[NormanS]

Jonathan, I think you are straining at a gnat. You'll have to admit that there are more cases of anchors dragging, than there are of chains breaking. Anyway, I'm exempt from your perceived problem, 'cos my chain is 10mm for a 20kg anchor.
Must admit I'm intrigued by your idea that an anchor can hold "too well". I'll try to remember that, the next time I'm anchored in Loch Seaforth in 74 knots of screaming wind!

 

[PaulJ]

Strictly speaking I suppose you are right and an increase in size of anchor should mean an increase in size/weight of chain. However the reason we use chain is not so much for it's strength as for it's weight and consequent ability to form a catenary and thus give a horizonal pull with the minimum of vertical component. Without having the figures in front of me I would think that as long as the chain is heavy enough for the size/weight of the boat, increasing the size of the chain wouldn't achieve much other than to weigh down the bows even more. The strength of the chain is not likely to be a problem. I imagine most people go one size bigger on the anchor just to be on the safe side....... I know I did!

The piont you make about the bigger anchor not digging in as well is an interesting one......... When we were cruising, for my admittedly heavy 39footer I used a 25Kg Delta which is one, possibly two sizes bigger than recommended. Whenever the water was clear enough I always snorkelled to check that the anchor was dug in and only on the softest of bottoms did it ever fully bury itself. Usually I could still see the tail end of the flukes despite have applied "substantial" reverse to dig it in. I hasten to add that I think the Delta is a terrific anchor and in eight years of cruising it only dragged on maybe three occasions and then only under very severe conditions. In the end I guess it is a matter of "you pays your money and you takes your choice" and as long as you pays enough you will usually get away with it.......

Paul

 

[vyv_cox]

However the reason we use chain is not so much for it's strength as for it's weight and consequent ability to form a catenary and thus give a horizonal pull with the minimum of vertical component. Without having the figures in front of me I would think that as long as the chain is heavy enough for the size/weight of the boat, increasing the size of the chain wouldn't achieve much other than to weigh down the bows even more. The strength of the chain is not likely to be a problem. I imagine most people go one size bigger on the anchor just to be on the safe side....... I know I did!

I suggest you read Jonathan's article on chain in the current Sailing Today, and maybe attend one of my anchoring talks, RYA and CA have hosted several recently. Catenary at fresh winds and above is effectively non-existent, also as calculated by Alain Fraysse some years ago. I show a large number of photos that demonstrate quite conclusively that all of the chain lifts of the bottom in winds of F6-7. One of these is of a 27 kg Manson Supreme with 12 mm chain attached to an Oyster 55. Even this extremely heavy chain, about 25 metres with a scope of 4:1, was fully clear of the bottom in gusts.

I also show the graph of results of the 2006 YM anchor tests. All anchors were 35 lb models. In a variety of bottoms Spade, Rocna and Manson Supreme all held at over 2 tons load on 3 out of 4 pulls. I cannot imagine that my deck fittings would hold this much, although my 8 mm chain certainly would.

 

[Neeves]

Jonathan, I think you are straining at a gnat. You'll have to admit that there are more cases of anchors dragging, than there are of chains breaking. Anyway, I'm exempt from your perceived problem, 'cos my chain is 10mm for a 20kg anchor.
Must admit I'm intrigued by your idea that an anchor can hold "too well". I'll try to remember that, the next time I'm anchored in Loch Seaforth in 74 knots of screaming wind!

I tried to mention modern anchors which are not prone to dragging (according to a previous thread), not pre-modern anchors, which are notorious for dragging). Nor am I saying that chains break - I'm pointing out the apparent contradictions. The best examples of holding too well - A Fortress anchor after a named storm, saves the vessel - not uncommon to be irretrievable - to me cheap insurance.

The focus is, apparently, having an anchor that will withstand your 74 knots - but there is no concern for the chain (or shackle). I just thought it was anomalous and wondered if the contradiction had crossed the consciousness of the proponents (of big anchors) and those who have followed the advice.

I also find it anomalous that we, myself included, complain about the cost of anchor and chain - yet many buy anchors that might be too big - with only 'gut feel' to support the extra expenditure and many perpetuate the myth of 'heavy' chain, which shows RYA teaching methods are successful!, without having tried the alternatives.

Basically purchases are determined by emotion not fact - and I think emotions can be questioned.

Norman, maybe you cannot fight emotional purchases, but I'll have a go!

Jonathan

 

[RichardS]

I'm not sure why this thread seems to be focussed on anchor weight.

When I bought my Rocna I went for a slightly larger one as I wanted the larger fluke area and as long as you stick to the same model then the two are related.

If Rocna had made the same anchor with the same weight but with larger flukes (but with no loss of strength!) I would have gone for the same weight / more fluke option.

Richard

 

[NormanS]

I suggest you read Jonathan's article on chain in the current Sailing Today, and maybe attend one of my anchoring talks, RYA and CA have hosted several recently. Catenary at fresh winds and above is effectively non-existent, also as calculated by Alain Fraysse some years ago. I show a large number of photos that demonstrate quite conclusively that all of the chain lifts of the bottom in winds of F6-7. One of these is of a 27 kg Manson Supreme with 12 mm chain attached to an Oyster 55. Even this extremely heavy chain, about 25 metres with a scope of 4:1, was fully clear of the bottom in gusts.

I also show the graph of results of the 2006 YM anchor tests. All anchors were 35 lb models. In a variety of bottoms Spade, Rocna and Manson Supreme all held at over 2 tons load on 3 out of 4 pulls. I cannot imagine that my deck fittings would hold this much, although my 8 mm chain certainly would.

The fact that all the chain is off the bottom, doesn't mean that there is no catenary. It is impossible to pull a chain (unless vertical) into a straight line, so even if all the chain is off the bottom, the pull on the anchor will be horizontal, unless you are determined to anchor with "lavvy" chain. The heavier the chain, the more effective the catenary. I know that you say that you can walk around your boatyard dragging your 8mm chain. Just for interest, during the winter, when I had my 10mm chain stretched out for marking, I tried dragging mine - no way!

 

[pmagowan]

I'm not sure why this thread seems to be focussed on anchor weight.

When I bought my Rocna I went for a slightly larger one as I wanted the larger fluke area and as long as you stick to the same model then the two are related.

If Rocna had made the same anchor with the same weight but with larger flukes (but with no loss of strength!) I would have gone for the same weight / more fluke option.

Richard

Yes, I think if we were working on the basis of a simple sinker then weight would be the primary factor but with anchors weight just happens to be proportional to the size. I suspect that you need a critical weight (maybe density is the correct term) to first sink the anchor, then get through any weed and then break into the substrate with the tip. If you have this then surely the prime factors are anchor design (to increase the liklihood of it burrying properly) and fluke size (to increase its holding power). I am not sure that anyone thinks that much about chain because it is not generally considered the weak link (forgive the pun) in the boats setup. I suspect that parts of the boat may start to fall off before a decent chain snaps and the wind would have to be pretty extreme. As others have said I think catenery is a red herring but may be helpful in the initial set of the anchor.

 

[Neeves]

Reports of chain breaking are minimal, or almost never. But chain stretching, permanent stretch, occurs at a bit over 2 times WLL. (and is reported). I gave a talk on 'anchoring' and 2 out of about 50 had had their chain stretch. If you have 50m of chain deployed and it stretches - it will not fit the gypsy - you will need to retrieve by hand until you can replace it. So for a G30 chain, which I understand is the common strength quality in the UK (and Oz), 8mm will start to stretch at about 1,500kg - which is well below the holding capacity figures that Vyv quotes above for pretty small anchors, 15kg - yet people whose yacht might be specified to carry a 20kg anchor and 8mm chain (and the charts are on the basis of Delta/CQR/Bruce) will not unusually buy a 30kg anchor - without considering that such an anchor could hold much more than 2t. As Vyv says - the bow roller will possibly collapse before even the 15kg anchor drags - so why the 30kg model without an upgrade to cater for the potential extra loads that the owner is anticipating.

Or put another wa - if you do not anticipate the load - why the bigger anchor, but I thought I'd try to pose the question the other way round

So - if you accept you need the extra hold - why ignore the other items that are part of the equation. I find it a contradiction and I wonderedd if it had been thought about and if so what is the reasoning.

Why weight - its the common measure of anchor size, its convenient. Only Spade a Rocna mention surface area (and I don't know how you measure surface area of an anchor). I think surface area more critical, than weight - but for a given design there is a rough correlation between weight and area (its not exact because steel plate is not necessarily available at the size (thickness) scaling needs).

Jonathan

 

[Neeves]

The fact that all the chain is off the bottom, doesn't mean that there is no catenary. It is impossible to pull a chain (unless vertical) into a straight line, so even if all the chain is off the bottom, the pull on the anchor will be horizontal, unless you are determined to anchor with "lavvy" chain. The heavier the chain, the more effective the catenary. I know that you say that you can walk around your boatyard dragging your 8mm chain. Just for interest, during the winter, when I had my 10mm chain stretched out for marking, I tried dragging mine - no way!

To me a well set anchor is one you cannot see (I accept that in a very hard seabed this might not be possible) and this is a commonly how I see, or do not see ) our anchor. When set we often have buried the anchor and 3 metres of chain. I believe other people have the same observation, their anchor disappears. When I see an anchor with the shank sitting above the seabed I see a poorly set anchor. If the anchor is well buried then the pull on the anchor is the angle of the shackle in the seabed - and this bears almost no relationship to the angle of the rode at the seabed. The rode can be at 0 degrees but the shackle angle could be 15 degrees. Calculation was made of the shackle angle on Fortress Chesapeake mud tests (the US Navy have programmes to make the calculations) and shackle angles of 45 degrees were evident against rode seabed angles of 8 degrees. This was quite extreme, the shackle I recall was at about 14' deep!

But its a standard notion - a buried anchor develops a reverse catenary and the deeper the anchor dives the more vertical the pull on the shackle.

The idea that catenary provides opportunity for a horizontal pull is only true of a poorly set anchor - otherwise its a fallacy (in addition to the idea a shallow catenary absorbs snatch loads). You can enjoy the positive effects of catenary but you need more and more and heavier and heavier chain - and most yachts afloat today have limited chain locker space and possibly an even more limited ability to carry extra bow weight.

My question was not focussed at using (buying) heavier chain (its unrealistic) but its not unrealistic to upgrade to G40, of G70 (and there is also a G50 grade) if you are sufficient motivated to buy that bigger anchor.

Jonathan

 

[NormanS]

Surely it's better to avoid "snatch" loads in the first place. A responsible skipper will take action to avoid them.
1. Don't anchor in places where there are likely to be big seas.
2. If anchoring in shallow water, where the benefit of catenary is lost, use a nylon snubber of adequate length.
3. Reduce yawing by using an anchor sail or possibly two anchors in a Vee.

One time when snatch loading may be less avoidable, is when raising anchor in gale conditions, but even then by taking it slowly, so that the anchor has a chance to break out before the chain is straight up and down, snatching can be reduced. And at least then, you are awake and ready to get underway.

 

[pmagowan]

Reports of chain breaking are minimal, or almost never. But chain stretching, permanent stretch, occurs at a bit over 2 times WLL. (and is reported). I gave a talk on 'anchoring' and 2 out of about 50 had had their chain stretch. If you have 50m of chain deployed and it stretches - it will not fit the gypsy - you will need to retrieve by hand until you can replace it. So for a G30 chain, which I understand is the common strength quality in the UK (and Oz), 8mm will start to stretch at about 1,500kg - which is well below the holding capacity figures that Vyv quotes above for pretty small anchors, 15kg - yet people whose yacht might be specified to carry a 20kg anchor and 8mm chain (and the charts are on the basis of Delta/CQR/Bruce) will not unusually buy a 30kg anchor - without considering that such an anchor could hold much more than 2t. As Vyv says - the bow roller will possibly collapse before even the 15kg anchor drags - so why the 30kg model without an upgrade to cater for the potential extra loads that the owner is anticipating.

Or put another wa - if you do not anticipate the load - why the bigger anchor, but I thought I'd try to pose the question the other way round

So - if you accept you need the extra hold - why ignore the other items that are part of the equation. I find it a contradiction and I wonderedd if it had been thought about and if so what is the reasoning.

Why weight - its the common measure of anchor size, its convenient. Only Spade a Rocna mention surface area (and I don't know how you measure surface area of an anchor). I think surface area more critical, than weight - but for a given design there is a rough correlation between weight and area (its not exact because steel plate is not necessarily available at the size (thickness) scaling needs).

Jonathan

That is a very odd way to look at it. The answer, I would have thought is that the limiting factor is holding power. The smaller anchor may have a 1500kg ideal circumstances rating but ideal circumstances are hard to come by. Anchors drag. This is the problem people face. I have never heard of a decent chain breaking in normal use on a yacht. I don't care if it stretches in a hurricane. I suspect that 9 times out of 10 the anchor dragging will be the issue and in those circumstances a bigger anchor may well reduce this risk. I.e. In any system you need to look at the weak link in real life not just the maximum ratings.

 

[vyv_cox]

I know that you say that you can walk around your boatyard dragging your 8mm chain. Just for interest, during the winter, when I had my 10mm chain stretched out for marking, I tried dragging mine - no way!

I wouldn't describe it as 'walking round'! I managed to move all 60 metres of it in a straight line by heaving backwards. Dragging it around in a loop was considerably easier, but then I was not pulling all of it.

 

[noelex]

So for a G30 chain, which I understand is the common strength quality in the UK (and Oz), 8mm will start to stretch at about 1,500kg - which is well below the holding capacity figures that Vyv quotes above for pretty small anchors, 15kg - yet people whose yacht might be specified to carry a 20kg anchor and 8mm chain (and the charts are on the basis of Delta/CQR/Bruce) will not unusually buy a 30kg anchor - without considering that such an anchor could hold much more than 2t. As Vyv says - the bow roller will possibly collapse before even the 15kg anchor drags - so why the 30kg model without an upgrade to cater for the potential extra loads that the owner is anticipating.

G30 8mm chain is not very strong and only suitable for smaller yachts. Even if we use this chain (WLL 818kg ultimate strength of around 3272 kg) is still well above the ultimate holding power of the 35lb anchors recorded in the 2006 test at some locations.

In the 2006 test quoted in this thread the lowest holding power of the three locations tested (New Brighton) gave the following results:

Rocna : 680 kg
Manson Supreme: 680kg
Sarca: 400 kg
Fortress: 350 kg
Spade : 225 kg
(Older generation anchors were worse)


The results for more difficult substrates tend to become erratic so I don't think we should take these individual results as gospel, but I think the low overall performance justifies Kelpie's insightful comment:

I would have thought that it was about increasing your chances of getting an anchor to hold on marginal seabeds rather than needing higher ultimate holding power in a good seabed.

Even the top performers in this substrate did not generate a lot of holding power, especially when we consider these are average results. Some individual sets were significantly worse

A larger anchor is not a panacea. It can still drag, but it will reduce your chances of problems. As well, it provides other advantages such as the ability to anchor at shorter scopes when necessary.

Chain strength is constant. It does not vary from location to location.

If the chain strength is adequate for severe conditions it will always OK (ignoring corrosion) irrespective of the substrate. This requirement is not difficult to achieve. Most boats comfortably exceed this standard, as shown by the very low incidence of chain breakages.

On the other hand a lot of boats drag in strong wind. I think we need to concentrate on anchor performance (rather than chain strength) if we are going to minimise problems.

 

[Neeves]

Most of the anchor shanks that have bent have done so at retrieval.

8mm chain is supplied as standard on some quite large, or to me they seem large, yachts. 40' would not be unusual and I've seen 8mm on 45' yachts. I'm not suggesting this is adequate - just it happens.

Noelex - what your holding capacity data demonstrates is that its not size but design that impacts hold. If you want to maximise hold, then have more than one design. For the Fortress to compete with Rocna the Fortress would need to be twice as big, in the Fortress tests in Chesapeake the Rocna would need to be 10 times as big to compete with Fortress. If you want to maximise hold in marginal seabeds - carry more than one anchor. You are right - we need to concentrate on anchor performance - but having that performance compatible with the rest of the ground tackle.

When I asked for the history of modern anchors dragging on this forum there was an absence of replies (describing anchors dragging) - and this was from a cross section of participants used in a cross section of seabeds with the recommended sized anchors, larger anchors and a few smaller anchors. I have not heard anyone yet complain their modern anchor was too small and were going to/had bought a bigger one.

I have yet to hear of anyone but Dashew actually anchoring at short scope in strong winds and documenting the experience. I oft hear it quoted but never see it documented, let alone photographed. I am very wary of anecdotal comments without any corroboration (at all). Few mention when quoting that his experience is of a Rocna anchor well over 100kg used on a 40t x 64' yacht with 240hp engine. None of us have the pulling power anywhere near that size nor that pulling power to weight ratio. I am sure he can set that anchor deeply - not so most of us with a maximum of 50hp available - which is why the oversize anchors I mostly see are shallow set with the shank protruding well above the seabed (the engine is too small to set the anchor). And short scope is not a well known technique to use to expect wind to set the anchor for you - I might even suggest its irresponsible suggestion without further evidence of success.

Jonathan

 

[pmagowan]

When you say 'size' are you using that interchangeably with 'weight'?

 

[noelex]

I might even suggest its irresponsible suggestion without further evidence of success.

Short scopes and strong winds are not a great combination, and not one I would recommend.

The focus on a larger anchor is often just on the improved holding capacity but in practice the ability to anchor in poorer substrates or at shorter scopes is a valuable bonus.

It opens up anchorages that would otherwise not be viable, or perhaps not deemed suitable for overnight.

It is a matter of degree, rather than absolute difference, but anchoring overnight (hopefully safely) and enjoying a peaceful sunset while some boats are forced to leave for a more "suitable location" is a nice advantage.

A larger anchor is by no means immune from dragging so it is important to excersise some judgment, but it does extend the viable anchorages (compared to an identical smaller anchor) for any given conditions.

 

[GHA]

Another rerun for a Neeves peeves

And not really on solid ground imho, is there really a recommendation for the majority of boats people on here sail, weekends out and a week or three in the summer to go a size or 2?
Don't think there is and wandering round marinas it's seldom you see a biggie. And even if you do it's often on a boat obviously more used to being on the move and living on the hook. Where the practice is commonplace and recommended by many extremely experienced cruisers is on a long term long distance boat. With regards to chain strength etc, in general these are people well versed in keeping the boat going and more than capable of reading data sheets along so that's not really a big deal. Also keeping the anchor loads down with various strategies / snubbers etc takes the strength argument largely out of it.

So do many weekend sailors go big? Can't be many, you never see them.