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Old 04-03-2013, 16:50   #1
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anchor scope

I read something in Sail the other day that caught me off guard. It was in the letters to the experts (and I think Don Casey gave the answer) and was about anchoring.

The thing that caught me was that he said there is no data that scope of more than 4:1 was of any use. Now I learned the standard 7:1 rule and accept that "all chain" of 4:1 when things are normal is enough. But in this case he is saying that 4:1 is as good as it gets and than more than that doesn't ready change the angle far as the anchor shank etc. and doesn't really help.

Once I started thinking about it I could not really think of any real studies or tests with higher scope and had just accepted the "more is better" thinking. Anyone have any links to something about scope far as tests?
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Old 04-03-2013, 17:02   #2
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pirate Re: anchor scope

Quote:
Originally Posted by Don L View Post
I read something in Sail the other day that caught me off guard. It was in the letters to the experts (and I think Don Casey gave the answer) and was about anchoring.

The thing that caught me was that he said there is no data that scope of more than 4:1 was of any use. Now I learned the standard 7:1 rule and accept that "all chain" of 4:1 when things are normal is enough. But in this case he is saying that 4:1 is as good as it gets and than more than that doesn't ready change the angle far as the anchor shank etc. and doesn't really help.

Once I started thinking about it I could not really think of any real studies or tests with higher scope and had just accepted the "more is better" thinking. Anyone have any links to something about scope far as tests?
OK Don... for your boat what is a 4:1 scope.. in say 20ft roller to bottom..
and how much chain for a 7:1 scope...
Sorry... this scope term is something I've only heard from US sailors...
I'm a Brit, dead thick... and also untrained in modern (post 60's) yachting terminology
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Old 04-03-2013, 17:06   #3
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Re: anchor scope

4:1 would be having 100 feet out when the depth from the sea botton to the bow roller is 25 feet.
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Old 04-03-2013, 17:08   #4
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Re: anchor scope

Scope is not a new term, at least in the US. It is the ratio of anchor rode out to water depth.
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Old 04-03-2013, 17:09   #5
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Re: anchor scope

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[FONT=Comic Sans MS][SIZE=3]OK Don... for your boat what is a 4:1 scope.. in say 20ft roller to bottom..
On Don's boat that would be 80 feet.

Back when Boatman61 was young, I learned that maximum efficiency was to be found at 5:1 for chain and 7:1 for nylon. 10:1 was something reserved for severe storm surge.
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Old 04-03-2013, 17:10   #6
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Re: anchor scope

Maybe this will help explain anchor scope and whats needed.

Anchor Scope Illustrated Photo Gallery by Compass Marine at pbase.com
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Old 04-03-2013, 17:18   #7
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pirate Re: anchor scope

And here we are again....
I learnt in boat lengths + depth...
Ergo 20ft + 65ft in Dons anchorage...
My boats 21ft.. weighs 2 tons... all chain + Bruce
35ft boat 130ft..
54ft boat.. 180ft..
More length = more weight = more chain... now ask me how often I drag..
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Old 04-03-2013, 17:28   #8
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Re: anchor scope

Just don't anchor like some of the folks in Mexico with 10-12 feet of water and 10-1 scope which screws it up for anyone else. If you have more than 5-1 scope your just a pain the the butt in most of the smaller anchorages.
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Old 04-03-2013, 17:33   #9
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Re: anchor scope

we are dragging off topic I think
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Old 04-03-2013, 17:45   #10
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pirate Re: anchor scope

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Maybe this will help explain anchor scope and whats needed.

Anchor Scope Illustrated Photo Gallery by Compass Marine at pbase.com
If my rode looked like the 1st example... I'd know for sure I was gonna drag...
Think I'll stick to neanderthal boat lengths of chain and stay put...
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Old 04-03-2013, 17:49   #11
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Re: anchor scope

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we are dragging off topic I think
Actually, I think boatman61 is making an interesting argument, that bigger boats require greater amounts of rode. To some extent that's the reason for adding the height of the roller to the formula. A big boat with a 10' high roller in 20' of water is going to have 120' of rode out at 4:1, while a small boat with only 2' between the roller and the waterline is going to have 68' of rode out at 4:1 in that same anchorage.

So which of those two boats is going to be in better shape at 4:1? Boaty seems to be arguing, indirectly, that 4:1 wouldn't be adequate in this situation for the larger boat.
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Old 04-03-2013, 18:08   #12
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Re: anchor scope

I find it hard to believe that Don wrote that because he's wrong, and he's usually right! There are lots of studies on this. It not only improves the angle of pull at the anchor, but it reduces the load on the anchor by putting either additional catenary and/or additional stretch in the line. Yes, catenary does go away after a certain load is reached, but it does have an effect at normal wind speeds. Besides, I have seen it in action hundreds or thousands of times when I've dropped anchors and played with the scope--too little and it doesn't hold so well, let out more the anchor holds just fine. I also see it in action every weekend in places like Cuttyhunk where due to the crowded conditions a lot of people short scope it and drag anchor during the 2am wind shift or thunderstorm.

Here's our own JonJo's article on this in Practical Sailor.
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Old 04-03-2013, 18:19   #13
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Re: anchor scope

And here's an article by Don Casey himself on this topic where he says increased scope helps, but no testing.
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Old 04-03-2013, 18:25   #14
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Re: anchor scope

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Good article. But we've got to get JonJo a byline.
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Old 04-03-2013, 18:25   #15
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Re: anchor scope

Im with Kettlewell on this one ! More is better ! I use all chain and try to use more then 4-1 if I can ! I hate to drag ! So will use 5-1 or better if Im by myself in a anchorage. With 4-1 I just don't feel comfortable sleeping soundly! LOL but with a little more Im a happy sailor! Of course I don't have one of those fancy anchors LOL maybe when I find one of these new style anchors that fits my dumb set up I might change my mind!
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