Anchor size vs Chain weight

GHA said:
Once the forces get up, when you really want a catenary, it does little.
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This takes me into a new line of thinking ......

Lets ignore the rope/ chain for a moment and consider just the anchor with a bar tight rode......

Clearly the safest situation is to have enough scope to keep the "upward component" of force due to the boat to a value less than the "digging in force" caused by the horizontal force and the angle of the flukes. ( which I assume must remain below horizontal in order to dig in and not out of the bed )

Could this be resolved into the equivalent of something like a polar diagram showing both force and the angle applied ...in this respect an efficient anchor is one which produces the most "dig" for least horizontal force

If this is correct then the limiting case is actually the shortest scope ( steepest angle) where the anchor will hold a given force without breaking out.

This fits with the concept that many anchors work tolerably with lots of chain but the newer designs actually dig in better and hence give more ultimate holding power at higher angles/ shorter scopes

One of the new MANTUS anchor videos show tests at a 2 :1 scope .and .it digs in very well...

http://mantusanchors.com/test-video/

Does this make sense ??
 
FWIW, have a look at the Fob Rock. I am just looking at the special edition of Voile magazine which writes (approx) : refined by the designers of the firm, Fob, this plough type anchor has performance, whilst remaining simple and affordable. This was confirmed by our tests with traction loads exceeding 2 tonnes which class it amongst the best ( my note : this is about twice the load of the Kobra 2) This result is even more positive when you note that its main rival at this level of performance, the Spade, is almost €200 more expensive. And it's made in France.

In commenting on the Rocna they say that it has a winning cocktail because its performance is at the same level as the Fob Rock....

The 16 kg Rock and the 15 kg Rocna are the same price in France - €370.
 
The deciding factor for us was that the Rocna wouldn't stow over our bow roller without modifications to the boat, and wouldn't fit into the anchor locker easily, if at all. The spade seems to fit if a CQR will. How would the Fob Rock score on this? I still carry a CQR as my spare bower and whilst a second Spade would be my preference I would like to keep costs down when I replace it.
 
typhoonNige said:
The deciding factor for us was that the Rocna wouldn't stow over our bow roller without modifications to the boat, and wouldn't fit into the anchor locker easily, if at all. The spade seems to fit if a CQR will. How would the Fob Rock score on this? I still carry a CQR as my spare bower and whilst a second Spade would be my preference I would like to keep costs down when I replace it.
Click to expand...

In terms of shape it looks similar to a Kobra.

http://www.calibramarine.com/shop/item.aspx?itemid=211
 
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I thought I might join in!

There are strong advocates for Spade, Rocna, Excel and Supreme - we all argue with each other over our personal preferences but we do agree that whichever of tbe 4 (sorry if I missed some) they are better than a CQR and Delta. The choice from the 4 might be determined by price, whtehr its available where you are, whether it fits - or which arguments seem the most sound. In terms of holding, they are as good as you will get. A budget option would be a Kobra - but I am suspicious of the strengths of its shank - but I have only tested a small one and seen bent bigger ones - so just something about which to be wary.

We have been using 8mm short link, 50m and 15kg Excel for a 38' cat, 6/7t we have not had any problems but we need new chain (all the gal has gone) and we will be biting the bullet and going for a new gypsy and 70m, 6mm (might go 80m) G7. There have been times when we have ignored a deep anchorage simply because 50m too short and adding rope (which we do carry) is too much of hassle at 2am. I'll report on the experience in the fullness of time.

Heavy chain is great when the winds are light, say upto 20 knots, and you are in a tight anchorage. The weight of the chain stops you drifting about. But you could achieve the same effect by simply dropping a second anchor off the bow such that it simply scrapes across the seabed. Any chain over, say 30 knots, will be off the seabed and get to 35 knots and most chains are bar tight (so having a heavy chain (just when you thought it was useful) is simply ballast). Most modern anchors, any of those 4, work well when loaded at an angle above the seabed, so when on long scope and bar tight chain. This is not an issue.

All chain is common as the complete rode is then equally abrasion resistant. If you were always anchoring in mud and sand - a mixed rode would do fine, thousands use a mixed rode and failure is conspicuous by its lack of reports.

People swear by heavy chain. Oddly thousands, 10,000s use a mixed rode whose combined weight is low - oddly none of them have an issue - go for as small a link as you can get away with.

With a clean slate, go for one of the 4 anchors (or other if I missed something), go for length of chain (rather than weight, or size, of link) commensurate with whatever strength you need, see below. Most people are restricted by gypsy (so check cost of a gypsy taking smaller link - (they must be available second hand? we will soon have an 8mm Muir Atlantic spare). Check gypsy against chain size, 5/16th link and 8mm are the same, similar, wire size, links might be different lengths!!

There are advocates of going up in anchor size, we have not found it necessary (and we have the windage of a 45' mono) but others swear by it. But the extra weight of a larger anchor is not a big issue (compared with the weight of chain you carry).

You will need a second anchor - it should be capable of replacing your 'primary' and being used as a primary. So your second anchor should be as good as the first, we carry a FX 23, alloy Spade (A80) but are to try the new alloy Excel, same size as our 15kg Excel). The alloy anchors are light, so can be deployed from a dinghy, they have similar holding capacity and all are demountable - so you can pack them 'anywhere'. Some have bad experiences of alloy - we think them excellent (hence carrying so many - all are the same 'size' as our primary). We carry one spare rode, 30m 8mm plus 40m 12mm hawser laid nylon (next time we would have anchor plait). When the chips are down we use 2 rodes in a V, others argue against such practice (but its our yacht, its our choice - and so far it has worked).

Most of this has a cost implication - but a decent anchor is worth the investment. People do swear by CQR, Delta etc but an equal number complain - no-one who owns one complains of one of the 4!

You need nylon snubbers, 10m minimum, forget the bridle length - its too short! Our snubbers are retired 11mm climbing rope - so far so good, only 6 months. We are trialing hollow braided dyneema as wear sleeves for our snubbers, something else we will report on in the future).

On chain - ignore Working Load Limit, or Safe Working Load data. Chain makers use 5:1, 4:1 and 3:1 safety factors for chain for the same application. 4:1 is a fairly common safety factor and I would go to Breaking load and divide by 4 (dividing by 3 gives a marketing advantage - I do not see that it is any safer!)

Sorry a bit brief and staccato,

Send me a PM and post for queries (I do not check the threads every day!) and I'll post replies on the thread

Jonathan
 
Neeves said:
… On chain - ignore Working Load Limit, or Safe Working Load data. Chain makers use 5:1, 4:1 and 3:1 safety factors for chain for the same application. 4:1 is a fairly common safety factor and I would go to Breaking load and divide by 4 (dividing by 3 gives a marketing advantage - I do not see that it is any safer!) ….
Click to expand...

Why do you say this? The chain is rated at a WLL/SWL which is a function of a factor added at the end of the design calculations when sizing chain for a desired WLL. It accounts for matters such as uncertainty in material specifications, manufacturing tolerances and assumed wear and tear in use (to give the product a useful life). Users of chain ignore this at their peril, the 5:1 guy may not have the same quality control as the 4:1 guy and he has allowed for this with his larger safety factor. Unless you understand how a safety factors is derived by the designer or the technical authority for a standard (that the chain is made to) it is impossible to arbitrarily apply a de-rating factor from "breaking load". This is why critical users of chain (or wire rope) demand actual tensile strength testing (from a length of their purchased chain/rope). In the thread on shanks you state that theoretical practice and real life practice are different; safety factors take into account real life uncertainties. Perhaps your reverse engineering work is leading you down a road where absolutes give you a sense of security. Remember your own words though.
 
Safety factors

G43 chain has been traditionally given a safety factor 3:1 by anyone that makes it. G3 chain has been given a safety factor of 4:1. Both G3 and G43 are made by the same people with the same care. Even they cannot recall why G43 has a safety factor of 3:1 (it has been suggested it might be that it was traditionally a Transport Chain, but this is not consistent as G7, also a Transport Chain, is 4:1). But to buy G43 on the basis of its 3:1 safety factor in the absence of other input? You would follow that route as you would not think to query, good luck. Its a bit like buying an anchor and assuming that if its on sale in a chandler it must be safe, or even if made by someone with a reputation that it must be safe - history shows otherwise.

If you can find a good reason why a G43 chain is any more safe than a G3 chain let the American chain makers know. But G43 will have as uncertain a performance as an anchor chain as G3. The raw material tolerance, manufacturing tolerances are identical.

My advise is to realise there is a difference and if you are cautious not to let this colour your judgement when you purchase. I'm simply pointing it out. I do note that some chain suppliers, offering G43 do not quote breaking Loads, but only WLL, or SWL (its not very transparent when the punter might not realise G43 has a safety factor of 3:1 but the G3 with which he is comparing is 4:1). - so my advise is look at the Breaking Load Limit and then compare.

In critical usage product is tested to its limit (not its WLL, or SWL) - when its critical look at the limit, funny I seem to have said something similar:)

As I say, Good Luck

Jonathan
 
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Neeves said:
.....
Heavy chain is great when the winds are light, say upto 20 knots, and you are in a tight anchorage. The weight of the chain stops you drifting about. But you could achieve the same effect by simply dropping a second anchor off the bow such that it simply scrapes across the seabed. Any chain over, say 30 knots, will be off the seabed and get to 35 knots and most chains are bar tight (so having a heavy chain (just when you thought it was useful) is simply ballast). Most modern anchors, any of those 4, work well when loaded at an angle above the seabed, so when on long scope and bar tight chain. This is not an issue.

.....

People swear by heavy chain. Oddly thousands, 10,000s use a mixed rode whose combined weight is low - oddly none of them have an issue - go for as small a link as you can get away with.

.....

Jonathan
Click to expand...

I have always been an advocate of heavy chain but I have to agree that our chain has been bar taut on many occasions. So what you are saying is that the primary reason for a long length of chain (apart from some snubbing effect) is purely to keep the angle of the anchor shank as low as possible ? The only other function of the chain is it's strength so it does not snap ! edit + anti chafe on bottom.

Am I understanding you correctly ?
 
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Storyline said:
I have always been an advocate of heavy chain but I have to agree that our chain has been bar taut on many occasions. So what you are saying is that the primary reason for a long length of chain (apart from some snubbing effect) is purely to keep the angle of the anchor shank as low as possible ? The only other function of the chain is it's strength so it does not snap ! edit + anti chafe on bottom.

Am I understanding you correctly ?
Click to expand...

Basically correct. This is not my suggestion, but one with which I have a lot of sympathy - if you had piano wire that was strong and abrasive enough enough it would work as a rode. You need length to keep the load on the shank as low as possible (because that is when the anchor works to greatest effectiveness) - but beyond about 7:1 the angle does not 'improve' much. You need long chain so that you can anchor in deeper water and still have that low angle. You might need chain if you anchor near coral or in areas with sand and rock (where a synthetic rode would wear - you can eat through synthetic overnight near the Great Barrier Reef.

But there is no doubt in 10 knots of variable wind the yacht with a light chain wanders about more than the one with the heavy chain - if you have light chain you would need to take this into account.

Vyv Cox I recall has been proposing the 'light chain' concept for years - I'm hardly original:)

Jonathan
 
I may have misled slightly, I'm not sure that Vyv has proposed 'light chain' he has proposed the idea that heavy chain and catenary are not the panacea that is offered by many - which is slightly different to proposing 'light chain'.

We need new chain - and it seems, to us a sensible option to follow.

Its about strength of chain vs size.

We have a relatively lightweight yacht (agreed high windage). We need a chain that is strong enough, a G7 6mm will be stronger than the 8mm G3. So strength is not an issue. 8mm chain weighs 30% more than 6mm (about - I'm reading imperial units and fudging) so we can carry 20m more, for about the same weight. Basically we have worked on the basis G3 has stood us in good stead, so it was adequate - we will not be compromising on strength as the G7 6mm is stronger. Recall also that G7 and G3 both have a 4:1 safety margin.

We are having G7 large links at each end, so as to take 'normal' shackles and can end for end. Our G7 chain will be Proof Tested, that's tested to twice Safe Working Load or half Min Breaking Strength and MBS is on the basis of the min to which the wire (from which the links are made) is made (it should always come out higher). Proof testing of anchor chain is fairly normal - and is something any can request, Peerless and Maggi, I think, do it as standard.

Part of the idea of heavy weight was catenary - snubbers replace that notion (and much more effectively as snubbers work at 40 knots (and 60 knots if you are unlucky and are prepared) when the catenary has all gone).

We will gain the negative of a bit of a wander at low wind speeds - but we are used to that - its a cat (and you can overcome this by dropping something off the bow to drag on the seabed if its an issue). If the windlass gives up, we have had the soleniod jamb twice, we can lift 6mm much more easily than 8mm.

It sounds controversial, but there are a number of people going from 13mm to 10mm or 10mm to 8mm. Its actually not that unusual.

Currently there are 2 international suppliers, Maggi (Italy), Peerless (US). Ours will be made by PWB, local to Oz - used to be part of Anglo American.

One issue we will be looking at is whether we lose corrosion resistance quickly. G7 should not be re-galvanised. This is not a big issue for us, primarily because it costs more for us to transport to the nearest galvaniser, acid wash, gal, return etc than buy new chain.

But if you have sat at anchor with the chain bar tight for a few days continuously it does underline the idea that maybe catenary is over rated.

Jonathan
 
Neeves said:
.....
Part of the idea of heavy weight was catenary - snubbers replace that notion (and much more effectively as snubbers work at 40 knots (and 60 knots if you are unlucky and are prepared) when the catenary has all gone). .....
Jonathan
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Could you explain 'snubbers' ? We lie to a snubber - for us it is a question of attaching some anchorplait (around 3-4 metres) and then veering chain so that the chain hangs loosely in a loop and the snubber takes the load. The attachment point of snubber to chain is usually at or just below the level of the water. Is that what you mean by snubber ? I should add that we also attach a chain hook to the chain should the anchorplait give up.
 
All this talk about chain going bar tight and catenary disappearing, presupposes that there is a constant pull between the boat and the anchor. In the real world there isn't, because unless you use an anchor sail, which keeps the boat head to wind, the boat's sheering around means that the load is constantly changing. That is how the catenary of heavy chain dampens out the load.

If your boat is light displacement, like a catamaran, then you need to think about lighter chain, mixed rodes, snubbers etc. For a traditional displacement vessel capable of carrying it, there is nothing better for a good night's sleep, than heavy ground tackle.
 
NormanS said:
All this talk about chain going bar tight and catenary disappearing, presupposes that there is a constant pull between the boat and the anchor. In the real world there isn't, because unless you use an anchor sail, which keeps the boat head to wind, the boat's sheering around means that the load is constantly changing. That is how the catenary of heavy chain dampens out the load.

If your boat is light displacement, like a catamaran, then you need to think about lighter chain, mixed rodes, snubbers etc. For a traditional displacement vessel capable of carrying it, there is nothing better for a good night's sleep, than heavy ground tackle.
Click to expand...

The catenary will do little to dampen snatch loading. For that you need an elastic element to the rode. A boat length of anchor plait works well - anchor plait stretches by about 25% under load.
The catenary alters the angle of pull at the anchor: even if your chain is bar taught it's not a straight line, and you don't need much curve to make a big difference to the angulation.

...and I'd decided I was going to leave this thread alone :D

John
 
Neeves said:
..... 8mm chain weighs 30% more than 6mm (about - I'm reading imperial units and fudging) so we can carry 20m more, for about the same weight. ....
Jonathan
Click to expand...
Bit more than 30% I think - in fact should be nearly twice as much (8 squared / 6 squared = 64/36 in theory!)
 
From Jimmy greens - anchor chain weight

[TABLE="class: techtable techdata, width: 1"]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Size[/TD]
[TD="align: center"]6mm[/TD]
[TD="align: center"]7mm[/TD]
[TD="align: center"]8mm[/TD]
[TD="align: center"]9.5mm[/TD]
[TD="align: center"]10mm[/TD]
[TD="align: center"]12mm[/TD]
[/TR]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Weight per metre[/TD]
[TD="align: center"]0.85 kg[/TD]
[TD="align: center"]1.1 kg[/TD]
[TD="align: center"]1.45 kg[/TD]
[TD="align: center"]2.2 kg[/TD]
[TD="align: center"]2.3 kg[/TD]
[TD="align: center"]3.8 kg[/TD]
[/TR]
[/TABLE]
 
So moving on a bit...
[TABLE="class: techtable techdata, width: 1"]
[TR]
[TH="bgcolor: #DDDDDD, colspan: 7, align: center"]Calibrated Anchor Chain - Strength & Weight Guide[/TH]
[/TR]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Size[/TD]
[TD="align: center"]6mm[/TD]
[TD="align: center"]7mm[/TD]
[TD="align: center"]8mm[/TD]
[TD="align: center"]9.5mm[/TD]
[TD="align: center"]10mm[/TD]
[TD="align: center"]12mm[/TD]
[/TR]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Weight per metre[/TD]
[TD="align: center"]0.85 kg[/TD]
[TD="align: center"]1.1 kg[/TD]
[TD="align: center"]1.45 kg[/TD]
[TD="align: center"]2.2 kg[/TD]
[TD="align: center"]2.3 kg[/TD]
[TD="align: center"]3.8 kg[/TD]
[/TR]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Approx Length per 20 kilos[/TD]
[TD="align: center"]24 mtrs[/TD]
[TD="align: center"]18 mtrs[/TD]
[TD="align: center"]13 mtrs[/TD]
[TD="align: center"]9 mtrs[/TD]
[TD="align: center"]8 mtrs[/TD]
[TD="align: center"]5 mtrs[/TD]
[/TR]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Weight per 30 metres[/TD]
[TD="align: center"]25 kg[/TD]
[TD="align: center"]33 kg[/TD]
[TD="align: center"]43 kg[/TD]
[TD="align: center"]66 kg[/TD]
[TD="align: center"]69 kg[/TD]
[TD="align: center"]114 kg[/TD]
[/TR]
[TR]
[TD="class: ltd, bgcolor: #EEEEEE"]Approx Breaking Load[/TD]
[TD="align: center"]22.7 Kn[/TD]
[TD="align: center"]30.8 Kn[/TD]
[TD="align: center"]40.2 Kn[/TD]
[TD="align: center"]-[/TD]
[TD="align: center"]62.8 Kn[/TD]
[TD="align: center"]72 Kn[/TD]
[/TR]
[TR]
[TD="class: nb, colspan: 7"]Kn = Kilo Newtons - Although not strictly the same you can translate 40.2 Kn, for instance, as 4 tonnes[/TD]
[/TR]
[/TABLE]



Assuming 60kg of chain, You get 41m of 8mm or or 26m of 10mm.
Asuming again say 5m of water, using John's formula..
Force = W(Lr^2-H^2)/(2H) where W is the weight of the rode (kg/m), Lr is the length of the rode (m), H = the height to which the rode is lifted (depth of water) (m)
Click to expand...

I make the force required to just lift all the chain off the seabed with 60kg of 8mm in 5m of water to be 192kg and for 60kg of 10mm - 120kg. (with a 0.8 Factor for chain weight in water)

Anyone care to check that? I'm wrong depressingly often ;)

But if right far from the real world looking at chain weight alone lighter is better.

Maybe :)
 
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