How much anchor chain?

noelex said:
... This type of slow transition gives the anchor plenty of time to shuffle. With an all rope rode the boat position would have changed very rapidly leaving little time for the anchor to rotate. In addition if the wind change is rapid, the boat can build considerable momentum increasing the chance that the anchor is going to to flip (although this is unlikely) rather than the more secure “shuffle” or slow rotation.

awc3ML5.jpg
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While I'm a believer in power setting the anchor (in large part to test the bottom), I also recognize that when the wind changes and blows hard, the anchor will have to move. The shank will have to come up to the surface to do this. Are there exceptions? I have not seen them when testing veered anchors. Will the anchor then shuffle and rotate, or break-out and re-set, or do something else? Which is better, an anchor that shuffles easily on the surface (big Mantus), one that sets very deeply but breaks out when it turns (smallish Fortress), or something in the middle (Rocna)? Spade is somewhere in there too, setting deeply but rotating more smoothly than some.

I don't claim to have an answer to this question or even a strong opinion. I think it is a fascinating puzzle. I like deep set anchors, but I recognize that in some circumstances they can and will be brought to the surface by rotation. Thus deep setting is not a cure-all. I'm also sure there are hybrid behaviors that I have not described.

Opinions?
 
noelex said:
Here is an example of an Ultra anchor rotating to a new wind direction. You can see the puffs of sand produced as the anchor slowly rotates. It is not doing a great job, with a very large list, but to be fair it was not set very well to begin with.

6JM6LmP.jpg


The chain is along the seabed so the anchor is being pulled at 0°, helping the anchor stay engaged with the seabed as it rotates. If this was on an all rope rode, the rope would be elevated at the scope angle (12° for 5:1), as with even a small force the only slightly negatively buoyant rope will have zero catenary.

If using all rope rode this 12° upwards force angle on the shank would increase the chance that the anchor will completely break out during the rotation.

Chain is very heavy. It is worth considering “spending” this weight elswhere when practical such as increasing the size of the anchor. Generally this will result in more benifits. However, I think it is wrong to percieve the weight is the chain as someting that has no value.
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Good example.

Also note that with grass, the root generally keep the shank and chain on the surface, and the fluke will be just under the roots. No deep burial. In fact, any bottom with significant debris on the surface will prevent deep penetration by buoying the shank and chain. I've seen sticks do this countless times.
 
Ok, more snorkelling needed on the wind force to start the boat wandering around the anchor not the chain. I’ll stick with my estimate of 5 knots wind for 10mm chain to slowly straighten the chain and to wander round the anchor but I’m now much less sure. I am still sure it’s a lot less than 20 knots.
 
RupertW said:
Ok, more snorkelling needed on the wind force to start the boat wandering around the anchor not the chain. I’ll stick with my estimate of 5 knots wind for 10mm chain to slowly straighten the chain and to wander round the anchor but I’m now much less sure. I am still sure it’s a lot less than 20 knots.
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I would not like to define a set windspeed (but 5 knots is too low). There are so many variables, particularly the substrate. Some bottom types can grip the chain quite well, while on smooth hard sand the chain will slide more easily.

The pattern is always interesting. You can see that there has been a small obstruction roughly where I have shown the red cross in the attached diagram. The yellow line indicates how the chain would have led around the obstruction to produce the arc centred on this point. The boat rotated about this obstruction for some time.

bwkX6r5.jpg


This type of pattern is not unusual. I often dive and find these “obstructions”. Typically they are surprisingly small, such as a clump of weed or a small rock. You can see by the multiple points in the “breadcrumb” trace that after the chain released itself from the obstruction the boat still moved backwards quite slowly.
 
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noelex said:
I would not like to define a set windspeed (but 5 knots is too low). There are so many variables, particularly the substrate. Some bottom types can grip the chain quite well, while on smooth hard sand the chain will slide more easily.

The pattern is always interesting. You can see that there has been a small obstruction roughly where I have shown the red cross in the attached diagram. The yellow line indicates how the chain would have led around the obstruction to produce the arc centred on this point. The boat rotated about this obstruction for some time.

This type of pattern is not unusual. I often dive and find these “obstructions”. Typically they are surprisingly small, such as a clump of weed or a small rock. You can see by the multiple points in the “breadcrumb” trace that after the chain released itself from the obstruction the boat still moved backwards quite slowly.
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What is the sampling rate?

Yes, interesting.

There is also a potential clue about catenary. The chain obviously had to have been on the bottom. Later, during the reversal, this is less clear, though it did snag a few more times.

What this says about nylon rode is less clear (nylon rode does not lay on the bottom, only some of the chain portion).
 
All of these posts about the advantages of chain are ignoring one very important point - the OP has a light performance boat, probably with no windlass. Hauling chain by hand will be inconvenient, to say the least,. The weight of chain on the bow will reduce performance.

So for him, where he plans to anchor, 5 m of chain and the balance with rope will be fine.
 
noelex said:
You make some good points Rupert. I dont think this stuff is simple. A couple of comments:


If you have an all chain rode and the chain is the normal recommended size for your boat (not thin and lightweight G70) the portion of the chain close to the anchor will stay on the seabed until quite late in the setting process.



If your ideas are based on this premise they may seem very persuasive its a pity they are:

Incorrect.

A 8mm chain in 6m of water at 5:1 scope - all the chain will be off the seabed with a tension of about 78kg (which is the tension in the rode at about 17 knots). If you don't believe the numbers (or the answer) plug the numbers into a catenary calculator and you will find theory and practice match. A 15kg anchor when fully set to its ultimate hold will have a hold of about 2,000kg. A decent rule of thumb for tension developed is about 100kg per 10hp of engine at full throttle.

All the chain, looking at the 8mm case and the 15kg anchor - a fair combination and at a realistic scope and depth - will be off the seabed as soon as the setting process commences. As the setting process develops and the chain straightens - then the chain will 'look' as near as makes no difference - straight (a bit like an all rope rode). When you reach the end of your power set and are using max revs for a few seconds that chain will lack catenary and the angle will not be much different to the all rope rode

How do I know - I've measured it.

You can change the numbers slightly - by having a smaller chain, or a larger chain - but the key word is 'slightly' (unless you are suggesting people use 12mm chain on a 35' yacht). When you check the numbers - plug different chain sizes into your calculator - and tell us the numbers - stop guessing

What is true is that a bigger anchor will not set as fully as a smaller anchor - under the same tension.

You quote your Vesper plot illustrating a slow change of angle under 20-25 knots of wind - at that wind speed with the same rode I mention - all the chain will be off the seabed at 17 knots - the chain will not slow the yacht, except in lulls - and in lulls the tension in the chain will be sufficiently low not to impact the anchor.

You can deploy more chain, that will change the numbers - except you may not have room to deploy more chain.

So - go back and rethink your ideas - and come up with another explanation that relies more on figures that are closer to correct - not wild guesses. You plausible explanations will be more palatable if you use the right numbers.

Jonathan

Noelex

I find it strange that on your previous yacht you used smaller sized G70 - not once did you mention the downsides of the smaller chain - and I do recall you made the occasional post on anchoring....

On your Photos of anchor thread - not once did you mention the downsides of your smaller chain. it was never mentioned. I can think of a number of occasions you actually recommended people convert to G70 - with no listing of the downsides.

You sound like one of those converts to a new political belief - evangelical and bigoted in your criticism of those who followed your old beliefs. It is also interesting that Dashew, on whom you based your original doctrine, and who has a high reputation as a blue water yachtsman and builder of 'expedition' vessels - does not appear to have changed his beliefs - he puts his money where his mouth has been for decades - and continues to use G70 chain on his own yacht and those he sells.

Part of your Mantra was - a bigger anchor is safe at short scope. Now I thought this dangerous nonsense (and implored you, without success to justify the statement) and in the absence of any support I assumed (and hoped others did as well) that it was nonsense and based on your fallible 'gut feel. I assume you have an underwater camera and this is such an interesting philosophy I assume you must have taken one or 2, or in keeping with your album, many 10s, of pictures of your, G70 rode, lifted at an angle of 10-15 degrees illustrating the security of your anchor - as just one facet of your support for the mantra.

Now is your chance

Or will it be another dangerous and inaccurate statement we will be expected to forgive and forget?
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I am only aware of one set of data, apart from limited work I have commenced, of the shackle angle of a ‘set’ leisure anchor which happens to include a comparison with the angles of the rode at the seabed. The data below is based on a mathematical analysis used by the US Navy in the development of fleet moorings.

The analysis was made by R Taylor who acted as a consultant for Fortress at the Chesapeake Mud tests a couple of years ago. R Taylor is now enjoying a well earned retirement having worked with the NCEL for most of his career - except for odd forays into consultancy. He holds patents on anchor designs, used by the US Navy, worked with Peter Bruce and some of the oil exploration companies and remains one of the best known anchor ‘experts’ in America. He appears to have been involved in some capacity in all of Fortress’ anchor tests.

There are 2 sets of data.

A Fortress anchor with the flukes set at 45 degrees at ultimate hold, of 2,100lb, with the shackle at 11’5” below the seabed.

At 5:1 scope the line angle at the seabed 15.2 degrees and at the shackle 49.7 degrees.

At 6.5:1 scope the line angle at the seabed 11.5 degrees and at the shackle 49 degrees

At 8.1:1 scope the line angle at the seabed 8.7 degrees and at the shackle 48.5 degrees.

He makes the comment that changes of scope have minimal effect on shackle angle.

Now these angles are illustrative but not very applicable as it is unlikely any of us will subject our anchors to a tension of approximately 1t.

He made a second calculation at 500lb tension, a more realistic tension and one many of us could acivieve in power setting out anchors - recall this is in mud and the shackle buried to 3’ (or around a metre). Note also this is the maximum tension developed by fixed fluke anchors - Spade, Rocna etc in the chosen test seabed.

At 5.1:1 scope the line angle at the seabed 12.1 degrees and at the shackle 35.9 degrees

At 6.5:1 scope the line angle at the seabed 9.1 degrees and at the shackle 35.4 degrees.

He again made the comment that changes of scope have minimal effect on shackle angle.

But the idea that scope determines the tension angle on the anchor needs to be modified and the tension angle is effectively independent of scope (in mud).

Jonathan
 
bbg said:
All of these posts about the advantages of chain are ignoring one very important point - the OP has a light performance boat, probably with no windlass. Hauling chain by hand will be inconvenient, to say the least,. The weight of chain on the bow will reduce performance.
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Errrr .... I thought that all those points, rather than being ignored, had been discussed ad nauseam. :confused:

Richard
 
So the rode is deflected by the seabed, no surprise there really.
Hence the shackle angle is not equal to rode angle.
But that does not, in any way, mean that the rode angle does not matter.
When the anchor is moving through the seabed as it digs in, the rode above the seabed needs to be more horizontal than the rode in the seabed, or there is no hozontal force to cut the rode through the seabed and bury the anchor.

Once the anchor and some chain are in the seabed, the shackle angle is not that fascinating, as we are not in the business of just pulling the anchor through the seabed, we are in the business of pulling the anchor and a (hopefully increasing) length of chain through the seabed.
I imagine the length of chain buried in the seabed will be a curve, probably a bit different from a true catenary....
 
a. The Fortress at 45 degrees is powerful but well known to be unreliable at setting. Fortress uses a default setting 32 degrees, as do other pivoting fluke angle makers. Scope anchors are a bit lower, depending on the make. The 45 degree setting is really a theoretical mooring setting of limitied practical interest to cruisers.

b. The Fortress design is sufficiently different from scope anchors that this data is really only of theoretical interest. It is well know that a storm buried Fortress can be nearly impossible to recover. They broke the rode off one in the Solomons Island tests... in soft mud!

Thus, we need to look at Mantus and Rocna data. They will show holding capacity drop offs at short scope. The more shallow the fluke angle, the greater the reduction. Additionally, the digging angle is NOT proportional to the fluke angle. By the time you get to Mantus (about 25 degrees fluke angle) the anchor will tolerate only limited shackle angle (10 degrees?) before leveling out and digging no further. Basically, the shank stays near or on the surface at short scope. Consider that the Fortress shackle angle was about 35 degrees. Simply subtract from the fluke angle and you get 10 degrees. Subtract 10 degrees from the 25-33 degree angle of most anchors and you get 15-23 degrees, or the up lift angle of short scope, with only minimal allowance for rode deflection. They can bury the rode, but not as far before they level out.

The other problem is that this reflects perfect burying behavior. In that case, any properly sized modern anchor will hold at relatively short scope. Easy. But we drag when something has gone wrong. The substrate has a hard underlying layer. The anchor hits a soft spot. The anchor has to reset and either clogs with mud or fouls on debris.

IMHO, what we are discussing is the remarkable mooring capabilities of the Fortress design. Important, but separate from reality.
 
thinwater said:
a. The Fortress at 45 degrees is powerful but well known to be unreliable at setting. Fortress uses a default setting 32 degrees, as do other pivoting fluke angle makers. Scope anchors are a bit lower, depending on the make. The 45 degree setting is really a theoretical mooring setting of limitied practical interest to cruisers.
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I think Thin you need to get out more.

We carry and use a FX 37 (the size bigger than recommended) with the flukes set at 45 degrees for application in thin mud. Nothing else works. The 'equivalent' fixed shank anchors (any of them - simply develop insufficient hold - as evidenced by the typical hold of 500lbs, say 200kg. This is equivalent to a wind speed of less than 30 knots.

As you mention the hold developed are ideal - and in real life might be less - but the Fortress at 45 degrees stands head a shoulders above the same anchor at 32 degrees and any of the fixed fluke anchors - Rocna, Supreme, Spade, Ultra, Mantus were all tested (by Fortress) and I can confirm our Excel is no better (and I have tried Spade, Rocna and Mantus ourselves).

I must confess I have never 'tested' a FX 37 and only used it in anger - that last time to secure us against a short lived, 30/60 minutes, frontal passage with 55 knot winds and lots of rain (named as a 'Southerly Buster' (Google it) in New South Wales).

We also carry a FX 16 set at 32 degrees - I cannot be bothered with the faff of changing angles and the FX 37 is simply too big, unnecessarily so, for sand.

But to suggest Fortress at 45 degrees is not for cruisers and only for 'moorings' is not substantiated by our actual experiences. We would not sail without one.

Frankly if I were laying a mooring - I'd use Danforth - they are cheaper. Interestingly there is a fixed mooring laid not far from our swing mooring - specially designed with Superyachts in mind. It is designed with 3 Danforth set at 120 degrees to each other with fluke seabed angles of 22 degrees - because it is easier to retrieve the anchors when the mooring needs to be serviced (one of the compromises of a shallow setting anchor - might crystallise a mind or two. The mooring has been approved by a Classification Society member.

What we have found is that where there are oyster farms, or wild oysters, there is a good chance of thin mud - and the need for the big Fortress.

Jonathan
 
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rgarside said:
It will be a catenary, but its shape will be a function of the shear strength profile of the material it is passing through.
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+1

And, as evidenced by the calculations, the angle of the shackle will be the highest of anywhere on the buried rode.

As Thinwater defines (in his post 131) - a shallow setting anchor will reach its ultimate hold at a much lower hold than one that sets deeply - Mantus being the classic example. You can overcome the shortcomings of a shallow setting anchor by oversize - but you will get more hold per kg of anchor with a better design - and maybe also get more hold for you buck (depends on how much (or littler) the anchor costs. Most of the time it might not matter - but in those 'difficult' seabeds it might not be too hard to exceed the hold of a recommended sized shallow setting anchor.

You can manipulate the shackle angle, slightly - by using thin chain, a smaller shackle (but you still need the shackle to be sufficiently strong) and removing the swivel - the Mantus swivel (despite being an amazing piece of clever engineering) is the biggest of the lot (and worst offender).

Jonathan
 
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For those who worship the cult of catenary some of this makes a good read:

http://www.cruisersforum.com/forums/f118/g70-chain-vs-g4-225019.html

If you cannot be bothered with the whole thread then post 28 is a good summary by Dockhead.

There is a lot of rubbish in the thread on the characteritics of various qualities of rode
- the blind trying to educate the blind - but then it is the internet.

Jonathan
 
Neeves said:
I think tThin you need to get out more.

We carry and use a FX 37 (the size bigger than recommended) with the flukes set at 45 degrees for application in thin mud. Nothing else works. The 'equivalent' fixed shank anchors (any of the - simply develop insufficient hold - as evidenced by the typical hold of 500lbs, say 200kg. This is equivalent to a wind speed of less than 30 knots.

As you mention the hold developed are ideal - and in real life might be less - but the Fortress at 45 degrees stands head a shoulders above the same anchor at 32 degrees and any of the fixed fluke anchors - Rocna, Supreme, Spade, Ultra, Mantuys were all tested and I can confirm our Excel is no better (and I have tried Sp[ade, Rocna and Mantus ourselves).

We also carry a FX 16 set at 32 degrees - I cannot be bothered with the faff of changing angles and the FX 37 is simply too big, unnecessarily so, for sand.

But to suggest Fortress at 45 degrees is not for cruisers and only for 'moorings' is not substantiated by our actual experiences.

What we have found is that where there are oyster farms, or wild oysters, there is a good chance of thin mud - and the need for the big Fortress.

Jonathan
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Then let's just say experiences differ. I tested at 45 and 32 side-by-side and found that 45 was POWERFUL when it worked... which was only about 60% of the time. 32 degrees was powerful enough and it always worked. You are right, if I needed more hold for a vicious storm I could and would use the 45 setting, and I would gladly take any extra time required to get it right.

My real point is that 45 degrees (Fortress) and 25 degrees (Mantus) are worlds apart. We agree on that.
 
Neeves said:
For those who worship the cult of catenary some of this makes a good read:

http://www.cruisersforum.com/forums/f118/g70-chain-vs-g4-225019.html

If you cannot be bothered with the whole thread then post 28 is a good summary by Dockhead.

There is a lot of rubbish in the thread on the characteritics of various qualities of rode
- the blind trying to educate the blind - but then it is the internet.

Jonathan
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Some of the difference is shallow vs. deep water. Much of it is strong wind vs. 30 knots (which is breezy but not strong at anchor). For those that don't like calculators, you would think that either simple trig (weight of about 1/2 of the rode in the center vs. a load of a ton or so) or the thought of really laying into a chain with a come-along (you did this, IIRC), would make it clear. I did something similar with kellets, just for fun, with the result that a kellet only works in quite light winds unless it weighs as much as a rode (over 100 pounds). In fact I sometimes use a kellet with rope rode, but ONLY to control swing and make the rode behave a little more like chain in very light winds. NOT because it absorbs shock.
 
thinwater said:
Then let's just say experiences differ. I tested at 45 and 32 side-by-side and found that 45 was POWERFUL when it worked... which was only about 60% of the time. 32 degrees was powerful enough and it always worked. You are right, if I needed more hold for a vicious storm I could and would use the 45 setting, and I would gladly take any extra time required to get it right.

My real point is that 45 degrees (Fortress) and 25 degrees (Mantus) are worlds apart. We agree on that.
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I confess I have never had an issue with the Fortress setting in thin mud (except for the mess it makes when you retrieve it - but then this is true of any anchor in thin mud). We would always power set, gently to start with and be patient. If you attack the seabed with aggression and speed - the anchor will not set - and simply swim through the soup.

The 45 degrees Fortress and 25 degrees Mantus - if this refers to fluke/shank angle I would agree, but its not really relevant as its not only fluke shank angle that it is important but how the fluke addresses the seabed, ideally 30 or 45/50 degree - depends on the seabed and anchor) - and fluke/shank angle is only part of the equation. Centre of gravity and centre of effort need also be considered - and Mantus and Fortress (and Mantus and many other anchors) are worlds apart to the detriment of the performance of Mantus (especially when looking at kg of hold vs kg of anchor weight).

Interestingly the new Viking, considered by some as a Mantus clone, - appears to have addressed the issue (so it is not a clone :) ) - based on an article the designer read in Practical Sailor, or so he said in Cruisers Forum. It is really easy to modify Mantus to make it perform much better - but then it would be a clone of a Viking :)

Jonathan
 
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