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  #1  
Old 03-31-2006
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Anchoring Scope

I am having a disagreement with a friend of mine who insists that the standard scope to use for anchoring is 7:1

Forgive me if I have this wrong, but I learned many years ago and have always used 6:1

Have things changed?

Does Chapman have anything to say about this? My copy is on the boat.


thanks in advance.
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Old 03-31-2006
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It is my belief there are too many variables to use a "standard scope" for anchor rode, but I do remember 7:1 as Chapman's recommendation. We have a 35# CRQ with 300 ft of chain and since most of Narragansett Bay is quite deep, we frequently anchor in 40 to 50 ft of water, even a bit deeper at times.

In most situations, I let out 200-250 ft in 40-50 ft, with a 5:1 scope. This seems more than adequate, especially compared to 280-350 ft using a 7:1 scope.

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Old 03-31-2006
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6 or 7 would be nice, but if you're in a popular anchorage, hard to get--the turning circle's too big. I recall that swinging room was usually the limiting factor, not what the "book" says. If I had the room, I'd go to 9, 10, or the bitter end of the rode, whichever's less.
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Old 03-31-2006
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Canadian Power and Sail Squadron teaches 7:1, but as others have said, swinging room is the realistic yardstick. In places where you can expect to lie in the same direction all night (i.e. in the trades), all the scope you can manage is best. If, however, as here on the West Coast the bays are small, generally deep and winds & currents go back and forth all the time, swinging room (and the practice of those already there) severely limits your options. Around here running sternlines ashore is a common practice to limit swinging but that too can be problematic in a strong cross-wind or cross-current situations.

Use common sense and keep a good eye (and ear) out for trouble overnight!
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Old 03-31-2006
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Not on the subject, but.......

Has anybody been to the airshow in Chicago? You talk about a comedy of errors. Nobody seems to know how to anchor. Boats are dragging everywhere. Its actually not funny, but I can't help from laughing.

We generally go 5;1 to 6:1 in the daytime for lunch or a barbeque, 7:1 for overnight. When in doubt; let more out.
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Old 03-31-2006
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the last chain link should remain horizontal !

[quote=administrator]I am having a disagreement with a friend of mine who insists that the standard scope to use for anchoring is 7:1

Forgive me if I have this wrong, but I learned many years ago and have always used 6:1

[quote]

In fact it will depend on the water height (the wind force and some other factors too.. )
- The principle would be to have at least the last link of the chain horizontal on the sea bottom (which means also the shank of the anchor horizontal too)

There is a very interesting Web page:
http://alain.fraysse.free.fr/sail/rode/rode_b.htm
where you can use an small Excell software to compute all these parameters and know, following the different conditions, which scope you should use to keep this last chain link horizontal..

Usually, I never use less that a 5:1 scope..

Safe anchoring

Alain

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Old 03-31-2006
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7:1 for rope rodes (with chain)
5:1 for chain

Thats the what the angle of flukes (etc.) makes with the bottom so that the anchor keeps digging .... instead of 'skipping' along the bottom or pulling out/up.
In other than 'light' conditions any less scope and the flukes, etc. will begin to come UP through the bottom instead of attempting to go deeper.

If you want to anchor with less rode, get an anchor with deeper 'setting' angles (flukes) or 'short shank' the stock with a connection further down/closer to the flukes.
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Old 04-01-2006
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Seems this depends on type of anchor, chain vs rode, conditions, and situation. Why deduce it to one figure when there are many times this wont apply? Seems Chapmans quotes a range of 7:1/8:1 as satisfactory but there are times when you would go more or less (no less than 5:1 - Chapmans reports anywhere from 5:1-10:1 when heavy). And dont forget to add the height of your bow in the equation.

Administrator: Apologies for the change of subject, but I have written a couple of PMs and severeal email to you over the last few weeks with no response. How does one contact you? I've run out of ways available.
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Old 04-01-2006
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Quote:
Originally Posted by abstrait
Seems this depends on type of anchor, chain vs rode, conditions, and situation
YES and.. NO..

Any anchor will be the most efficient with an horizontal pullling rode..
which means an infinite lenght..

As soon as the line is shortened.. the efficiency will be reduced:
- to 85% for a 10/1 scope
- to 55% for a 4/1 scope
- etc..
Due to the catenary and the weight of a chain rode, a part of it will be lying horizontaly on the bottoms. When the wind will be building up, the rode will progressively straighten itself, up to the point where the last link of chain will no longer be horizontal and the shank of the anchor will take an angulation to the sea bed, loosing some of its efficiency.

You should always, when possible, adapt the rode lenght in order to keep the last chain link horizontal, which is quite easy to perform, using the right formula ( http://alain.fraysse.free.fr/sail/ro...hesis/synt.htm)
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