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post #41 of 84 Old 11-10-2013
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Re: Caternary & Chain...

We seem to be all over and around the point here.

We are only discussing one senerio: when dragging is due to wind strong enough and consistent enough that all of the chain is well of the seabed. Testing demonstrated that this starts at ~ 15-20 knots and by the time it's blowing hard (>40kn) the chain has effectively no centenary to absorb shock. Thus, we need enough scope and an effective snubber.

The reason for making the point is that this dragging senario is particularly bad in that it is often...
* too strong for us to make any changes,
* re-anchor or move,
* if we hit shore we may loose the boat, and
* sudden. One minute the chain was helping, the next it is not.
It is also bad because most of us don't experience it often and thus don't get to practice. Practice is high-stakes.

Strategies that have worked for many years in lighter airs and in protected spots fail. They means NOTHING regarding success in stronger winds, since the rules change. The angle of the shank is up and shock absorption fails, both at the same time and with little warning. This is the point.

If you are dragging due to tide shift or in lighter winds, there are more serious problems. Lotsa chain isolates us from basic unsound anchoring practices, since it will sink in the mud and that helps. There local places I go that have terrible holding but work find if I use a lot of chain. But I know that chain means nothing when it blows hard... I will be off and drifting, so I make other plans if wind is expected.

(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber




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post #42 of 84 Old 11-10-2013
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Re: Caternary & Chain...

In this area (west coast of B.C.) we cannot rely on shallow depths in most cases and rarely have the room for 10-1 scope. Many bays are small, crowded or both. Often there is 15' or 20' of depth close in where there is not much swinging room, move out and the depth is very quickly over 100'.

To quote Steve Dashew regarding anchoring in B.C.'s Desolation Sound

"...surrounded by a lot of 40' to 50' yachts. Water is 55' deep at half tide.We set the big Rocna at 2.5 to 1 scope and then with a firm bite shorten scope to 1.6 to 1 scope. If you calculate the angle off the bottom, combine the chain length with Wind Horse's length, we need about 150' of swinging room. Compare this to a 40' yacht anchored with a normal sized anchor. They will need at least 4-1 and probably 5-1 scope. Take their length and add to it 240' of chain, allow for angles and you have a radius of at least 220' at a minimum. The smaller boat with its undersized (by our standards) anchor takes a lot more room than the bigger boat with its oversized anchor. Is there a lesson here? Not only does it work in crowded anchorages, but it benefits you in secluded spots which might otherwise be too tight with a normal anchor. The anchor, regardless of design, will set faster, being bigger. This means it drags less before it digs in. This reduces the risk of fouling debris."

From another Dashew post: How big should the anchor be?
Yachts in the 30'-40' range - 60lbs
Yachts in the 40'-50' range - 80lbs


A large bay with 10 - 15' depth and lots of swinging room - I wish! I think from this and the PS test anchor weight is everything. Forget the catenary that will quickly disappear in a blow and keep the rode - whether all chain or a combination of chain and rope - light as long as it has adequate strength. This is probably most important for those without a windlass as they (me as well) have to limit the overall weight to something manageable. With a windlass an all chain rode and an oversized anchor is not as much of an issue.

Steve Dashew is very concerned with overall weight and its effect on performance, whether sail or power. His chain has been Grade 70 for many years.

Brian
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Last edited by mitiempo; 11-10-2013 at 01:17 PM.
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post #43 of 84 Old 11-10-2013
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Re: Caternary & Chain...

Quote:
Originally Posted by Maine Sail View Post
Just use more scope when it gets snotty. 5:1 can be adequate under many conditions, with many anchors, but some really need a shallower shank angle to continue to hold...

I think this data only serves to confirm what Peter Smith has been saying for years, put the weight in the anchor, not the chain....

The less scope you have the easier it is to lift the rode and minimize or nearly eliminate caternary...... The less scope you have the easier it is to shock load deck fittings. The less scope you have means the more elastic your snubber needs to be...
Well yes, but we knew (at least most of should have) this already.

All that we really have learned with this "new" info is that the bad habits we have talked ourselves into because we were on all chain are just excuses to not follow the old rules of anchoring.

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post #44 of 84 Old 11-10-2013
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Re: Caternary & Chain...

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Originally Posted by SVAuspicious View Post
Vocabulary is important. Catenary doesn't have anything to do with what you describe. .......
Then look it up in Webster.

A catenary is not only a dip in a line but also something in the form of a catenary. I was referring to the latter and you clearly understood what I meant.


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post #45 of 84 Old 11-10-2013
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Re: Caternary & Chain...

Quote:
Originally Posted by Minnewaska View Post
Then look it up in Webster.

A catenary is not only a dip in a line but also something in the form of a catenary. I was referring to the latter and you clearly understood what I meant.
Sorry - that I figured out what you meant when you misused a word doesn't mean your usage is okay. That simply isn't what a catenary means, particularly in a discussion relating to structural strength. Vocabulary IS important. Language is all we have to communicate with and if we don't use it properly we don't communicate effectively.

What (I think) you were describing is a valid and applicable factor in where a boat lies at anchor in light air. The word you used is not appropriate.

A catenary is:
Quote:
the curve assumed approximately by a heavy uniform cord or chain hanging freely from two points not in the same vertical line. Equation: y = k cos h ( x / k ).
The four dictionaries I checked concur. I did not find any definition that uses the terminology "similar."

Of course dictionaries do track common usage no matter how misleading. I'm still wound up over considering 'presently' and 'currently' as synonyms so maybe it's just me.

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Last edited by SVAuspicious; 11-10-2013 at 04:54 PM. Reason: updated punctuation
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post #46 of 84 Old 11-10-2013
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Re: Caternary & Chain...

Quote:
Originally Posted by SVAuspicious View Post
Sorry - that I figured out what you meant when you misused a word doesn't mean your usage is okay. That simply isn't what a catenary means, particularly in a discussion relating to structural strength. Vocabulary IS important. Language is all we have to communicate with and if we don't use it properly we don't communicate effectively.

What (I think) you were describing is a valid and applicable factor in where a boat lies at anchor in light air. The word you used is not appropriate.

A catenary is:

The four dictionaries I checked concur. I did not find any definition that uses the terminology similar.

Of course dictionaries do track common usage no matter how misleading. I'm still wound up over considering 'presently' and 'currently' as synonyms so maybe it's just me.
Come on professor. Looking this up took 5 seconds. Note 2.

Quote:
Definition of CATENARY

1: the curve assumed by a cord of uniform density and cross section that is perfectly flexible but not capable of being stretched and that hangs freely from two fixed points

2: something in the form of a catenary
catenary adjective
Catenary - Definition and More from the Free Merriam-Webster Dictionary


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Re: Caternary & Chain...

Quote:
Originally Posted by Minnewaska View Post
2: something in the form of a catenary
What you describe is most assuredly not in the form of a catenary beyond third order issues. Catenary is simply not relevant to what you describe and insisting on an inappropriate word simply takes away from what is otherwise a a valid point about the behavior of ground tackle in light air.

Clearly you don't like that. Fine. I won't belabor the point any further.

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post #48 of 84 Old 11-10-2013
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Re: Caternary & Chain...

I can't find the reference right now, but Rod Stephens of S&S is on record discouraging the use of a chain rode. If I recall, he believed chain stresses fittings and does not add to holding strength.

Hard to argue with Rod or his brother!

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Re: Caternary & Chain...

Quote:
Originally Posted by SkywalkerII View Post
I can't find the reference right now, but Rod Stephens of S&S is on record discouraging the use of a chain rode. If I recall, he believed chain stresses fittings and does not add to holding strength.

Hard to argue with Rod or his brother!

Skywalker
Hence the need for a snubber.

Brian
Living aboard in Victoria Harbour
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post #50 of 84 Old 11-10-2013
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Re: Caternary & Chain...

I think what is missing in this discussion about all chain or chain/rope is that the lighter chain/rope will be a straight line to the anchor, and therefore starting to lift the shank, in much less wind than the all chain. If it takes 160 lbs of force to lift the chain in PS's test it would take a small fraction of that to lift a length of nylon line and 30-40' of chain. So it would seem to follow that the same dynamics on the anchor would happen sooner (much lower wind speeds) than with all chain. Increasing the scope would lower the angle of pull somewhat but would not add weight (catenary) at all compared to all chain. No argument that when the wind speed increases to the point where the chain is bar tight there will be no catenary, but 300' of 3/8" chain is going to keep the pull on the anchor horizontal at much higher wind speeds than an equivalent length of nylon line. So I would assume that the same anchor wouldn't start dragging until the wind maintained enough force to lift the chain.
We use a 66 lb Bruce with 300' of 3/8" BBB as our main anchor on a 40' 24,000lb boat.

John
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