How Chain Size Impacts the Catenary

[SvenG]

I decided to explore the importance of chain size (weight) using calculations rather than anecdotal info. I ended up being surprised to find out how important the chain really is.

The reason the chain helps anchor performance is that it can help make the load on the anchor 100% horizontal, rather than have the pull on the rode pull/twist the anchor shank up from the bottom and possibly break the anchor free.

I've attached a graph of a typical catenary. If the anchor is attached at any point right of where the rode becomes parallel with the bottom the anchor will only experience horizontal loads at the loading that set up that catenary curve, or less (assuming the anchor isn't turned around).

There are any number of catenary calculators out there, I just happened to chose the one at www1.iwvisp.com/download/pub/spreadsheet/anchor.xls . I updated that one to allow me to compute catenarys for rodes up to 400' but otherwise didn't really make any changes to it. I attempted to check the math and as far as I can tell it agrees with others' calculations.

I did not explore catenarys for mixed rodes or using a kellet, but I ran cases for 1/2" line, and 1/4", 5/16" and 3/8" chain. The weight per foot in water would be slightly less of course, the line might even float.

I ran cases for 10', 30' and 50' depths.

I also ran both 7:1 scope and 3:1 scope examples. I assumed that the rode was attached at the water-line to keep it simple as all I was after was the relative loads, not absolute loads.

The number in the 7:1 and 3:1 scope tables show how many pounds of horizontal force can be applied at the water level of the boat and still keep the anchor flat on the bottom.

It shouldn't have been surprising, but it is pretty clear that with an all line rode you are yanking the anchor up as soon as you get around 10 to a 100 pounds of horizontal force. If the anchor is big enough or set well enough that force could of course be ok.

Another observation which seemed intuitively obvious (but I'd never done the numbers before) is that at 50' depth a 3:1 all chain scope will hold almost as well as a 7:1 all chain scope at 10' depth.

Maybe nobody else will find these numbers interesting but I did. If anyone has a pointer to a better analysis I'd love to look at it (as long as it isn't armwaving).



-Sven

[David M]

Sven,
I find it very interesting. I think most sailors know this intuitively. It is really nice for you to provide the actual numbers. Thanks.

[cal40john]

From Earl Hinz's book, 3/8" chain is suggested for 40-50' boats. ABYC table (in same book) for horizontal loads for a storm anchor which they say is for 42 knots of wind range from 2,400 to 3,200 lbs. So your calculations show the chain is pulling up at 1660 lbs, significantly lower than storm conditions. So not much different from the articles found on the Rocna site, but they approach it from the opposite side. Chain weight doesn't really matter since the catenary is gone when you are in bad storm conditions.

Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats

More articles (that have been repeatedly pointed out)
Anchors and Anchoring - Articles on Boat Anchors

John

[craigsmith]

You are focusing on the point at which the angle of pull on the anchor goes slightly positive. This point in reality is totally unimportant. Any decent anchor will handle a very high vertical component of pull. In other words you're using data that's irrelevent. Garbage in garbage out.

www.petersmith.net.nz/boat-anchors/catenary.php

[SvenG]

Originally Posted by cal40john

Chain weight doesn't really matter since the catenary is gone when you are in bad storm conditions.

The catenary is never gone, that is a physical impossibility.



-Sven

[SvenG]

Originally Posted by craigsmith

You are focusing on the point at which the angle of pull on the anchor goes slightly positive. This point in reality is totally unimportant. Any decent anchor will handle a very high vertical component of pull. In other words you're using data that's irrelevent. Garbage in garbage out.

Would you like to try that again ?



-Sven

[craigsmith]

Originally Posted by SvenG

The catenary is never gone, that is a physical impossibility.
-Sven

Of course it isn't, but it goes close enough. Read the material people are referencing you to...

Here's another one:
Tuning an Anchor Rode

You are imagining on some unexplained presumption that the point at which the angle of pull goes positive is important, and basing all your conclusions around that. It's totally fallacious. All that matters, and the data you should be using, is 1) when the anchor is likely to drag, and 2) what forces the boat is likely to put on the rode.

[SvenG]

Like I said, "long as it isn't armwaving".



-Sven

[Maine Sail]

Originally Posted by SvenG

The catenary is never gone, that is a physical impossibility.



-Sven

Craig,

It seems we may need to see some of those pictures of bar tight, no caternary left, all chain rodes again.. Impossible?

[craigsmith]

Well he's right there's still some catenary in them, it requires infinite tension (i.e. you'll break it first) to fully straighten anything under the influence of gravity. But you can get very close fairly easily. It's just an academic red herring, a wood-for-the-trees issue.

The mistake here is repeated frequently, assuming that the point at which the chain lifts off the seabed is somehow of importance.

There was a big long thread on YBW.com recently where a guy did exactly the same thing, only more involved with a long document written in a quasi-academic style. He at least established a basis for his data: he wrote that anchors can only sustain their own weight in terms of vertical pull before they will fail, so this was the right point to use, and the angle of pull must remain horizontal... Pointing out the independent example of West Marine's 2006 testing where anchors like the Rocna 15 (33 lb) sustained over 5,000 lb-force pulls at 5:1 scope (therefore 1,000 lb vertical component less any benefit from catenary [trivial]) didn't go down well.

Oh alright then:

[SvenG]

Why is it that every single anchoring thread on CF has to be about the soft bottom Rocna miracle anchor ?

Is there really no other aspect of anchoring anywhere that is worth discussing ?

Wow.



-Sven

[hummingway]

Originally Posted by craigsmith

The mistake here is repeated frequently, assuming that the point at which the chain lifts off the seabed is somehow of importance.

Hmmm ... some but apparently not all sailors intuitively get the original point. Certainly the Rocna salesmen don't get it.

A couple of thoughts. It would seem some what obvious that the greater weight of chain on the bottom the more wind before force is applied to the anchor? The more swinging causes an anchor to reset the greater chance of an anchor dragging? So miracle qualities of the rocna anchor not withstanding it would seem that more weight in the chain might provide better holding in some conditions?

Not all anchoring situationa re the same. This isn't just a function of bottom but also currents, tides and changing winds. Given a situation where a tide is going to turn you around 180 degrees in the course of a night it would seem to me that catenary has a role to play in anchoring.

Another point that is made in the rocna advertising that beyond 2/3 of scope the effect of chain diminishes but unless you anchor in the same depth every night I guess you better prepare for the worst case scenario, wouldn't you agree? So ... what depth do you decide to have 2/3 of (so you can then spend the extra money on a wonderful new hefty rocna)?

[mesquaukee]

Originally Posted by craigsmith

Well he's right there's still some catenary in them, it requires infinite tension (i.e. you'll break it first) to fully straighten anything under the influence of gravity. But you can get very close fairly easily. It's just an academic red herring, a wood-for-the-trees issue.

The mistake here is repeated frequently, assuming that the point at which the chain lifts off the seabed is somehow of importance.

There was a big long thread on YBW.com recently where a guy did exactly the same thing, only more involved with a long document written in a quasi-academic style. He at least established a basis for his data: he wrote that anchors can only sustain their own weight in terms of vertical pull before they will fail, so this was the right point to use, and the angle of pull must remain horizontal... Pointing out the independent example of West Marine's 2006 testing where anchors like the Rocna 15 (33 lb) sustained over 5,000 lb-force pulls at 5:1 scope (therefore 1,000 lb vertical component less any benefit from catenary [trivial]) didn't go down well.

The vertical component is irrevalent as long as the blade of the anchor is angled downward into the sea bed, ie. it will cut down deeper into the seabed as the load increases.

[cal40john]

Originally Posted by SvenG

Why is it that every single anchoring thread on CF has to be about the soft bottom Rocna miracle anchor ?

Is there really no other aspect of anchoring anywhere that is worth discussing ?

Wow.



-Sven

I didn't armwave, I pointed out data from Hinz's book, and referenced catenary data that happened to be from the Rocna site. You chose to focus on nitpicking on no catenary is impossible rather than taking my meaning of essentially no catenary.

The Rocna was used as an example, but it could have as easily been pointed out that 7:1 scope is widely used as it is the minimum scope to provide an 8 degree upward pull angle on the anchor. This goes back to the discussion of whether the point at which the lead pulling horizontally is important or not.

Ignore the word Rocna, tell me what points brought up don't make sense.

John

[SvenG]

Originally Posted by mesquaukee

The vertical component is irrevalent as long as the blade of the anchor is angled downward into the sea bed, ie. it will cut down deeper into the seabed as the load increases.

Do the vectors and you will see that it depends on the angle of the pull and actually reinforces the point about the importance of the chain rode.



-Sven