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Old 02-12-2009, 01:51   #31
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Hmmm...I think your confusing two separate trains of thought in his post...so I will let him answer your question himself... I see no reference to scope at all.
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Old 02-12-2009, 03:04   #32
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Quote:
Originally Posted by sec906 View Post
Read again: he calculates 2 meters, I calculate 14 meters. So who's right and why?
I don't count the air between the water and the deck as depth.

Scope, to me, means the water depth.


Its more relevant and understandable using 'normal' scopes:

If one is in 2 meter water and wants 3:1 he lets out 6 meters rode. But as he is standing on deck at 2 meters above water only 4 meters are in the water giving a scope of 2:1.

So I am saying the freeboard height should be included: 8 meters = 3:1 plus 2 meters.

But to make it easier I just put that first 2 meters(a constant) in the initial 10m measurement.

I don't think the whole depth is to the bow roller, just to the water surface.


Of course putting out only 8m of rode in 2 m of water is going to drag, so in shallow water I use about 10:1 scope.

Lesser ratio for deeper

I havent tried my windlas to pull up 30m of 10mm chain as I havent anchored that deep. 15 to 20 would be my max without the thought of the back breaking snubber to winch manual lift!




For thenew person it should also be recognised that too much scope out, especially if its rope rode can be very bad in crowded anchorages!!
Fine in strong winds but bump, bump in the calm night against other boats!


Hope this 'elps

PS [edit] I NEVER use less than 10m of chain out. More like I never use less than 20m. We currently are on anchor watch as I have the low tide in about 15 minutes and I think I will have about 20 to 30cms depth under the kee.l Less than 1 foot! We currently have 30 meters of chain out in 2.2 meters water. I feel safe from dragging, just not safe from grounding!!!!!!!!!!!!!!!!!!!!


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Old 02-12-2009, 05:25   #33
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I need a more clear expalnation: if the boat has spun several times....

Quote:
Originally Posted by Hud3 View Post
I'm on the "one anchor" school, the exception being when anchored in a reversing tidal situation.

If you feel you need two anchors, a way to solve the problem of the two rodes wrapping around themselves is to attach a snatch block to your primary rode, lead the secondary through it, and then let out enough of the primary to take the snatch block 5-10 feet under the surface. That way, even if you get some wrap, releasing the snatch block easily undoes it.
How does the snatch block unwind it? There seems to be a basic geometry limitation here.

The only cure I have found is to have a shorter rode that I connect to the primary rode. That has worked every time, without hastle. The "shorter rode" is long enough for 7 or 10:1 scope, it is just shorter than 200'. Since I use a long bridle and anchor in 7-10' of water, that means about 70'.
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Old 02-12-2009, 05:47   #34
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Quote:
Originally Posted by MarkJ View Post
I don't count the air between the water and the deck as depth.

Scope, to me, means the water depth.


Its more relevant and understandable using 'normal' scopes:

If one is in 2 meter water and wants 3:1 he lets out 6 meters rode. But as he is standing on deck at 2 meters above water only 4 meters are in the water giving a scope of 2:1.

So I am saying the freeboard height should be included: 8 meters = 3:1 plus 2 meters.

But to make it easier I just put that first 2 meters(a constant) in the initial 10m measurement.

I don't think the whole depth is to the bow roller, just to the water surface.
EVERY book I've read differs on this point of anchoring with good reason. All of them include bow freeboard in the scope calculation. (You should also consider tidal range, but that's another issue.) Why is that? One of the most critical times in anchoring is when so much force is being exerted on your boat as to pull even a heavy chain taut between your anchor and the bow. Anchor scope is calculated so that the anchor will have a reasonable angle of inclination to the bottom in this scenario. You can visualize an imaginary right triangle with one edge being the rode, one edge dropping vertically from your bow, and one edge extending horizontally from the anchor to meet the vertical edge. Your "scope" is ratio of the hypotenuse over the rise (depth + freeboard). If you base the scope on (depth + freeboard), this angle will always be the same when the rode is taut. Many modern anchors require a very low angle of inclination to be effective.

The issue is actually more complicated than this, though, because the bottom is never flat. Hence, when you anchor so that the anchor sets in rising ground you can theoretically get away with less scope (be careful though), but if you anchor in falling ground you would need more. Since most folks don't like to do trig while they anchor, many opt to be conservative. Anchoring is a complex issue.
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Old 02-12-2009, 06:04   #35
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Originally Posted by marujo.sortudo View Post
. All of them include bow freeboard in the scope calculation.
What? Me wrong? Hells-bells!

We should put the question out to the crusty old buggers of this forum for adjudication!

I looked up Wikepeadia nautical glossary and it didn't even include scope! LOL


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Old 02-12-2009, 06:42   #36
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Scope is normally measured from the deck of the boat to the bottom. So if your freeboard at the bow is five feet you have to add that to the water depth. That's if you're deploying a bow anchor and are concerned with this "scope" thing.

Myself, I just put a whole lot of chain out. As long as there's room. I try to anchor in ten feet or less (not difficult to find skinny water in the Bahamas) and usually put 100 feet of chain in the water.

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Old 02-12-2009, 07:01   #37
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Of course putting out only 8m of rode in 2 m of water is going to drag, so in shallow water I use about 10:1 scope.

Lesser ratio for deeper
Mark
The rode doesn't go straight down to the water from the deck, it goes off at the same angle you are setting for the scope. I think you are compensating by increasing the rode in your formula for shallow water, whereas by the way I would calculate it you're probably maintaining closer to the 7-1 scope.

2 meters freeboard plus 10-1 scope for 2 meters of water is 22 meters of rode.

(2 meters freeboard plus 2 meters of water) times 7-1 scope is 28 meters of rode.
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Old 02-12-2009, 07:13   #38
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Google to the rescue?

The Annapolis book of seamanship - Google Books


Annapolis Book of Seamanship, page 306: "Scope is the ratio between the amount of rode let out and the distance from the deck to the water's bottom (or the freeboard plus water depth)..."
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Old 02-12-2009, 12:43   #39
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1) If you get a chance to add some more chain, and are in waters clear enough to snorkel and look at your anchor it might help with the sleeping. I'm a relative neophyte at anchoring, but just came back from the BVIs on a bareboat charter, and I slept like a baby at anchor there. The 40ft Beneteau had 100ft of chain, and I hooked on the 15 foot of snubber line when at anchor. The waters were crystal clear. After anchoring I would snorkel and inspect the anchor and watch the chain as the boat sailed back and forth in the trades. It was very reassuring to see how even the bigger gusts barely shift the chain, the last 20-30 feet up to the shank lying flat and unmoved.
Granted, the conditions there are ideal with steady trade winds and sandy bottoms. But if you get a chance to study your ground tackle at work it can be very comforting.
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Old 02-12-2009, 12:59   #40
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I use the depth from the bow roller to calculate scope. I once used a snubber that was attached just above the waterline and then I used that as the reference.

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Old 02-12-2009, 16:43   #41
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Well, thanks to Sec909 for first pointing my mistake out and marujo.sortudo for confirming, and for the rest of you for your 2 crusty cents worth.

I shall go back to junior boat school and start with a rubber duckie in a bath tub. I jus' got no bath tub!

Of course it must be from the bow roller. If one imagines a ship where the freeboard is 10 meters anchoring in 2 meter water the angle to the anchor stock would just pull it out.


Thanks all, its food for thought. Its never given me a problem as we lay out so much chain, but great for the future etc.


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Old 02-12-2009, 18:52   #42
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You guys are scaring the new folks....

We really DO know what we're talking about.... usually .... when no one checks up on us..... OK maybe not so much.... what was that splash I just heard honey?.... honey? ..... HONEY??!!

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Old 02-12-2009, 19:12   #43
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You will eventually learn to trust your gear and your selves enough to not only sleep well at anchor, but to love it [and each other] in the process. I have spent far more nights at anchor than at a dock. It is good you relegated the danforth to some more appropriate use. You current gear is good enough. I always sail without auxiliary power, so I anchor the old Bahamian style. I sail in, pick my anchorage, set up my anchor and rode for quick drop, then run down on the spot and drop the anchor under sail. The rode stops the boat and pivots it on the hawse. I then quickly drop the job, but wait on the main while I take visual bearings on some spot and wait a few minutes until I know I am not still moving. Then I drop the main, tidy all up. I use a 20 ft line attached to the waterline as a snubber. I wrap a rolling hitch on the anchor rode and let the rode out until I am riding to the snubber, then cleat off the anchor rode with a bight hanging down. Now I am riding to a line from my waterline, thus increasing scope, decreasing roll in a chop, and removing all chafe from my anchor rode. Of course, without a bowsprite there is usually no need for the waterline hardware, so you may not be able to do this. The rest of the system may work for you. My best advice is to read, read, read. And stay on the hook as much as you can. Congratulations on taking the plunge that so many people fear to do. Be careful about using your engine where there are lots of anchor rodes in the water. Anchor some place where you are alone and practice doing it all under sail. When you get that down, your confidence will really take a boost. For good reason. Way to go, sailor!
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