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04-03-2013, 16:50
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#1
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S/V rubber ducky
Join Date: Jul 2007
Location: Florida cruising currently
Boat: Hunter 410
Posts: 18,807
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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?
__________________
It is OK if others want to do it different on THEIR boat
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04-03-2013, 17:02
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#2
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Seaman, Delivery skipper


Join Date: Jan 2010
Location: PORTUGAL
Posts: 28,534
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Re: anchor scope
Quote:
Originally Posted by Don L
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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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
__________________
It was a dark and stormy night and the captain of the ship said.. "Hey Jim, spin us a yarn." and the yarn began like this.. "It was a dark and stormy night.."
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04-03-2013, 17:06
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#3
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S/V rubber ducky
Join Date: Jul 2007
Location: Florida cruising currently
Boat: Hunter 410
Posts: 18,807
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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.
__________________
It is OK if others want to do it different on THEIR boat
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04-03-2013, 17:08
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#4
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Moderator Emeritus

Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,401
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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.
__________________
David
Life begins where land ends.
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04-03-2013, 17:09
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#5
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CF Adviser
Join Date: Dec 2008
Location: sausalito
Boat: 14 meter sloop
Posts: 7,260
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Re: anchor scope
Quote:
Originally Posted by boatman61
[FONT=Comic Sans MS][SIZE=3]OK Don... for your boat what is a 4:1 scope.. in say 20ft roller to bottom..
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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.
__________________
cruising is entirely about showing up--in boat shoes.
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04-03-2013, 17:10
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#6
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Registered User

Join Date: Dec 2007
Boat: Mahe 36, Helia 44 Evo, MY 37
Posts: 5,731
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Re: anchor scope
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04-03-2013, 17:28
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#8
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Registered User
Join Date: Sep 2012
Posts: 5,921
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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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04-03-2013, 17:33
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#9
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S/V rubber ducky
Join Date: Jul 2007
Location: Florida cruising currently
Boat: Hunter 410
Posts: 18,807
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Re: anchor scope
we are dragging off topic I think
__________________
It is OK if others want to do it different on THEIR boat
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04-03-2013, 17:49
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#11
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CF Adviser
Join Date: Dec 2008
Location: sausalito
Boat: 14 meter sloop
Posts: 7,260
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Re: anchor scope
Quote:
Originally Posted by Don L
we are dragging off topic I think
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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.
__________________
cruising is entirely about showing up--in boat shoes.
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04-03-2013, 18:08
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#12
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Registered User
Join Date: Jul 2006
Posts: 3,945
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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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04-03-2013, 18:19
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#13
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Registered User
Join Date: Jul 2006
Posts: 3,945
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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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04-03-2013, 18:25
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#14
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CF Adviser
Join Date: Dec 2008
Location: sausalito
Boat: 14 meter sloop
Posts: 7,260
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Re: anchor scope
Quote:
Originally Posted by Kettlewell
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Good article. But we've got to get JonJo a byline.
__________________
cruising is entirely about showing up--in boat shoes.
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04-03-2013, 18:25
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#15
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Registered User
Join Date: Nov 2011
Location: Montegut LA.
Boat: Now we need to get her to Louisiana !! she's ours
Posts: 3,421
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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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Bob and Connie
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