Caternary & Chain... - Page 3 - SailNet Community
Old 11-08-2013 Thread Starter
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Re: Caternary & Chain...

I did this a while ago to attempt to illustrate what scope looks like. It was done to scale using the shank length of my CQR as the shank in the illustration....

Anchor Scope Illustrated

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Last edited by Maine Sail; 11-08-2013 at 08:24 AM.
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Old 11-08-2013
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Re: Caternary & Chain...

Quote:
Originally Posted by denverd0n View Post
True, but that was the only picture that I could find easily to illustrate the calculations I was doing. I calculated the angle at A based on a 5:1 ratio. That is, a triangle where length of "a" would be 1, "c" would be 5, and "b" would be 4.9. That angle, when the chain is straightened out, would be 11.5 degrees.

Of course there will be some catenary, so in reality the angle at A would be somewhat less. Still, the point remains that the angle at A, even at "only" a 5:1 ratio, is pretty small.
I just thought your valid point needed a better illustration to clarify. Mainsail's link shows it well. The real crux is the design of the anchor as it pertains to angle of the flukes to ground. If the scope with 0 caternary reaches the upper limit of the anchor's ability to keep digging, then the anchor starts to detach the flukes from their design angle but this angle would be very large. Danforth and Fortress type anchors have the pivot point to make angle of rode even more secondary once the flukes are digging in. I know my Danforth can get REALLY buried over time, more so than my CQR or Bruce. With a straight pull on a 5:1 scope, a good anchor should bury deeper even if the chain is not dragging along the bottom. The actual "caternary effect" may be next to 0 as Mainesail is saying. I guess the real question is at what angle does an anchor begin to stop digging in and rise out of the bottom.

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Last edited by smurphny; 11-08-2013 at 09:42 AM.
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Old 11-08-2013
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Re: Caternary & Chain...

I get the impact of a gale and 4 ft seas in the anchorage. However, the impact of wind on rope v chain over the entire scale, not just when the first link is lifted, is going to be more applicable for the everyday cruiser in sheltered anchorages.

Without looking above, i think it was shown it takes 19 knots of wind to lift the first link of chain. However, the rope would be pulled taut by then. That must be two different angles on the shank for identical conditions.

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Old 11-09-2013
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Re: Caternary & Chain...

It's hard to argue with reputable test results but common sense appears to be my problem.

So the fact that the test showed that 190 lbs of pull will lift the chain to it's end should come as no surprise - after all, 100 feet of 8mm chain only weighs about 120 lbs.

In my case I use 10mm chain which weighs about 156 lbs so on a direct extrapolation my boat would have to impose a pull of about 270lbs before the end of the chain begins to lift.

What I don't know is what windspeed it takes for my boat to impose a 270lb pull on the chain. Neither do I care.

What I do know is that on a 7 to 1 scope, I have more than once sat through 45 knots without a drag of the anchor. But it was not a lee shore so there were no waves.

What I also know is that boats around the world have anchored millions of times using the above chains with scopes of 5-1 and 7-1 without dragging - they didn't know about this test. Up to now neither did I.

Will it change the way I anchor or my ground tackle? Not a chance. And I will continue to find places to anchor in places where the waves are not 3 feet and the wind speed is less than it takes to lift my anchor out of the ground.

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Old 11-09-2013
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Re: Caternary & Chain...

What I understand from this (and I have had this opinion for quite a while) is that for a given boat/anchoring system the weight is much more effective in the anchor than the rode. As long as there is enough strength in the chain extra weight means nothing. While the chain is needed in many locations for abrasion resistance the anchor weight is more important.

Some (Steve Dashew for one) use grade 70 chain coupled with a custom Rocna anchor. Their current Rocna is 250 lbs on a lean 83' powerboat named Windhorse. A Sundeer 64 weathered a hurricane in Grenada a few years ago without dragging while lying to a 176 lb Bruce anchor. Steve regularly anchors with short scope - 2.5 to 1, often less and hasn't dragged. Here is a link to a blog article by Steve on anchor sizing. SetSail» Blog Archive » More on Anchors and Sizing

My current setup is (for a 27' CS) 80' of 5/16" grade 30 plus rope and a 25lb CQR, which will be changed to a 33lb Rocna. I would use lighter chain but already have the 5/16. Were I to replace the chain it would be to 1/4" G40, and maybe reduce its length to 40' or 50'. I do not have a windlass.

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Old 11-09-2013
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Re: Caternary & Chain...

Forgot to add this.

Here is a list of anchors used by Steve over the years:

Intermezzo (50' ketch)...........100 lbs
Intermezzo II (62' cutter).......150 lbs
Sundeer (67' ketch)...............180 lbs
Beowulf (78' ketch)................225 lbs
Windhorse (83' unsailboat).......250 lbs

To quote Steve "when people start to laugh at the size of your anchor you're off to a good start"

Brian
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Old 11-09-2013
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Re: Caternary & Chain...

There was an article in Pacific Yachting based in Vancouver BC a year or so ago. It showed that an all chain rode in shallower water was not as good as a combo chain/line rode! As shown, a 5-1 chain will lift such that the anchor will pull out, meanwhile a line roade at 7 or even 10-1 will still pull straight.

Where an ALL chain rode did seem to help, was deeper anchorages at lower scopes. Here the chain wt would help. Otherwise, the article pretty much showed that an all chain setup was not as good as a chain/line setup. Anchors were the same size in this test/article!

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Old 11-09-2013
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Re: Caternary & Chain...

Marty, I read that article as well. Basic conclusion was that when catenary is needed the most, as in a strong blow, it didn't exist as whatever rode is used it is bar tight.

Brian
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Old 11-09-2013
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Re: Caternary & Chain...

Quote:
Originally Posted by mitiempo View Post
To quote Steve "when people start to laugh at the size of your anchor you're off to a good start"
great quote!
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Old 11-09-2013
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Re: Caternary & Chain...

What seems to be missing from the discussion is the fact that there is a widely variable amount of pull. Wave action, gusts, calms, the boat sailing at anchor: all these make the stress go from 0 to X pounds of pull. There is never a steady load on the anchor system. That's where the use of some sort of shock absorber in the line from boat to anchor raises a whole new set of parameters. Don't underestimate the pounds of stress a 35-40' boat can exert when swinging in a 30 knot wind. Nigel Calder's Cruising Handbook has a good chart of anchor stresses that shows 5/16" chain to be VERY marginal for this range of boats. I highly recommend that book.

Aside from the coral abrasion issue, there is a lot to be said for a rope/chain rode. Not only that but it is MUCH easier to "feel" the anchor set when using rope/chain. With all chain it is almost impossible to feel the anchor grabbing and work it in by hand because the chain wants to go straight down and create so much caternary that there is little indication on the boat end as to what the anchor is actually doing.

Last year I went to mostly all chain (120' of 3/8" G4) which doesn't get to the rope in most anchorages but I'm seriously considering shortening that again to maybe 40' of chain as I had before. It would eliminate the pita of having to deal with a snubber contraption and make it a lot easier on the hands (no windlass).

Alberg 35: With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship.
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