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Old 06-01-2012, 21:58   #1
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Chain Versus Rope Scope ?

I understand the why of having a 5-1 or 7-1 scope when using chain, but why would the same hold true for rope?

With chain, the weight acts as part of what holds you in place, along with the anchor. With rope, that is not the case.

It would seem to me, that once you have enough scope out for the anchor to set as it is designed, that the rest would only be needed to allow for the variance in wave or swell action, along with tides.

In other words, once you have allowed enough scope for that, any additional over and above should be irrelevant.

What am I not understanding about the concept?
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Old 06-01-2012, 22:37   #2
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Re: Chain versus rope scope?

I am not the best person to answer this...
But, the weight of the chain helps hold the anchor down to "bite" at the correct angle. Not having any chain will make it difficult for the anchor to stay down and set.
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Old 06-01-2012, 22:53   #3
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Re: Chain versus rope scope?

scope is a ratio of rode length to depth from rode attachment point. It is the same whether rope or chain. If the pull is strong the catenary of chain adds little holding, unless the depth is great, and the elasticity of rope helps holding, which is why you need a rope snubber with chain. The main advantage of chain is not holding, it is cut/abrasion resistance. Main disadvantage is weight.
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Old 06-01-2012, 23:24   #4
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Re: Chain versus rope scope?

My understanding of scope is that the more you have the more horizontal load you put on the anchor and the less vertical load. The less vertical load you have, the less likely the anchor is to pull out. Think of the angle formed between the rode and the bottom, at a 1:1 scope the angle is 90 degrees and the load is all vertical, no horizontal and no holding power. At a 5:1 scope the angle is only about 12 degrees (I think?) and much more of the load is horizontal. If you could have the rode completely flat on the bottom you would have all horizontal load and maximum holding power for the anchor. Adding weight to the rode wether it is chain or a kellet increases catenary in the rode and reduces the angle even more increasing the holding power.
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Old 07-01-2012, 02:14   #5
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Re: Chain versus rope scope?

Quote:
Originally Posted by SailFastTri View Post
scope is a ratio of rode length to depth from rode attachment point. It is the same whether rope or chain. If the pull is strong the catenary of chain adds little holding, unless the depth is great, and the elasticity of rope helps holding, which is why you need a rope snubber with chain. The main advantage of chain is not holding, it is cut/abrasion resistance. Main disadvantage is weight.
+1

Exactly right. Scope is all about the geometry of pull on the anchor. The higher the angle of pull, the more the anchor will tend to be pulled out of the seabed.

It used to be thought that the catenary of chain altered the angle of pull and aided holding. By the same logic some people used to slide "angels" or "chums" down their anchor rode, to keep the rode at a lower angle to the anchor.

Then someone proved (there's an article about it somewhere; I forget) that neither chain nor angels helps holding, because as the load increases, the chain becomes straight anyway -- that is, catenary from the weight of chain disappears exactly at the moment when you need it.

Still, of course, after a certain weight of chain, the load required to straighten it out does get pretty large. My main anchor road is 330 feet of 1/2" chain, which weighs nearly a thousand pounds. It's true that in extreme conditions it won't be any better than rope, but in other conditions a thousand pounds of chain does help. Besides lowering the angle of pull, the weight of chain also helps dampen snatch loads, but again -- only if the load is not so great as to make the rode bar-tight.

So rope is probably better in every way except for one crucial aspect -- chafe. Chafe can kill you in extreme conditions, so this is not a minor matter.
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Old 07-01-2012, 11:04   #6
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Re: Chain versus rope scope?

Great clarification.

Who ever said there is no such thing as a stupid question was right; thanks all.
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Old 07-01-2012, 11:49   #7
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Energy think energy. Chain absorbs energy while it is being lifted expending energy. This is useful if it's blowing snot and the boat is snatching at the hook. The snubber does a similar energy distribution. once everything is tawt the energy transferred from boat cleat to hook is the same wether it be chain or rope. The ground had a better distribution of energy if the anchor pulls into density. Pulling up had less density. That's how I understand it. The idea of transferring energy is fundamental in all parts of sailing.
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Old 07-01-2012, 11:57   #8
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Re: Chain versus rope scope?

My understanding is that once an all chain rode is tight (straight) it has no ability to absorb shock anymore whereas rope can still stretch a bit more and absorb additional shock.
From all of my experience as a mechanic I can tell you that shock is the best way to loosen something that's tight.
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Old 07-01-2012, 12:17   #9
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pirate Re: Chain versus rope scope?

Quote:
Originally Posted by sww914 View Post
My understanding is that once an all chain rode is tight (straight) it has no ability to absorb shock anymore whereas rope can still stretch a bit more and absorb additional shock.
From all of my experience as a mechanic I can tell you that shock is the best way to loosen something that's tight.
And my understanding is this is when one lets out more chain... change the ratio's... 5.1.. 7.1 is not written in stone...
Apart from E Coast... sorry could not resist...lol
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Old 07-01-2012, 12:38   #10
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Re: Chain versus rope scope?

Chain or line rode, both have their advantages. I went with 60' of chain with the rest multiplait rode. Reason being, my environment here and for now.

I try to anchor in 20-25' of water, calculating for a low tide. Drop anchor and all the chain to the bottom, then just lift enough to feel the chain end come off the bottom. And then back down on that until the boat stops.

Once I have the anchor set this has been 100% secure, so far. There's enough chain down to act like all chain but not so heavy to haul around on a lightweight vessel. Plus I get the snubber effect.

If I had coral to worry about then I'd go all chain and may do so before heading South. Although, one can still use this method buy lifting the chain about 10' off the bottom, in fair weather. I really don't like to anchor in coral anyway. I prefer to dive down and tie off to a large coral head and leave a float, if I plan to stay for awhile or come back.
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Old 07-01-2012, 12:41   #11
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This thread proves how hard it is to do away with all the silly talk about the importance of chain weight. It was 20 years or so ago that Dashew stated something like every pound of weight in the anchor provides 10-100x as much holding as a pound of weight in or on the rode.

Amd still I see guys tying killets and what have you to their hefty chains which all is attached to a silly little anchor.

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Old 07-01-2012, 12:58   #12
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Re: Chain versus rope scope?

Quote:
Originally Posted by s/v Jedi View Post
This thread proves how hard it is to do away with all the silly talk about the importance of chain weight. It was 20 years or so ago that Dashew stated something like every pound of weight in the anchor provides 10-100x as much holding as a pound of weight in or on the rode.

Amd still I see guys tying killets and what have you to their hefty chains which all is attached to a silly little anchor.

cheers,
Nick.
I really don't see the weight of the anchor as any help other then to get it started into the bottom. It's the amount of surface area it can grab and keep (plow down) that I look at. The chain is there to keep the anchor shank as horizontal to the bottom as possible. After all, the way to retrieve the anchor is to lift the shank straight up and over, releasing it from the bottom.

So with that, one needs enough chain to keep the boat from lifting the shank, and that's where scope comes in. The harder the blow the more scope, and then maybe a killet or two.
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Old 07-01-2012, 12:59   #13
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Re: Chain versus rope scope?

i love how folks who think chain doesnt do anything much find when they use 30 ft of chain with a 45 pound anchor that they ALWAYS drag anchor. when more chain is added there is no dragging. go figger.
i used 100 ft chain with my ericson and never dragged. i use 100 per cent chainh rode with formosa and dragged only one time, when wind exceeded the recommended max speed for anchorage i was in.
of course, everyone has a different story, and there is no allegedly correct answer-- but i found that chain rode works well. rode of rope doesnt.
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Old 07-01-2012, 13:04   #14
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Quote:
Originally Posted by delmarrey

I really don't see the weight of the anchor as any help other then to get it started into the bottom. It's the amount of surface area it can grab and keep (plow down) that I look at.
Sure, but without the weight, you would rip it apart. Fluke area and weight go hand in hand. Aluminium isn't suitable for primairy anchors so I leave that out.

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Old 07-01-2012, 13:07   #15
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Quote:
Originally Posted by zeehag
i love how folks who think chain doesnt do anything much find when they use 30 ft of chain with a 45 pound anchor that they ALWAYS drag anchor. when more chain is added there is no dragging. go figger.
heheh

So, instead of adding more chain, adding that same weight of the extra chain to the anchor, by buying a bigger one, would increase the holding power MUCH more than adding that chain. And that is the thing, people do not accept the option because they refuse to upgrade the anchor itself.

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