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I think it's really down to scope the anchor's ability to bury itself and the sea bed.

 

For a 10:1 scope, the example Chris has used, the % of the force in the rode that acts in the horizontal direction will be 99.5%. The % of the rode force in the vertical direction will be 9.9%. If the anchor is of a type that digs in easily and the sea bed has enough sheer strength, the downward force created as a result of the flukes digging in due to the horizontal pull, only has to have a vertical component of greater than 9.9% of the force in the rode.

 

When you reduce the scope to 5:1, the angle is increased to 11 degrees, which will give 98% of the rode force in the horizontal direction, but the vertical component has now doubled to almost 20%.

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Start with the Rocna Knowledge Base, they give a dissertation on cantenary on their site as well as links to others. Lloyds have test results also as do some American sites, Coast Guard I think. Also quite a bit of computer modelling by people designing mooring systems for oil rigs.

And I have some as well.

I read that it is almost impossible to pull chain bar tight - the forces required are enormous. However it is relatively easy to pull chain almost bar tight. I read why and don't understand the physics.

So I took my newly acquired at great expense 10m of 8mm chain and shackled one end to a lamp post. And correct, no way could I pull it bar tight. But, at a wild guess 50kg of pull got it tight to a mid point deflection of about 8cm, measured from a string line, nowhere near enough I would suggest to provide any useful cantenary in a rode and not enough that any of it would lie flat on the seabed. 8mm chain weighs 1.47kg per meter, 14.7kg for my 10 meter length. Put it into seawater with an average SG of 1.125 then it weighs 87% of what it does in air, about 12.6kg so I can see that chain weight does not do much in an anchor rode.

I am not saying that the anti cantenary effect brigade are correct, but they do come out with a lot of test data and computer modelling whereas the the pro cantenary brigade use the 'everybody knows' argument.

As I mentioned in an earlier post, 'everybody knows' that the chain component of a rode should equal the boat length. Well I don't know why this should be. But since everybody else knows this perhaps someone could enlighten me as to the correlation between the two.

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Ok Chris, now put 30m of warp on it, and try to pull it tight enough to lift the chain off the ground. No way you'll be able to do this, it will take several tonnes, if you are 5m or less above the anchor point.

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This is the test set up. Not much but the best the village of Lay St. Remy has to offer.

I can pull the chain tight to an estimated 8mm deflection. With 20m of 16mm braid... I can pull the chain off the ground but not continuously. I need to overcome the stretch factor in the polyester braid which makes it difficult. Be interesting to try with some Spectra but I don't have any of that.20190623_091651.jpg20190623_091624.jpg

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I'm picking the caternary effects of chain, and especially chain with some stretchy rode, is not so much in the static state of pulling, but in damping out any shock loading, which only occurs in the dynamic state of a boat dancing and tugging on the system when its blowing dogs off chains.

 

So in a round about way, it is damn near impossible to set up a test for the dynamic state, but it is the extra tugs, or shock loads, that pop anchors out. Once the anchor is out, its loads easier to drag (required force is lower). Anyway, what I'm meaning to say is that static tests wont show the benefits of the caternary. The real benefits would be in looking at the peak force applied to the anchor, in a dynamic state. The key issue isn't the angle of chain to the seabed at the anchor (on the proviso you've got half enough scope out) but the maximum force experienced by the anchor.

 

This would also be why all the 'no chain' arguements provide lots of analysis but no conclusion in response to the 'it just works' brigade. The no chain arguements aren't getting to the root cause of anchors popping out.

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unless you are going to do some open water motoring ( Issel meer perhaps ? ) I fail to see the need to worry too much, unless, as you have already said, if you are in a flood river you can end up in a farmers paddock and Joke wouldn't allow that,i'm sure!  ;-)   ;-)

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There are lots of conclusions drawn by the no-chain brigade. In fact the conclusion arrived at is that in shallow water, say the 5 -8 meter mark, chain cantenary does absolutely nothing in terms of improved holding. There is some benefit in deeper water. In fact they state that an all chain rode will assist the anchor in dislodging because in heavy weather an all chain rode is as good as bar tight so there is no give in it therefore shock loadings are transmitted directly to the anchor. And I can confirm that anchored in two cyclones. In the second I rode to all chain to prevent chafe in the fair lead but was obliged to put in 8 metres of nylon to stop the pretty vicious snatching. This was on 30 metres of chain in 4 metres of water. That's where my questioning of chain started. Don't forget, none of this applies to an old school anchor. With the new high hold anchors the holding power of the anchor on its own far exceeds any increase in holding power that can be added to it by the chain. This is the reason that modern anchors are designed to withstand a certain degree of upward pull on the stock without any big degradation in holding power. Ask me to verify that and I can't - after the best part of a day reading and being directed ever onwards from one site to the next, I can't find it back.

So the opinion of the no-chain brigade, and that's a misnomer, obviously they advocate a few metres of chain for chafe and to limit the boat sailing around its anchor, but their opinion is that the answer lies in simple geometry, and an 8:1 scope is the magic number. This amount of scope is not needed in light weather of course but when it starts blowing its knickers off, there are big reductions in the seabed/shank angle down to 7° at 8:1 which is well within the capability of modern anchors to handle. Beyond that it's a case of vanishing returns as geometry shows.

But Steve is right. In my case it's all academic as I have bought my piece of chain to shackle onto the anchor which I will never use, but the authorities are now content.

But it interests me because I like to understand.

Disclaimer. Whilst I lean towards doubting the huge improvements in holding power attributed to a chain rode, I don't have the mental agility required to critically assess all the data that I have read.

Before we came to France for the first time we were a bit nervous because we were told watch out for the French, unhelpful rude arrogant buggers. And be careful on the canals. They dump their old cars, washing machines... many are almost clogged up with rubbish, everybody knows that. And don't ride your bike in France, you won't survive a week - woohoo, put another chalk mark on the dashboard, got another one! And never leave your boat unattended on the Canal side. They'll steal everything off it, everybody knows that.

Well none of that has happened to me, nor any of the other many canallers we have met.

Twice now I've asked the 'why' question on the chain length to boat length rule that everybody knows is correct, but no answers have been forthcoming.

Is this because although everybody knows, in fact nobody knows?

Research. Research. Research. Arrive at your own conclusions. You don't want to do something just because, well, that's the way we do it round here.

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Now't wrong with research, except with todays glut of information you can often end up more confused than when you started! Ask an expert? well they are just like the rest of us, they rely on the same research as the rest of us, except they stop researching earlier, either they have found the holy grail or they think OK that's good enough, or "perhaps" if I say it authoritatively enough they (the punters) will believe / accept it as gospel????

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Having dived on my anchor a few times over summer - anchored in 5 metres or less of water with plenty of chain out - not often have i actually ever seen the anchor taking the load apart from when rafted up and its on a funny angle (One of many anchors).
Other than that normally anchor is side ways and chain weight on the ground and catenary is taking the load.
Vindex 350 with 8mm chain and a 50lb manson surpreme (i like to sleep at night and also raft up often).

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We have stopped for the night on the Meuse, a very nice spot but no bollards. So, made fast to a tree aft and to our baby pool anchor forward.

Here's a picture. Now don't tell me that chain is required on a rode!!20190624_144304.jpg

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Now you've said that out loud Murphys Law would suggest that some stage over night a rat will chew through your line :) :)

He He. Early morning coffee on the back deck. Still here..

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The chain is required to set the anchor in a bit of wind without it bouncing around and ressetting when the tide and wind turns. You dont swing around in a canal.

 

We have a small aluminium anchor on a couple of meters of chain that we take ashore and bury deep in the sand that holds fine. It is great for puliing the back of our boat close to shore to hop on and off.

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Yep, more load at boat end.

Some energy is absorbed in moving the weight of chain, some in caternary effect, some in moving the chain thru the water column, across the seabed, and probably some other stuff I have not thought of....

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OK here's a question that can play with the head a little bit. 

 

 

You've anchored your boat using a 5:1 scope using an all chain rode. There is some weather so there is some load right thru the system.

 

One end of the anchor chain is connected to a load cell on the boats stem head. The other end of the chain is connected to a load cell on the end of the anchor.

 

Do both load cells show the same load or does one show a higher load than the other?

The one with the greatest vertical component will show the greatest load, i.e. the one on the stem head. If all the chain is off the seabed, the stem head load cell will show the weight of chain as a force greater than the anchor load cell.

Gravity and all that. Its why in calm conditions the boat can anchor on chain only, and happily swing around without setting the anchor.

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hmmmm... presumably the load generator end, the boat, gets bigger spikes from the major jolts from gusts etc

 

which gets attenuated down the chain through caternary? action 

 

BUT

 

if a situation occurred were the chain was bar-tight 

 

then anchor end would up carry the load of the boat + chain into a tiny area of

 

not good

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